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ROADSIDE DESIGN GUIDE
PAGE 1
01 INTRODUCTION FIRST EDITION -DECEMBER 2016
1 INTRODUCTION
1.1 Overview
In 2010, the Abu Dhabi Department of Municipal Affairs and Transport commenced with the “Unifying and
Standardizing of Road Engineering Practices” Project. The objective of the project was to enhance the
management, planning, design, construction, maintenance and operation of all roads and related
infrastructures in the Emirate and ensure a safe and uniform operational and structural capacity throughout
the road network.
To achieve this objective a set of standards, specifications, guidelines and manuals were developed in
consultation with all relevant authorities in the Abu Dhabi Emirate, including the Department of Municipal Affairs
(DMA) and Urban Planning Council (UPC). In future, all authorities or agencies involved in roads and road
infrastructures in the Emirate shall exercise their functions and responsibilities in accordance with these
documents. The purpose, scope and applicability of each document are clearly indicated in each document.
It is recognized that there are already published documents with similar objectives and contents prepared by
other authorities. Such related publications are mentioned in each new document and are being superseded
by the publication of the new document, except in cases where previously published documents are recognized
and referenced in the new document.
1.2 Purpose and scope
The guidance supplied in this document, the Abu Dhabi Roadside Design Guide (ADRSDG), is based on
established international practices. This especially includes the American Association of State Highway and
Transportation Officials (AASHTO) Roadside Design Guide [1] and is supplemented by recent research and
documents prepared by the CEDR funded ‘SAVERS’ [2] project, the United Kingdom, Austroads [3] and Dubai
[4].
This Guide has been prepared to provide uniform practices for government transport agencies within the Abu
Dhabi Emirate and consultant personnel preparing studies, reports and contract road plans for these agencies.
The designer should use this document to develop roadside designs that meet the operational and safety
requirements while preserving the aesthetic, historic, and cultural resources of an area. This Guide will be
updated regularly as new data and experience with best practices become available.
Roadside design is defined as the design of the area between the outside shoulder edge and the right-of-way
limits. The Abu Dhabi Emirate is reducing the number of fatalities year on year, however, roadside crashes
account for a significant portion of the total fatal highway crashes. Approximately 25 percent are the result of
a single vehicle run-off-the-road crash. This emphasizes the importance of providing as safe a roadside design
as practical.
1.3 The application of the Abu Dhabi Roadside Design Guide
The guidance presented in this document is intended for the roadside design of new construction and major
reconstruction of roads and streets located within the Abu Dhabi road network. Local municipalities are
encouraged to adopt these guidelines to ensure uniformity of roadside designs
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01 INTRODUCTION FIRST EDITION -DECEMBER 2016
within the Emirate. Design guidance is provided, and is considered applicable, for all facility types in both urban
and rural locations.
The designer should attempt to meet all criteria and practices presented in this Guide; however, it should not
be considered a standard that must be met regardless of cost and impacts. Designers must exercise good
judgment on individual projects and, frequently, they must be innovative in their approach to roadside design.
Designers should review the listed references to gain an understanding of the basis for the selected roadside
design criteria. The guidance provided in this document should not be a substitute for good engineering
knowledge, experience, or sound judgment. The concepts, designs and philosophies presented in this Guide
may not be applicable to every project. Each project is unique and offers an individual opportunity to enhance
that particular roadside environment.
The amount of monetary resources available for all roadside safety enhancements is limited. The objective of
designers is to maximize roadside safety on a system-wide basis with the given funds. Accomplishing this
objective means addressing those specific roadside features that can contribute the most to the safety
enhancement of that individual project. Given that objects and slope changes must be introduced at varying
points off the pavement edge, the enhancement of roadside safety involves selecting the “best” choice among
several acceptable design alternatives. This document is intended to represent the spectrum of commonly
available roadside design alternatives.
1.3.1 Background to Some of the Key Approaches Adopted
One of the main aims of this Guide is to provide guidance on roadside design based on international best
practice which fits best to reflect the local conditions, requirements and applications. To help achieve this, a
team of internationally recognised, independent road safety specialists have carried out site visits around the
Abu Dhabi Road Network, reviewed the existing processes and assessed the local conditions in collaboration
with the Abu Dhabi Department of Municipal Affairs and Transport. A summary of these assessments were
presented in the “Abu Dhabi Roadside Design Guide – Inception Report” (See Appendix B).
An important outcome of the Inception Report was recommendations for some key approaches, which helped
form the basis of this Guide. An overview of these key approaches is presented in the following sections. For
more detail, please refer to Appendix B.
1.3.2 A Performance Based Approach Rather Than a Prescriptive One
During the development of Vehicle Restraint Systems (VRS), the number and type of systems available to the
roadside designer/engineer were limited. The limited number of designs would have distinct performance
characteristics and therefore the designer/engineer could prescriptively refer to these systems. For example a
designer/engineer could expect a concrete or a cable barrier to perform at a certain performance level. As
alternative systems were developed, the available types of different systems increased substantially, and now
there are many different systems available on the market together with variants of the same original design,
all with slightly different performance characteristics. As the number of the alternative designs increased, the
gaps between the performance levels of the original VRS designs were filled by new products. Therefore it is
no longer valid to use a product-specific prescriptive approach, where reference is only made to a number of
specific designs.
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01 INTRODUCTION FIRST EDITION -DECEMBER 2016
A prescriptive approach, which focuses on the use of a limited number and type of systems, is not necessarily
the best one. The lack of choice can force a designer/engineer to use systems which may not be the optimal
solution for a specific site in terms of performance and cost. This sort of prescriptive approach not only limits
the ability of the designer to use engineering judgment but it also makes it difficult for new systems to enter
the market. It furthermore leads to less of an understanding from the engineer, which may lead to inappropriate
systems and arrangements being used in certain locations.
In order to overcome these issues, current international best practice is to identify the performance
requirements and physical constraints of a site under consideration and then let the engineer/designer chose
the most appropriate solution based on proven properties of impact tested and approved systems. Therefore,
this version of the manual is based on the required performance specifications, rather than prescriptive
descriptions of certain products. In order to achieve this, rather than listing the design details of certain products
available on the market, the proposed layout of the roadside design guide was organised to explain, for each
VRS type, in basic terms:
• The main types of systems available on the worldwide market;
• The performance classifications of the products based on impact testing;
• The selection criteria based on the performance classifications of the product.
1.3.3 The Use of Proprietary over Non-Proprietary Systems
The term ‘proprietary’ means that the system has been independently designed and successfully tested by a
VRS manufacturer, who is ultimately responsible for the design of the system. Conversely, non-proprietary
systems have often been developed by National Road Authorities and/or Universities, who have subsequently
published the drawings of the system and, as such, these can be manufactured by anyone.
Whilst many of the non-proprietary systems have historically performed well, many of these systems were
developed a number of years ago, and were tested to older versions of the testing standards. Due to budgetary
constraints within the original developers of the systems, these systems are often not updated, nor are they
supported in terms of future development and product refinement.
This is not the case in the commercial domain of the proprietary systems, where development of systems
continues on a frequent basis, with product designers keen to develop more effective products than their rivals,
testing and certifying products to the latest standards. This competition and the continuous development have
led to proprietary products which perform better (higher containment levels with less deflection and lower
acceleration severity) than their non-proprietary predecessors.
For these reasons, it is now common practice for many countries to install proprietary systems, with a number
of countries, such as the UK, USA (FHWA) and Ireland maintaining a list of ‘accepted’ vehicle restraint systems
products. Therefore the use of proprietary systems is recommended over non-proprietary ones.
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01 INTRODUCTION FIRST EDITION -DECEMBER 2016
1.3.4 Use of a Single Testing Standard
In the past, barriers successfully tested to the requirements of USA NCHRP350 [5]/MASH [6], European
EN1317 [7], and non-proprietary systems were permitted for use on the Abu Dhabi road Network, and in many
other countries. However, the testing parameters for these two types of international systems are very different.
As a result, no comparison or equivalence can be assumed between systems tested to the different standards.
This can lead to, for example, inappropriate transitions between systems, and unknown levels of performance
where the performance of one system depends on that of another (for example where the performance of a
safety barrier relies on the performance of a terminal). For this reason it is recommended that systems which
are tested only to a single standard are permitted for future use on the Abu Dhabi road network.
1.3.5 The Use of NCHRP350 and MASH Tested Products Instead of
EN1317
The road network and the vehicle fleet in Abu Dhabi are more similar to an American one rather than a
European one. The adoption of the American testing standards NCHRP350 and MASH are felt to be more
suitable for the Abu Dhabi road network, over the European test standard EN1317 for the reasons explained
in the following sub-sections. However, it is felt that the knowledge of the European test standard EN1317 [7]
can still be useful to designers/engineers, especially when comparing VRS products tested to different
standards. The Appendix A provides a brief overview of the EN1317. The information provided includes the
test types and performance classes and test criteria for barriers, bridge parapets, terminals, transitions and
crash cushions.
1.3.5.1 Weight of Vehicles used in NCHRP350, MASH & EN1317
Vehicles used in NCHRP350 and more recently MASH, represent larger and heavier vehicles, such as pickup
trucks, which are more common in the USA; whilst EN1317 uses smaller and lighter vehicles which are more
common in Europe. This can be seen by comparing Test Level-3 of MASH and NCHRP350 to containment
level N2 of EN1317 as these are the most commonly used base performance criteria for America and Europe,
respectively.
In MASH, a product should perform satisfactorily during impact testing to achieve the basic test level TL-3
using a 1,100 kg passenger car and a 2,270kg pickup truck. While in EN1317 to achieve an N2 containment
level, a barrier should perform satisfactorily using a 900kg passenger car and a 1,500kg passenger car (see
Appendix A). There is currently no vehicle type in EN1317 which represents an SUV or a pickup truck, which
are common in the Abu Dhabi vehicle fleet.
As a result, it can be said that American testing standards MASH and NCHRP350, offer a better representation
of the vehicle fleet, more specifically enabling vehicle restraint systems to prove their ability to safely contain
and redirect (under standard testing conditions) the high percentage of SUVs in Abu Dhabi; while the vehicle
classes in EN1317 do not represent the SUVs and therefore are not guaranteed to work on impact with these
types of vehicles.
1.3.5.2 Height of Vehicles used in NCHRP350, MASH & EN1317
The weight of the vehicles used in NCHRP350 and MASH are a better representation of the SUVs which
represent a high percentage of the vehicles used in Abu Dhabi. However, it is not only the
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01 INTRODUCTION FIRST EDITION -DECEMBER 2016
weight of the vehicles that make American standards a better fit for Abu Dhabi, but it is also the height, more
specifically the height of the centre of mass, required for the test vehicles.
Vehicle rollover is one of the more common types of incidents observed in Abu Dhabi. Vehicles with a higher
centre of mass, such as SUVs are more likely to roll over. The minima required centre of mass locations for a
pickup truck for testing to the basic TL-3 containment levels in MASH & NCHRP350 are 710mm and
700±50mm, respectively. In EN1317 the highest vehicle used to test a N2 normal containment level, requires
to have a centre of mass location 530mm above the ground. As can be seen from these numbers, EN1317
test vehicles are far from representing the higher centre of mass seen with SUVs, which are common in Abu
Dhabi. Therefore, there is no guarantee that an SUV, which is likely to have a centre of mass higher than
530mm, will be contained by an N2 containment level barrier.
1.3.5.3 Impact Angles used in NCHRP350, MASH & EN1317
Another important difference between the American and European testing standards is the angle of impact.
A good comparison can be made between the base performance levels of TL-3 for MASH/NCHRP350 and N2
for EN1317. MASH uses an impact angle of 25˚ for car and pickup truck impacts, to demonstrate a successful
TL-3 classification. EN1317 on the other hand, uses an impact angle of 20˚ for TB11 and TB32 tests, which
are required to demonstrate a successful classification of N2.
The roads in Abu Dhabi, in general, appear to be wider than the European ones, with expressways regularly
featuring up to 4 lanes in each direction with generous shoulder areas provided on each side of the road.
Research shows that in similar conditions, impacts with roadside barriers are more likely to occur with higher
angles of incidence on wider carriageways. For this reason it is believed that the larger impact angles used in
MASH and NCHRP350 may provide a better representation of the actual impacts that are likely to occur on
the Abu Dhabi road network.
1.4 Scope of the ADRSDG
1.4.1 Overview
This Guide has been developed to enable practitioners to follow a step-by-step risk-based process to
understand and mitigate the risks posed by hazards, integrating International best practice.
This has been achieved by first introducing the user of the Guide to the Risk Mitigation Approach, explaining
the concept of the clear zone, and assisting users in the identification of hazards. The following chapters
provide information on experience-based advice and internationally recognised practices, to assist Engineers
in dealing with the hazards present. This includes details of the various product types available on the market
to mitigate road user risk. An overview of the ways in which these solutions can be assessed on an economic
basis is then presented. The final chapter of this Guide deals exclusively with the risks existing within the urban
environment, and ways in which such risks should be assessed. An overview of the content of each chapter
is presented below:
1.4.2 Chapter 2 “Risk Mitigation Approach”
This chapter discusses the hazard mitigation process and the forgiving roadside approach to road safety. The
Chapter also introduces the clear roadside concept and its application to roadside design.
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1.4.3 Chapter 3 “Concept and Calculation of Clear Zone”
This chapter gives further details of the clear zone concept, providing advice on how the clear zone distance
should be calculated for straight sections, foreslopes, backslopes and for curved sections of road.
Consideration is also given for determining the clear zone for medians and high risk hazards. To aid
understanding, examples of clear zone calculations are also detailed.
1.4.4 Chapter 4 “Identification of Roadside Hazards”
Once the user of this Guide has followed the procedures within Chapter 3 to determine the clear zone for a
particular road scheme, location or hazard, Chapter 4 assists the designer in determining whether objects
and/or features within the clear zone are hazards. This includes the identification of risks to third parties.
1.4.5 Chapter 5 “Passively Safe Support Structure and Traversable
Objects”
This chapter investigates ways in which hazards can be made passively safe or traversable (i.e. less
hazardous to road users in the event of an impact). Advice is given on the types of commercially available
systems and technologies which exist, and how these should be selected and applied.
1.4.6 Chapter 6 “Description of Roadside, Median and Bridge Barriers”
If a hazard cannot be made passively safe or traversable, one of the most common road safety devices is the
barrier. However, there are many different types available and this chapter gives an overview of the three
different types available (flexible, semi-rigid and rigid). Details of the testing procedures for barrier systems are
then explained.
1.4.7 Chapter 7 “Selection and Application of Roadside, Median, and
Bridge Barriers”
This chapter firstly gives an overview of the way in which roadside, median and bridge barriers should be
specified, in terms of their containment level, deflection characteristics, impact severity level and both
maintenance and inspection requirements.
Secondly, the chapter provides guidance on how the barrier systems should be used on the roadside in terms
of the length required and where this should be sited, how to place the barrier laterally at the side of the road,
and how to flare back the end of the barrier to reduce road user risk. Guidance on barrier foundations, such
as compaction requirements, push-and-pull test and common foundation mistakes to avoid, are also given.
Specific guidance is also presented for roadside barriers (in terms of barriers which are installed on sharp
horizontal curves), median barriers (in terms of emergency and maintenance crossings and glare screens),
and bridge barriers (in terms of minimum height requirements and fixation to bridge decks).
1.4.8 Chapter 8 “Motorcyclist Protection Systems”
This chapter gives details of the different types of motorcyclist protection systems (MPS) available (continuous
and discontinuous), and gives details of the impact testing requirements for such systems. An explanation of
the performance classifications resulting from this testing is also given, together with a description of how to
use the performance classifications for the application of the
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01 INTRODUCTION FIRST EDITION -DECEMBER 2016
products. Specific guidance is given with regard to locations where a positive cost/benefit could result from the
application of an MPS.
1.4.9 Chapter 9 “Terminals”
This chapter gives details of the different types of terminals available (ramped down end, full height, flared and
buried), and gives details of the impact testing requirements for such systems. An explanation of the
performance classifications resulting from this testing (for example gating and non-gating terminals) is also
given, together with an explanation of how to use the performance classifications for the application of the
products. Specific guidance is provided with regard to the site grading for terminals.
1.4.10 Chapter 10 “Crash Cushions”
This chapter gives details of the different types of crash cushions available (redirective/ non- redirective,
sacrificial/reusable and both low maintenance and self-restoring crash cushions). The chapter gives details of
the impact testing requirements for such systems, and an explanation of the performance classifications
resulting from this testing (for example gating and non-gating terminals). An explanation of how to use the
performance classifications for the application of the products is also outlined. Specific guidance is provided
with regard to the site grading for crash cushions.
1.4.11 Chapter 11 “Transitions”
This chapter gives details of the different types of terminals available and gives details of the impact testing
requirements for such systems. An explanation of the performance classifications resulting from this testing is
also provided, together with an explanation of how to use the performance classifications for the application of
the products. Specific guidance is provided with regard to the design of terminals, and examples of good and
bad practice are outlined.
1.4.12 Chapter 12 “Economic Assessment”
The chapter discusses the use of economic analysis to make roadside safety decisions, and provides an
overview of the economic assessment process which should be undertaken. The first part of the chapter
focuses on the assessment of economic feasibility for alternative roadside treatment options. A step by step
guide into benefit/cost ratio (BCR) analysis is presented. Guidance is provided into the prediction of expected
number of crashes at a site, prediction of decrease in the number of crashes due to a safety treatment and the
estimation of monetary benefits associated with roadside crashes prevented. Example calculations are
included for each step of the BCR analysis. A wide range of crash modification factors is provided to help
designer/engineers estimate the reduction in the number and/or severity of crashes due to specific roadside
safety treatments.
The second part of the chapter focuses on treatment prioritization methods. Guidance is given on treatment
assessment and ranking based on cost-effectiveness, risk reduction and non-monetary considerations.
1.4.13 Chapter 13 “Urban Roadside Design”
This final chapter of this Guide identifies risk relating to the specific case of urban areas, and provides
overarching guidance as to how the risks in these areas differ and, hence, which additional measures should
be considered when mitigating risk to road users, and to vulnerable users of the road corridor (such as
pedestrians).
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01 INTRODUCTION FIRST EDITION -DECEMBER 2016
Guidance is provided on the lateral offset required between the roadway and the roadside hazards, for different
road configurations, such as curves, merge locations and junctions. Information is also provided on the specific
applications for pedestrian and bicyclist facilities. Finally, specific guidance is provided into the application of
common urban roadside features, such as curbs, pedestrian barriers, street furniture, etc.
1.5 References
[1]AASHTO, Roadside Design Guide, 4th Edition, Washington D.C.: American Association of State Highway
and Transportation Officials, 2011.
[2]CEDR, “SAVERS (Selection of Appropriate Vehicle Restraint Systems) - WP1: Defining the Different
Parameters which can Influence the Need and Selection of VRS (Unpublished Report),” Conference of
European Directors of Roads, 2014.
[3]Austroads, Guide to Road Design Part 6: Roadside Design, Safety and Barriers, Sydney, NSW: Austroads,
2010.
[4]Roads & Transport Authority, Roadside Design Guide for Dubai, First Edition, Dubai: RTA, 2008.
[5]NCHRP, “NCHRP Report 350, Recommended Procedures for the Safety Performance Evaluation of
Highway Features,” Transportation Research Board, National Research Council, Washington DC, 1993.
[6]AASHTO, “Manual for Assessing Safety Hardware,” Ammerican Association of State Highway and
Transportation Officials, Washington DC, 2009.
[7]CEN , “EN 1317 Road Restraint Systems - Part 2: Performance classes, impact test
acceptance criteria and test methods for safety barriers including vehicle parapets,” CEN (European
Committee for Standardization), Brussels, 2010.
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02 ROADSIDE RISK MITIGATION FIRST EDITION -DECEMBER 2016
2 ROADSIDE RISK MITIGATION
2.1 Introduction
There are many reasons why vehicles may leave the road and potentially encroach on the roadside. These
include:
• Driver fatigue or inattention;
• Excessive speed;
• Crash avoidance;
• Roadway conditions (e.g. pavement deterioration);
• Vehicle component failure;
• Poor visibility; and
• Driver impairment.
When a vehicle runs off the road, it may reach a hazard, collide or overturn; all of which may result in injuries
or even fatalities. These casualties may be reduced by making every roadside flat, traversable and free of
obstacles; therefore giving enough space to errant vehicles to regain control and return to the road with a
reduced likelihood of injury. However, in reality this is not always possible due to physical and economic
constraints. Engineers and designers often have to find an optimal solution; one that finds a balance between
the maximum amount of safety and economic feasibility.
A good way of achieving this is to evaluate each roadside on a case by case basis with a risk based approach.
Not every road has the same probability of a vehicle running off the road, nor does every roadside hazard
have the same level of consequences, if reached by an errant vehicle. It is important to identify and prioritise
the sites with a higher level of risk and apply the necessary countermeasures to keep the risk to a reasonable
level.
This approach forms the basis for the roadside design guides and standards of many countries around the
world [1]. In the UK the decision for roadside treatments is based on The Road Restraint Risk Assessment
Process (RRRAP); a software based tool which aims to decrease the level of risk for the evaluated area to “As
Low as Reasonably Practicable” [2]. In Germany, the decision on whether to implement a roadside barrier is
based on the probability of a vehicle running off the road and the level of risk posed by different type of hazards
[3]. In the United States, a recommended practice is the evaluation of different roadside treatment options with
Roadside Safety Analysis Program (RSAP); a risk based benefit/cost analysis tool [4].
Following International best practice, the Abu Dhabi Roadside Design Guide is structured as an easy to use
and understand roadside risk mitigation tool.
This chapter presents an explanation of the concept of risk from a roadside safety perspective and the
recommended risk mitigation approach of this Guide.
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2.2 Definition of Risk from a Roadside Safety Perspective
There are many definitions of hazard and risk, but for the purpose of roadside safety design a hazard can be
described as a roadside feature or object that can cause physical, economic, time- based or strategic harm or
loss. Risk is the chance, high or low, that somebody or something will be harmed by the roadside hazard.
Risk, as shown in Figure 2.1, is directly related to the likelihood of the hazard being reached by a vehicle and
the resulting consequences if the hazard is reached.
Figure 2.1 - Risk from a roadside safety perspective
As shown in Figure 2.1, the likelihood of a roadside accident depends on the probability of a vehicle running
off the road and the probability of the errant vehicle subsequently reaching the hazard if it does leave the
carriageway. Run-off-the road probability is related to parameters such as traffic volume and horizontal curve
radius; while the probability of an errant vehicle reaching a hazard depends on factors such as the distance of
the hazard from the edge of the travelled way, speed of the errant vehicle and roadside topography.
When a vehicle reaches a hazard, the most obvious consequences are the ones to the occupants of the
vehicle, in the form of physical harm and economic loss. But some hazards, if reached by errant vehicles can
have consequences for third parties as well. For example, an errant vehicle reaching the opposite side of a
dual carriageway can cause serious harm to the people travelling on the other side. An errant vehicle entering
a water reservoir may contaminate the drinking water needed by many others.
Engineers and designers can mitigate the level of risk by controlling either or both likelihood and/or
consequences of a roadside accident. The following sections present the methodology to achieve this.
2.3 Abu Dhabi Roadside Design Guide Risk Mitigation Approach
2.3.1 Outline of the Risk Mitigation Approach
Figure 2.2 presents the risk mitigation approach adopted for this Design Guide. As can be seen from the figure,
the corresponding chapter/s for each step is also presented.
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02 ROADSIDE RISK MITIGATION FIRST EDITION -DECEMBER 2016
Figure 2.2 – Abu Dhabi Roadside Design Guide risk mitigation approach
The following sections present further explanation on each step of the hazard mitigation approach.
2.3.2 Step 1 - Understand the Area under Evaluation
The first action the designer/engineer needs to take is to gather information and understand the conditions at
the site being evaluated. The area under evaluation may be a section of roadside / median, or it may even be
a specific hazard with a known history of accidents. As defined in Section 1.3.1, the area of evaluation may be
a part of the design of a new construction or a major reconstruction of an existing road.
Information gathered at this stage will not only be necessary in the following steps of the risk mitigation process,
but it will also enable the designer/engineer to establish an overall
Design the Optimal
roadside treatment
Step 7
Choose the Optimal
treatment option
Step 6
Chapter 12
Assess & Rank
treatment options
Step 5
Shield Delineate
Safe
Roadside
Provide Recoverable Make Passively
Relocate
Remove
Chapters 5 to 11
Identify applicable
treatment options
Step 4
Identify the hazards
within clear zone
Step 3
Calculate the
clear zone
Step 2
Chapter 4
Understand the area
under evaluation
Step 1
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02 ROADSIDE RISK MITIGATION FIRST EDITION -DECEMBER 2016
understanding of the site and hence will enable them to make more informed decisions in each of the following
steps of the risk mitigation process.
The following are some example questions which can guide a designer/engineer into a better understanding
of the area:
• What are the traffic characteristics? Traffic information such as volume and speed are not
only necessary for the clear zone calculations (See Section 2.2.3), but they are also useful
during the assessment of treatment options (See Section 2.2.5). For example the
designer/engineer may consider a motorcyclist protection system (MPS) installation
depending on the volume of motorcyclist traffic; subject to engineering judgment. However,
depending on the traffic speed, they may not always be able to gain the full benefits from
these systems, as they are designed to work up to certain impact speeds. (See Chapter 8).
• What are the physical characteristics of the road and the roadside? Geometric
characteristics such as horizontal curve radius and the gradient of the side slope are not only
necessary information for the calculation of the clear zone (See Section 2.2.3), but they may
also become physical boundary conditions; limiting the type of countermeasures, which may
be applicable to the site. For example, relocating a hazard (See Section 2.2.5.2) may not be
a possible option if there isn’t enough physical space for this, and all mitigation measures,
such as crash cushions and terminals have a defined road space requirement, both for their
installation, and their operation.
• What is the history of accidents on the site? This document is designed to give guidance
to designers/engineers on the mitigation of most common roadside risks, based on
international best practice. However, every site is different and should be evaluated
individually. Studying the accident history can reveal local problems, which would help in a
more focused risk mitigation approach.
• What and why are the potential hazards located on the roadside? Identifying the
potential hazards on the roadside is one of the required steps of the risk mitigation approach
(see Section 2.2.4). But understanding the nature and reasons why they are located on the
roadside is also important for identifying applicable countermeasures. For example, a
roadside sign may need to be within a certain distance to the roadside, and therefore
relocating it beyond the clear zone may not even be an applicable countermeasure.
Figure 2.3 introduces an example of a typical roadside hazard in Abu Dhabi. Examination of this example case
will be followed through in the subsequent step-based analysis. The figure represents some of the information
that needs to be gathered at Step 1.
2.3.3 Step 2 – Calculate the Clear Zone
The second step of the hazard mitigation process is the calculation of the clear zone. The clear zone is the
area beside the road, along which the majority of the errant vehicles are expected to regain control. This is the
area which should ideally be kept clear of any hazards; hence leading to the title “Clear Zone”.
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Figure 2.3 – Understanding the area under evaluation
Clear Zone effectively represents the “likelihood” element of the risk formula shown in Figure 2.1, as it is a
measure of how far errant vehicles are likely to travel along the roadside. It therefore gives an idea of the
likelihood of a hazard being reached, depending on parameters such as traffic volume, design speed,
horizontal curve radius, gradient of side slope and the distance of the hazard from the edge of the travelled
way. By controlling one or more of these parameters, the designer/engineer may be able to decrease the
likelihood of a hazard being reached; hence decreasing the risk posed by the hazard.
Figure 2.4 shows the required clear zone area for the example roadside and it can be seen that the sign post
lies within it. Hazards lying within the clear zone are more likely to be reached by errant vehicles than the
hazards lying beyond it.
Chapter 3 presents a detailed explanation of the concept and the calculation of recommended clear zones.
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Figure 2.4 – Clear zone area
2.3.4 Step 3 – Identify Hazards within Clear Zone
Step 3 in hazard mitigation process involves the identification of all roadside hazards within the clear zone and
consideration of high-risk hazards beyond the clear zone, for example a railway line. The road designer should
identify all roadside hazards within the area of interest (based on clear zone widths). However, it is not always
straightforward to understand if and when a roadside feature becomes a hazard. For example, a roadside ditch
may be considered a hazard or a traversable roadside feature, depending on its geometry. A tree may be
considered a hazard or not, depending on the diameter of its trunk. A shallow pool of water may become a
hazard with floods following seasonal rain.
To help the designer/engineer with identifying hazards within the clear zone, Chapter 4 presents a detailed
explanation of the types and properties of the most common types of roadside hazards. The following types of
hazards are explained in detail within Chapter 4:
• Foreslopes (Embankments);
• Backslopes(Cutting Slopes);
• Ditches;
• Transverse Slopes;
• Trees;
• Overhead Gantries and Cantilevers;
• Other Sign Supports;
• CCTV Masts and Luminaire Supports;
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• Concrete Foundations Protruding from the Ground;
• Bridge Piers, Abutments and Portals;
• Bridge Railing Ends & Ends of Concrete Barriers;
• Above Ground Equipment;
• Culverts, Pipes, Headwalls;
• Pedestrian Fences and Walls;
• Retaining Walls;
• Noise Barriers;
• Bodies of Water;
• Adjacent Roads and Carriageways;
• Storage of Hazardous Material;
• Places of Frequent Pedestrian Activity / Places of Public Gathering;
• Cycle Lanes;
• Structures at Risk of Collapse;
• Rail Lines
• Speed cameras.
Hazard identification effectively relates to the “consequences” element of the risk formula shown in Figure 2.1,
as it is a measure of how severe the consequences would be if an errant vehicle reaches a hazard. Hazards
that lie within the clear zone, as previously shown in Figure 2.4, pose a risk to road users and this risk should
be mitigated through one of the treatment options explained in the following step.
2.3.5 Step 4 – Identify Applicable Treatment Options
Where hazards exist within the clear zone (or outside the clear zone in the case of high- consequence
hazards), potential treatment options should be identified so that their effectiveness in reducing the risk
associated with the hazard can be assessed.
The following are the basic treatment options that should be considered:
• Remove the hazard;
• Relocate the hazard;
• Make the hazard passively safe or traversable;
• Shield the hazard with a longitudinal barrier or crash cushion;
• Delineate the hazard;
• Design roadside safety (refer Austroads Sections 4.4 bullet point no: 3).
These options are listed in an order of decreasing desirability from a safety perspective; i.e. it is more desirable
to completely remove a hazard than shielding it with a barrier, if costs and physical attributes are not
constraints. Details of each of these approaches are given in the subsequent sections.
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2.3.5.1 Remove the hazard
The first approach in decreasing roadside risk is to remove the hazard. This is the most desirable treatment
option from a safety perspective as it completely eliminates the risk of a roadside accident through eliminating
the consequences if a vehicle were to run-off-the road.
This option, although desirable from a safety perspective, may not always be physically possible, as the hazard
may be an item of essential roadside infrastructure, or it may not be cost effective, as the costs of completely
removing the hazard may not be justified by the benefits. See Chapter 12 for more information on Economic
Assessment. Figure 2.5 follows the previous example and shows the hazard removed from the clear zone,
therefore eliminating the risk.
Figure 2.5 – Hazard removed from the area
2.3.5.2 Relocate the hazard
If removing a hazard is not physically possible or cost effective, the second option to be considered is the
relocation of the hazard beyond the clear zone, where it is less likely to be reached by an errant vehicle.
Relocating a hazard may not always be physically possible due to right-of-way or other physical constraints.
For example a roadside sign may have to be within a certain distance from the travelled way, so that it is clearly
visible by all travelling vehicles. Relocating some of the roadside features may be too expensive to justify the
benefits. Figure 2.6 follows the previous example and shows it with the hazard relocated beyond the clear
zone.
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Figure 2.6 – Hazard relocated beyond the clear zone
2.3.5.3 Provide recoverable roadside
This treatment option refers to the provision of a recoverable roadside between the road and the hazard, which
would assist the driver to regain the control of the vehicle once it has left the road, before reaching the hazard.
This type of treatment is related to the likelihood part of the risk model, more specifically to the probability of
an errant vehicle reaching the hazard, once it runs off the road. An example to this kind of treatment is flattening
of a foreslope between the edge of the travelled way and a hazard. Errant vehicles travel further on steeper
foreslopes and therefore a wider clear zone is required. By flattening the foreslope, designer/engineer may
decrease the required clear zone to a point where the hazard is left outside. Therefore the risk of a vehicle
reaching the hazard would be considerably less. Section 3.3.3 provides further guidance on the effects of
roadside topography on the required clear zone.
2.3.5.4 Make the hazard passively safe or traversable
Another treatment option is to make the hazard either passively safe or traversable. For example, it may be
possible to replace a fixed sign post with a crash tested passively safe alternative, such as slip-base system
(see Figure 5.1 in Chapter 5). A cross-drainage culvert may be made traversable by installing an appropriate
grate to cover the opening (see Section 4.3.2.9).
Unfortunately, not every roadside hazard can be made passively safe or traversable; therefore this alternative
may not always be applicable. Chapter 5 provides guidance on the use of available passively safe roadside
hardware and the ways of making roadside features traversable.
Figure 2.7 shows the example roadside with the sign post still within the clear zone, but with the post made
passively safe through the use of a slip base system.
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Figure 2.7 – Hazard made passively safe
2.3.5.5 Shield the hazard with a longitudinal barrier or crash cushion
If none of the previous alternatives is physically possible or cost effective, the alternative of shielding the hazard
with an appropriate VRS (or a longitudinal barrier or crash cushion) should be considered. At this point the
designer should remember that vehicle restraint systems, although designed to provide controlled impact, are
a hazard themselves and they should ideally be used only if the consequences of hitting the VRS is likely to
be less than the consequences of reaching and/or impacting the hazard behind.
VRS systems have their own physical requirements, which they need in order to perform as designed, such
as enough clear space behind, minimum length of installation, etc. The roadside under consideration may not
always have the required physical space or it may not always be cost effective to install VRS for hazards that
are less likely to be reached.
Chapters 6 to 11 provide comprehensive information on vehicle restraint systems; i.e. roadside and median
barriers, parapets, terminals, transitions and crash cushions. Information on the general principals of these
systems, testing requirements and detailed design properties are also provided.
Figure 2.8 shows the example roadside with the sign post being shielded by a roadside barrier.
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Figure 2.8 – Hazard shielded with a roadside barrier
2.3.5.6 Delineate the hazard
The final treatment option is to delineate the hazard; i.e. make it more visible to the motorists and make them
aware of the danger. This may be achieved by using reflective material, as shown in Figure 2.9, and/or by
warning signs. This treatment option does not provide any physical protection but it may still be better than
doing nothing, where other options are either physically impossible or not cost effective.
Figure 2.9 – Hazard delineated
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2.3.6 Step 5 – Assessment & Ranking of Treatment Options
2.3.6.1 Overview of Treatment Assessment & Ranking Process
Once the potential treatment options are identified, each option should be assessed from a perspective of
physical applicability and economic feasibility. The options which are both physically applicable and
economically feasible should then be ranked based on the amount of risk reduction they provide, their benefit
cost ratio and other non-monetary considerations should be determined for a final decision. An overview of the
treatment assessment and ranking process is shown in Figure 2.10. Further detail on each individual step of
the process is presented in the following sections.
Figure 2.10 – Overview of treatment assessment & ranking process
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2.3.6.2 Assess Physical Practicability
After a hazard and the possible safety treatments are identified, an initial evaluation to the physical
practicability of the treatment should be carried out. A treatment option may not be reasonably practicable for
reasons such as:
• Constructability of the treatment;
• Right of Way Limitations;
• Insufficient physical space for the treatment to function as intended;
• Intended function of the hazardous object.
Some of the more obvious of the physically impracticable or extremely difficult to apply treatment options may
be related to the removal of a hazard. For example for coastal roads, the sea may be considered as a
continuous hazard, if the water is located within the clear zone distance, removal of such a hazard would be
considered extremely difficult, if not entirely impossible.
Some treatment options may not be possible due to the right of way limitations. For example a hazard may not
be able to be relocated beyond the clear zone, if the end of the clear zone area lies outside the right-of-way
boundaries.
Some treatment options may not be practicable due to restrictions in physical space available. For example
“shielding a hazard with a barrier” is only possible if the distance between the back face of the barrier and the
hazard is less than the deflection distance of the barrier (eee Section 7.3.5). If not, the impacting vehicle would
still reach the object behind as the barrier deflects. If the hazard is simply too close to the travelled way,
designer/engineer may not be able use certain types of barriers, due to physical limitations of the product and
the physical space available on the site.
Sometimes the intended function of a roadside hazard may be a reason for a treatment to be impracticable.
For example “relocation of the hazard beyond clear zone” may not be possible for a road sign as the sign
should be within a certain distance from the road to fulfil its intended function. Similarly “removal of the hazard”
may also not be an option for the same hazard.
For some treatment options, assessing the physical practicability is relatively straightforward. Treatment
options, which are either extremely difficult to apply or simply impracticable may be discarded from the risk
mitigation process, or the treatment may be modified and reassessed again.
Treatment options which are physically practicable should be carried over to the next phase of the process.
2.3.6.3 Assess Economic Feasibility
Once the physically impracticable treatment options are eliminated, the remaining should be assessed for their
economic feasibility. The expected benefits of a treatment (i.e. the anticipated reduction in the frequency and
severity of injuries, reduction in economic loss, reduction in losses due to traffic disruptions, etc.) should be
more than the associated costs (i.e. construction costs, maintenance costs, etc.) for its application. In other
words, the Benefit/Cost Ratio (BCR) of any safety treatment should be more than 1 (see Chapter 12 for
Economic Analysis).
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Any treatment option with a BCR less than or equal to 1.0 should be discarded from the risk assessment
process and the following phase should continue for options with a BCR greater than 1.0.
2.3.6.4 Rank Treatment Options
After economically feasible treatment options are identified, they should be compared and ranked from a risk
reduction and an economic perspective. Ranking is useful to identify the optimal treatment option.
Rank by Risk Reduction:
Different safety treatments provide different levels of reduction in overall risk. For example, as explained in
Section 0.1, removing a hazard usually provides a higher risk reduction than shielding the hazard with a VRS.
For this reason, alternative treatment options should be ranked by the amount of risk reduction they are
expected to provide. The amount of risk reduction can be quantified in terms of the expected reduction in crash
frequency, injuries and property damage. Example methods of carrying out such a ranking are presented in
Chapter 12. At this point the road authority or the designer/engineer may choose to set a certain level of risk
reduction as a minimum and eliminate any treatment options which do not provide the minimum desired level
of risk reduction.
Once the treatment options are ranked in risk reduction order, options that provide a higher level of risk
reduction should be given greater consideration. However the amount of risk reduction is not the only factor to
be used to determine the optimal solution. The treatment which provides the highest level of risk reduction
may also be the most expensive. In such a case, a second alternative may provide an acceptable level of risk
reduction for a better economic value. For this reason, the treatment options should also be assessed from an
economic perspective:
Rank by Benefit Cost Ratio
As explained above, the amount of risk reduction is not the only important factor when deciding upon the
optimal safety treatment. Treatment options which provide an acceptable level of risk reduction should also be
evaluated and ranked from a cost effectiveness perspective. BCR is a good indicator of project value. However,
simply comparing the BCR of different treatment options to each other may be misleading. This is because
BCR is a ratio of the benefits of a specific treatment to its costs and it does not necessarily provide a meaningful
comparison between the benefits of different treatment options.
For example, “delineation of a hazard” is often the cheapest treatment option and due to its low cost compared
to its potential benefits, it is usually the option with the highest BCR. However, this high BCR does not
necessarily mean that the benefits gained from delineation is more than the benefits gained from a more
expensive option, for example shielding the hazard with a barrier. A barrier would cost more, but its benefits
would also be greater than for delineation. Therefore, a solution can provide higher benefits, but with a lower
BCR.
In such a case an incremental BCR analysis may be applied to rank the treatment options economically. For
incremental BCR and more on Economic Analysis, see Chapter 12.
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The economic ranking of treatment options is essential, but is not the only consideration for deciding upon an
optimal solution. The decision should also be based on the assessment of non- monetary considerations.
2.3.6.5 Assess Non-Monetary Considerations
In most cases, the main benefits of applying a roadside safety treatment can be quantified in monetary terms;
i.e. the monetary gains expected due to a reduction in crash frequency, severity, and the associated repair
costs.
However, there are some factors which may be influential in the decision about which safety treatment to adopt
which cannot be quantified in monetary terms. Examples of these considerations include:
• Aesthetics;
• Public demands and perception of road safety improvements;
• Air quality, noise, visual intrusion or other environmental considerations;
• Road user needs.
As these considerations cannot be quantified, their effect in the final decision should be evaluated on a case
by case basis through engineering judgment. For example, aesthetics may be of a significant importance in
the selection of a certain type of treatment over the other alternatives around areas of natural beauty, land
marks, major tourist attractions, etc. while it may not be an important factor in a remote rural area of no special
importance.
Environmental concerns may be of significant importance in areas such as natural reserves, natural protection
areas such as source of drinking water, in conservation areas for a certain species of animal, etc.
In some cases the designer/engineer may choose to apply a certain treatment option to satisfy the public
demand, although it may not be the best option from an economic perspective. For example, one may choose
to install a motorcyclist protection system on a certain location to satisfy the demands of a motorcyclist action
group.
Non-monetary considerations can be of significant importance in the final decision; however the
designer/engineer should always ensure that an adequate level of safety is provided.
2.3.7 Step 6 – Choose the Optimal Treatment Option
Once the risk reduction and BCR ranking, and the non-monetary assessment of treatment options are
complete, the designer/engineer should rank the preferred treatment options based upon the risk reduction,
cost effectiveness and non-monetary considerations. As each site is different, engineering judgment should
be used to find the optimal solution that fits the needs of the particular site, as shown in Figure 2.11.
2.3.8 Step 7 – Design the Optimal Roadside Treatment
The final step in the risk mitigation process is the detailed design of the appropriate (optimal) roadside
treatment. This step may include the design of just a single treatment or the whole safety improvement scheme
along a section of road which addresses different types of hazards in one
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go. In some cases Abu Dhabi DoT Standard Drawings will provide the necessary detail, whereas the road
design layout will show the location of the treatment as well as information not covered by the standard
drawings. Necessary guidance for the design of individual roadside safety treatments is provided between
Chapters 3 to 11 of this Guide.
Figure 2.11 – Selection of optimal treatment option through engineering judgment
The designer/engineer of a specific treatment should consider that the final design of the roadside treatment
can affect the other responsible bodies which provide roadside infrastructure items, such as signage and
lighting. Lateral extensions of the clear zone or hazard corridor should be shown on the final plans so the other
responsible bodies can assess the final design from their own perspective.
The final design should include, but not be limited to the following information:
• All hazards for which a treatment warrant has been identified;
• The treatment options chosen for those hazards;
• The priority of the treatment options.
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2.4 Abu Dhabi DoT Product Approval Process
Prior to the use of any proprietary vehicle restraint system on the Abu Dhabi road network, the system must
have been accepted for use by the Abu Dhabi Department of Municipal Affairs and Transport, or their
representatives. This is to ensure that the product has been successfully tested to appropriate standards
(NCHRP350 and MASH), and that sufficient consideration has been given to the local conditions and demands
of the Abu Dhabi road environment. Whilst a large number of systems currently exist on the market, these
have often been developed with the local requirements of the USA, Australasia and (in some cases) European
road conditions in mind, not necessarily those of Abu Dhabi. A list of those products deemed to be acceptable
for use on such roads is available.
In order for a product to be listed, an application must first be filed with the Abu Dhabi Department of Municipal
Affairs and Transport, or its representatives. The Department will then issue a proforma detailing questions
regarding the suitability of product for the local conditions in Abu Dhabi. This must be completed in a
satisfactory way, supported by evidence where possible.
For each product, the manufacturer will also be required to supply a set of full (i.e. not summary) impact test
reports and videos of their system to the Department of Municipal Affairs and Transport, or its representatives,
for assessment against the relevant testing standard (either NCHRP350 or MASH). This should be
accompanied by any other relevant supporting evidence which may include:
• Inspection, maintenance and repair requirements;
• Installation manual;
• Restrictions on the use of the product, and compatibility with other products;
• Details of any modifications made to the product since it was tested, and any supporting
evidence/independent approval of the modifications made;
• Details of agreements with local distributors;
• Drawings and specifications for the system;
• Details of any in-service performance evaluation (including impacts with higher and faster
vehicles than specified in the testing standards);
• Details of any durability/environmental testing;
• Promotional literature;
• Any other information supporting the application.
Note that whilst the acceptance of a product in another territory (e.g. by Federal Highways in the USA or by
the award of a CE mark within Europe) may be considered as part of the approval process, this will be no
guarantee of acceptance for use by the Abu Dhabi Department of Municipal Affairs and Transport due to its
local needs and road conditions.
2.5 Summary and Conclusions
This chapter explains the recommended risk mitigation approach for Abu Dhabi. It also provides an overview
of how the manual should be used in general, by referencing the related chapters of the manual for each step
of the risk mitigation procedure.
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Risk, from a roadside safety perspective, is defined as the probability, high or low, that somebody or something
will be harmed by a roadside hazard. Roadside risk is directly related to both the likelihood of a roadside
accident and the consequences of it. Designers/engineers can mitigate the risk by controlling either or both
the likelihood or consequences of a run-off-the road accident, through the recommended treatment options.
The recommended risk mitigation approach consists of the following stages:
1. Understand the area under evaluation;
2. Calculate the clear zone;
3. Identify the hazards located within the clear zone;
4. Identify applicable treatment options to mitigate risks from the hazards located within the
clear zone. Recommended treatment options include:
o Remove the hazard;
o Relocate the hazard;
o Provide recoverable roadside;
o Replace the hazard with a passively safe system;
o Shield the hazard with a VRS;
o Delineate the hazard;
5. Assess and rank applicable treatment options:
o Assess physical applicability;
o Assess economic feasibility;
o Rank treatment options economically;
o Assess non-monetary considerations;
6. Choose the optimal treatment option through engineering judgment;
7. Design the appropriate (optimal) roadside treatment.
An overview of the DoT product approval process is also given in this chapter. Prior to the use of any
proprietary vehicle restraint system on the Abu Dhabi road network, the system must have been accepted for
use by the Abu Dhabi Department of Municipal Affairs and Transport, or its representatives.
2.6 References
[1]CEDR, “SAVERS (Selection of Appropriate Vehicle Restraint Systems) - WP1: Defining the Different
Parameters which can Influence the Need and Selection of VRS (Unpublished Report),” Conference of
European Directors of Roads, 2014.
[2]TD19/06 Design Manual for Roads and Bridges, Volume2 Highway Structures: Design, Section
2 Special Structures, Part 8, The Highways Agency, Transport Scotland, Welsh Assembly Government,
The Department for Regional Development Norther Ireland, 2006.
[3]FGSV, Traffic Management Work Group, “Guidlines for passive protection on roads by vehicle
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restraint systems,” FGSV Verlag GmbH, Koln, 2009.
[4] AASHTO, Roadside Design Guide, 4th Edition, Washington D.C.: American Association of State
Highway and Transportation Officials, 2011.
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3 CONCEPT AND CALCULATION OF CLEAR ZONE
3.1 Introduction
The “Clear Zone Concept” is a key part of this Guide’s risk mitigation process, as it provides engineers and
designers an easy to use tool for the assessment of risk of a roadside accident for selected road environments.
The chapter starts with a brief look at the concept of clear zone, its origins and its evolution over time. This is
followed by a look at the factors which may affect the required clear zone distance for different roads.
The Clear Zone Calculation Model for this Guide is presented in detail and followed by examples on clear zone
distance calculations.
3.2 The Clear Zone Concept
“The clear zone is the unobstructed, traversable area provided beyond the edge of the through travelled way
for the recovery of errant vehicles”. [1]
In an ideal world, providing unlimited, flat and obstacle-free areas along every road would completely eliminate
the problem of roadside accidents. However, in reality this is neither economically viable, nor physically
possible. For this reason, engineers and designers should assess the level of risk along each roadside and
find an optimal design solution; one which provides a balance between the amount of forgiving roadside
provided and the economic feasibility of the selected application.
Accordingly, AASHTO became the first organization to promote the idea of providing clear recovery areas
along highways. In 1974, the AASHTO document known as the “Yellow Book” stated that, “for adequate safety,
it is desirable to provide an unencumbered roadside recovery area that is as wide as practical on a specific
highway section. Studies have indicated that on high- speed highways, a width of 9 m or more from the edge
of the through travelled way permits about 80 percent of the errant vehicles leaving the roadway to recover.”
[2]. The idea of providing 9m wide clear zones was trialled by several highways agencies, and it was
understood that a constant width of recovery space is not always the optimal solution for roads with different
characteristics. Errant vehicles travel further along the roadside with increased traffic speeds, sharper
horizontal curves and steeper side slopes. Therefore 9m clear zone was not always enough for errant vehicles
to recover safely. On the other hand it was understood that 9m is too wide and economically not viable for
roads with lower speeds and traffic volumes.
For these reasons, in 1977 AASHTO modified its earlier clear zone concept by introducing variable clear-zone
distances based on traffic volumes, speeds and roadside geometry [3]. This new approach was well received
and became widespread. Many countries around the world today either use the exact AASHTO variable clear
zone model or modified versions of it to fit the specific needs of their road networks. [4]
The clear zone distances recommended by AASHTO Roadside Design Guide [1] are based on empirical
research data, which was later extrapolated to fill in the gaps of the database. Therefore, they are intended as
reference points rather than definitive values. It should be noted that these
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clear zone values, although considered sufficient for majority of errant vehicles to safely regain control, are not
enough to stop all 100% of errant vehicles. For example, the recommended clear zone widths may not be
enough for an over-speeding errant vehicle to safely regain control. A clear zone, where even the over-
speeding errant vehicles would regain control before reaching the end, would require a considerably wider
space. However, considering the low probability of such incidents, providing this extra space is not always
economically viable. However, The Abu Dhabi DoT lately extended the speed range of AASHTO Table, shown
in Table 3.2, up to 140 km/h. The values given were only extracted by interpolating the trend of previous speed
ranges given by AASHTO.
The relationship between the distance from the edge of the travelled way to a hazard and the proportion of
errant vehicles that will be able to reach the hazard has been the subject of several researches. One of the
most comprehensive datasets of run-off-road accidents was collected during late 1970s in Canada by P.
Cooper [5]. Cooper’s research involved weekly observations of wheel tracks on grass-covered roadsides of
rural highways of various functional classes, where he looked at the distance travelled by errant vehicles.
Cooper’s encroachment data was later re-analysed for development of the RSAP [6], and the relationship
shown in Figure 3.1 was derived. Figure 3.1 shows the proportion of errant vehicles which are expected to
travel over a certain distance from the edge of the travelled way during a run-off-road accident. The relationship
is presented for two-lane undivided roads, multi-lane divided roads and for the combination of both undivided
and divided roads. It can be seen from the figure that, as the distance from the edge of the travelled way
increases, the proportion of errant vehicles which are likely to reach the distance decreases.
Figure 3.1 shows that clear zone distances recommended by AASHTO are likely to allow enough space for
approximately 85% of the errant vehicles to stop or regain control. In other words, roughly 15% of errant
vehicles could still reach a hazard beyond these distances. A considerably wider space is required to ensure
that more than 85% of the errant vehicles will stop within the clear zone. For example for divided road,
increasing the proportion of vehicles which would stop within the clear zone from 85% to 95%, would require
the clear zone distance to increase from roughly around 12m to 20m. To take the same ratio from around 85%
to around 100%, the clear zone distance should almost be tripled from 12m to 30m. The considerable increase
in the required clear zone to cater for every single possible incident may not always be justified due to
economic reasons. Therefore, the designer should always use engineering judgment on a site-by- site basis
when deciding the acceptable clear zone distance.
However, some hazards, especially the ones where third parties may be affected, would yield more severe
consequences if reached by an errant vehicle. Examples of these are chemical plants, school playgrounds,
areas of public gathering, source of drinking water, etc. The consequences of a vehicle reaching these hazards
would be so high, that even a low likelihood of a vehicle reaching them would pose a significant risk (see
Section 2.2). For such high consequence hazards, the extra width required to increase the 85% mark to 95-
100% may be justified.
This approach of providing wider clear zones for higher consequence hazards is adopted by several countries
around the world. For example, the Norwegian VRS Manual [7] recommends up to doubling the clear zone
distance for higher risk hazards, while the German guidelines [8] recommend 8, 4 and 3m of extra clear zone
space for high risk hazards located close to highways of 100, 80 and 60km/h posted speed limit, respectively.
This approach forms the basis of the clear zone calculation model for the ADRSDG.
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Distance from the edge of travelled way (m)
Figure 3.1 – Distance from the edge of travelled way vs proportion of errant vehicles which may
reach it by highway type [6]
3.3 Factors Affecting the Clear Zone Distance
The required clear zone distance along a particular road is related to several factors. These factors are
explained briefly to provide an insight into how the clear zone calculation model works.
3.3.1 Traffic Volume
Traffic Volume is an important factor which affects the required clear zone distance, as it is directly related to
the exposure level of a roadside. Probability of a vehicle running off the road increases as the number of
vehicles passing through the area increases.
Traffic volume does not directly affect the distance travelled by an errant vehicle. However, there is an indirect
effect, as the probability of a faster vehicle running off the road increases with increased traffic volume. For
this reason, the required clear zone distance goes up with increased volume of traffic.
3.3.2 Design Speed
Design speed is another important factor, which affects the required clear zone distance. Design speed
determines the distance up to which an errant vehicle will travel and therefore the likelihood of the vehicle
reaching a hazard within a certain distance. For this reason, the required clear zone distance goes up with
increased design speed.
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3.3.3 Roadside Topography
Roadside topography has an important effect on how far an errant vehicle would travel; i.e. the probability of
an errant vehicle reaching a hazard within a certain distance. Due to the change in its potential energy, an
errant vehicle is likely to travel further distance along steeper downhill slopes,
i.e. foreslopes, and is likely to travel less distance along steeper uphill slopes, i.e. backslopes.
3.3.3.1 Foreslopes
According to their gradient, foreslopes are usually divided into three categories:
Recoverable Slopes:
Recoverable slopes are those on which a motorist can retain or regain control of a vehicle. Slopes equal to or
flatter than 1V:4H, as shown in Figure 3.2, are considered recoverable. Smooth, compact slopes with no
significant discontinuities and no protruding fixed objects are required from a safety standpoint. Slope should
be rounded so that an encroaching vehicle remains in contact with the ground. Also the toe of the slope should
be rounded to improve reversibility by an errant vehicle.
Figure 3.2 – Recoverable slope
Non-recoverable Slopes:
A non-recoverable foreslope is defined as one that is traversable but from which most vehicles will not be able
to stop or return to the roadway easily. A foreslope with gradient between 1V:4H and 1V:3H, as shown in
Figure 3.3, is considered as non-recoverable, as long as it has firm compacted surface or if it is treated with
concrete, rip-rap, etc. Conversely if it has a loose sandy surface which may cause a vehicle to overturn, a
foreslope with gradient between 1V:4H and 1V:3H should be considered as a hazard rather than a non-
recoverable slope. See Chapter 4 for more information on hazard assessment of foreslopes.
An errant vehicle reaching a non-recoverable slope would continue at least until it reaches the end of it. For
this reason, fixed obstacles along such slopes should be avoided and a clear runout area should be provided
at the base. This is also why, for the calculations, the total width of a non- recoverable slope is added to the
clear zone distance.
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Figure 3.3 – Non-recoverable slope
Critical Slopes:
A foreslope with gradient steeper than 1V:3H, as shown in Figure 3.4, is considered as a critical slope; one on
which an errant vehicle has a higher probability to overturn. For this reason a foreslope is considered as a
hazard by itself. Critical slopes which are located within the clear zone should ideally be flattened. If this is not
possible, or economically viable, a barrier is typically used.
3.3.3.2 Backslopes
Figure 3.4 – Critical slope
A backslope in a cut section may be traversable depending on its relative smoothness and the presence of
fixed obstacles. It may not be a significant obstacle if the front slope between the roadway and the base of the
backslope is traversable (1V:3H or flatter) and the backslope is obstacle-free. However, a steep, rough-sided
rock cut normally should begin outside the clear zone or be shielded. A rock cut normally is considered to be
rough-sided when the face will cause excessive vehicle snagging rather than provide relatively smooth
redirection.
3.3.4 High Risk Hazards
The type of a hazard does not affect the probability of an errant vehicle reaching it. However, it is still an
important factor for the determination of the required clear zone distance as it affects the overall risk. As
explained in Section 3.2, some hazards yield higher consequences than others in case of an errant vehicle
reaching them. Since risk is a product of likelihood and consequences (see Chapter 2), these high
consequence hazards can still pose a considerable risk for even low
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probability of a vehicle reaching them. Therefore, high risk hazards affect the required clear zone distance,
as they can economically justify the provision of wider clear zones.
3.4 Calculation of Clear Zone Distance
3.4.1 Clear Zone Model
The clear zone is calculated by using the following formula, as detailed in Table 3.1:
Where:
Table 3.1 – Calculation of clear zone distance, Cz
Calculation of Clear Zone Distance, Cz
Cz=(Bcw x Mc)
Bcw, Base Clear Zone Width See Table 3.2
Mc, Modification Factor for
Outside of Horizontal Curves
Straight Sections & Inside of Curves Mc=1.0
Outside of Curves See Table 3.3
3.4.2 Base Clear Zone Width
The base clear zone width is the recommended clear zone width from the edge of the travelled way for straight
road sections, and it is determined on the basis of ADT, Design Speed and Side Slope, by using Table 3.2.
The clear zone distances presented in Table 3.2 are based on the clear zone distances suggested by AASHTO
Roadside Design Guide [1]. To cater for the needs of the Abu Dhabi Road Network the AASHTO clear zone
values are extrapolated for design speeds up to 140km/h.
The clear zone values presented in Table 3.2 are based on empirical research data, which was later
extrapolated to fill in the gaps of the database. Therefore, they are intended as guidance values rather than
definitive values. It should be remembered that, as explained in Section 3.2, some vehicles can still travel
further than these recommended clear zone distances. Therefore, designer/engineer should assess the risk
for each site on a case-by-case basis, using engineering judgment.
In Table 3.2, a two-way ADT should be used for single carriageways and one-way ADT should be used per
direction of dual carriageways. The traffic volumes should be based on a 20-year projection from the
anticipated date of construction. For recommended clear zone widths over 15m, an additional assessment
should be carried out to justify the required empty space, as these are likely to cross the right-of-way limitations.
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Table 3.2 - Recommended base clear zone width, Bcw (in metres)
Design Year ADT
Design
Speed
(km/h)
Under 750 750-1500 1500-6000 Over 6000
Foreslopes
60 2.0 - 3.0 3.0 - 3.5 3.5 - 4.5 4.5 - 5.0
80 3.0 - 3.5 4.5 - 5.0 5.0 - 5.5 6.0 - 6.5
1V:6H 90 3.5 - 4.5 5.0 - 5.5 6.0 - 6.5 6.5 - 7.5
100 5.0 - 5.5 6.0 - 7.5 8.0 - 9.0 9.0 - 10.0
or
flatter
110 5.5 - 6.0 7.5 - 8.0 8.5 - 10.0 9.0 - 10.5
120 6.5 - 7.0 8.5 - 10.0 10.5 - 11.5 11.0 - 12.5
140 9.5 - 10.5 12.5 - 14.0 15.5* - 16.5* 16.0* - 17.5*
60 2.0 - 3.0 3.5 - 4.5 4.5 - 5.0 5.0 - 5.5
Steeper 80 3.5 - 4.5 5.0 - 6.0 6 - 8.0 7.5 - 8.5
90 4.5 - 5.5 6.0 - 7.5 7.5 - 9.0 8.0 - 10.0
than
1V:6H to
100 6.0 - 7.5 8.0 - 10.0 10.0 - 12.0 11.0 - 13.5
Flatter
110 6.0 - 8.0 8.5 - 11.0 10.5 - 13.0 11.5 - 14.0
than
1V:3H
120 8.5 - 10.5 10.5 - 13.5 13.0 - 16.5* 14.5 - 18.0*
140 13.5 - 15.5* 15.5* - 20.0* 19.0* - 25.0* 20.0* - 27.0*
Backslopes
60 2.0 - 3.0 3.0 - 3.5 3.5 - 4.5 4.5 - 5.0
80 2.5 - 3.0 3.0- 3.5 3.5 - 4.5 4.5 - 5.0
1V:3H 90 2.5 - 3.0 3.0 - 3.5 4.5 - 5.0 5.0 - 5.5
100 3.0 - 3.5 3.5 - 4.5 4.5 - 5.5 6.0 - 6.5
or
steeper
110 3.0 - 3.5 3.5 - 5.0 5.0 - 6.0 6.5 - 7.5
120 3.5 - 3.5 3.5 - 5.0 5.5 - 6.0 6.5 - 7.5
140 4.0 - 4.5 4.0 - 6.0 6.0 - 7.0 8.0 - 9.0
60 2.0 - 3.0 3.0 - 3.5 3.5 - 4.5 4.5 - 5.0
Steeper 80 2.5 - 3.0 3.5 - 4.5 4.5 - 5.0 5.5 - 6.0
90 3.0 - 3.5 4.5 - 5.0 5.0 - 5.5 6.0 - 6.5
than
1V:6H
100 3.5 - 4.5 5.0 - 5.5 5.5 - 6.5 7.5 - 8.0
Flatter
110 4.5 - 5.0 5.5 - 6.0 6.5 - 7.5 8.0 - 9.0
than
1V:3H 120 5.0 - 5.5 6.5 - 7.0 7.0 - 8.0 9.0 - 10.0
140 6.5 - 7.0 8.0 - 8.5 9.0 - 10.0 11.5 - 12.5
60 2.0 - 3.0 3.0 - 3.5 3.5 - 4.5 4.5 - 5.0
80 3.0 - 3.5 4.5 - 5.0 5.0 - 5.5 6.0 - 6.5
1V:6H 90 3.0 - 3.5 5.0 - 5.5 6.0 - 6.5 6.5 - 7.5
100 4.5 - 5.0 6.0 - 6.5 7.5 - 8.0 8.0 - 8.5
or
flatter
110 4.5 - 5.0 6.0 - 6.5 8.0 - 8.5 8.5 - 9.0
120 5.5 - 5.5 7.5 - 8.0 9.0 - 10.0 10.0 - 10.5
140 7.0 - 8.0 10.0 - 10.5 11.5 - 13.0 13.0 - 13.5
* Clear Zone distances over 15m should be evaluated on a site-by-site basis to justify the extra space
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3.4.3 Modification for the Outside of Curves
The base clear zone width (Bcw) values in Section 3.4.2 assume a tangent alignment. However, horizontal
curves may increase the angle of departure from the roadway and thus increase the distance the vehicle will
need to recover. The designer should adjust the tangent values to provide wider clear zones on the outside of
horizontal curves. Table 3.3 provides recommended modification factors for the outside of curves (Mc). A value
of Mc=1.0 should be used for straight sections and insides of curves, as a modification is not necessary for
these.
3.4.4 Clear Zone on Combination of Slopes
3.4.4.1 Variable Slopes
Sometimes a combination of two different side slopes may be used on a roadside, as shown in Figure 3.5. A
common application is a relatively flat recovery area immediately adjacent to the roadway followed by a steeper
foreslope. This type of application, often called as ‘barn roof’, can provide the flat recovery area by using less
material and space than a single continuous slope would require.
Figure 3.5 – Variable Slope
If the clear zone width required for the first slope does not reach the second one, no extra consideration is
necessary, as an errant vehicle would stop before reaching the second slope. If the clear zone width required
for the first slope extends beyond the first and overlaps into the second slope, the clear zone width for the
steeper slope should be used.
Table 3.3 – Modification factor for the outside of curves, Mc [1]
Radius (m)
Design Speed km/h
60 70 80 90 100 110 120
900 1.1 1.1 1.1 1.2 1.2 1.2 1.3
700 1.1 1.1 1.2 1.2 1.2 1.3 -
600 1.1 1.2 1.2 1.2 1.3 1.4 -
500 1.1 1.2 1.2 1.3 1.3 1.4 -
400 1.2 1.2 1.3 1.3 1.4 - -
300 1.2 1.3 1.4 1.5 1.5 - -
200 1.3 1.4 1.5 - - - -
100 1.5 - - - - - -
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3.4.4.2 Drainage Channels
Drainage channels are typically variable slopes with at least one foreslope and one backslope, as shown in
Figure 3.6. Drainage channels, depending on their geometry, can become hazards by themselves. This can
be checked from Figure 4.8 and Figure 4.9 in Chapter 4.
Drainage channels, which are not considered to be hazards, should be treated as variable slopes, and the
clear zone should be calculated accordingly, as explained in Section 3.4.4.1.
3.4.5 High Risk Hazards
Figure 3.6 – Drainage channel
High risk hazards are the ones where people other than the vehicle occupants (third parties) may be harmed
if reached by an errant vehicle. Chapter 4, Section 4.4 presents more detail into the identification of some of
the more common high risk hazards. Following are some of the examples to high risk hazards where 3rd
parties
may be harmed:
• Adjacent roads and carriageways (Section 4.4.1)
• Storage of hazardous material (Section 4.4.2)
• Places of frequent pedestrian activity / places of public gathering (Section 4.4.3)
• Cycle Lanes (Section 4.4.4)
• Structures at Risk of Collapse (Section 4.4.5)
• Rail Lines (Section 4.4.6)
As explained in Sections 3.2 & 3.3.4, the recommended clear zone distances in Table 3.2 are expected to
provide enough space for approximately %85 of the errant vehicles to stop or regain control. However, roughly
%15 of errant vehicles may still travel further than the recommended clear zone distance. It is possible to
ensure a safe stop for a higher percentage of errant vehicles by providing even wider clear zones. However,
for regular roadside hazards, the costs of providing the extra space may not always be justified by the benefits
gained.
However, in the case of 3rd
party hazards, the consequences can be so high, that even a 15% probability of
an errant vehicle reaching the hazard may result in an unacceptable level of risk. Therefore, if a high risk
hazard is located in the vicinity of the travelled way, the designer/engineer should check for distances which
are even further than the clear zone width recommended in Table 3.2. Based on the information presented in
Figure 3.1, it is recommended that the risk posed by any 3rd
party hazard located up to 30m to the travelled
way should be assessed using engineering judgment. If the assessment suggests that the likelihood of a
vehicle reaching the 3rd
party hazard is too high, then the hazard should be removed, relocated to a further
distance or
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shielded by an appropriate barrier. Considering the high consequences of the hazard, a high containment level
barrier should be used (see Chapter 7).
3.4.6 Other Considerations
3.4.6.1 The Use of Engineering Judgement
As explained in the previous sections, the recommended clear zone widths presented in this chapter are based
on empirical research data, which was later extrapolated to fill in the gaps in the database. Therefore, the clear
zone distances recommended in this chapter should only be taken as guidance, rather than definitive values.
The designer/engineer should assess each site on a case-by-case basis applying engineering judgment and
a justification should be provided for the final decision.
3.4.6.2 Departures from Standards
A Departure or Departure from Standard can be described as a non-compliance with a Mandatory Requirement
of a Declared Standard. The clear zone distances presented in this chapter are given as reference points
rather than definitive values. However, the designer/engineer should aim to provide at least the minimum
recommended clear zone width for each site. Where special circumstances arise and the straightforward
application of the technical requirements cannot be achieved or justified for some reason, such as the
environmental impact, exceptional layout situations or cost, users are encouraged to come forward with
Departures or to propose additional criteria (for aspects not covered by existing documents) based on a
reasoned assessment.
Full justification for the grounds of the proposed departure must be forwarded to Abu Dhabi Department of
Municipal Affairs and Transport at an early stage in design. The departure must demonstrate that the risk level
of the proposed solution is as low as reasonably practicable. Formal approval of the proposed Departure must
be received before incorporation into the design and the commencement of construction. Departures from
Standard are determined on an individual basis and a decision regarding a Departure for one location must
not be assumed to apply to any other site, even for similar situations.
3.5 Example Clear Zone Distance Calculations
3.5.1 Example 1 – Simple Recoverable Side Slope
Calculate the clear zone and evaluate the location of the electrical box.
Road Type: Multilane Divided ADT:
10,000 per direction Design Speed:
120km/h Horizontal Alignment: Straight
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Solution:
Step 1 - Determine Base Clear Zone Width (Bcw)
Using Table 3.2 for:
• A foreslope of 1V:10H (i.e. 1V:6H or flatter)
• Design Speed: 120km/h
• ADT: 10,000 (i.e. over 6,000)
Bcw = 11.0 to 12.5m
Step 2 – Modifier for Outside of Curves
• This is a straight section of road; a curve modification is not necessary. Mc = 1.0
Step 3 – Calculate the Clear Zone Distance (Cz)
Cz = Bcw x Mc
Cz = 11.0 to 12.5m
Step 4 – Comment on the location of the electrical box
As shown in the figure below, the electrical box lies within the minimum recommended clear zone distance of
11.0m from the edge of the travelled way.
The following treatment options should be evaluated:
• Remove the electrical box
• Relocate the electrical box beyond the 11.0 to 12.5m clear zone
• Use a passively safe electrical box
• Shield the electrical box with a barrier
• Delineate the electrical box, for example with reflective material.
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3.5.2 Example 2 – Side Slope on the Outside of a Curve
Calculate the clear zone and evaluate the location of the tree.
Road Type: Two Lane Undivided ADT:
5,000 for both directions Design Speed:
60km/h
Radius of Horizontal Curvature: 900m
Solution:
Step 1 - Determine Base Clear Zone Width (Bcw)
Using Table 3.2 for:
• A foreslope of 1V:4H.(1V:6H to 1V:4H)
• Design speed: 60km/h
• ADT: 5,000 (1,500 – 6,000)
Bcw = 4.5 to 5.0m
Step 2 – Determine Modifier for Outside of Curves (Mc)
Using Table 3.3 for:
• Radius: 900m
• Design speed: 60 km/h,
Mc = 1.1
Step 3– Calculate the Clear Zone Distance (Cz)
Cz = Bcw x Mc
Cz = (4.5 x 1.1) to (5.0 x 1.1)
Cz = 4.95m to 5.5m
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Step 4 – Comment on the location of the tree
• As shown in the figure below, the tree lies outside both minimum and maximum
recommended clear zone distance. Therefore no action is required.
3.5.3 Example 3 – Variable Side Slope
Calculate the clear zone for the variable side slope.
Road Type: Multilane Divided ADT:
8,000 per direction Design Speed:
100km/h Horizontal Alignment: Straight
Surface Condition of the Side Slope: Firm Compacted Soil
Solution:
Step 1 - Determine if the Non-Recoverable Side Slope (1V:3.5H) is a Hazard by Itself:
The surface of the side slope is firm and compacted, therefore, it should be considered as a non-
recoverable slope rather than a hazard by itself (see Section 3.3.3.1)
Step 2 - Determine Base Clear Zone Width (Bcw) for each side slope
Using Table 3.2 for both side slopes:
• For foreslope 1V:10H Bcw = 9.0 to 10.0m
• For foreslope 1V:8H, Bcw = 9.0 to 10.0m
• Foreslope 1V:3.5H is a non-recoverable slope. An errant vehicle would continue until the
bottom of the slope.
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Step 3 – Modifier for Outside of Curves
• This is a straight section of road; a curve modification is not necessary. Mc = 1.0
Step 4 – Calculate the Clear Zone Distance (Cz)
• For side slope 1V:10H, Cz = (9.0 x 1.0) to (10.0 x 1.0) = 9.0 to 10.0m
• For side slope 1V:8H, Cz = (9.0 x 1.0) to (10.0 x 1.0) = 9.0 to 10.0m
• Because foreslopes 1V:10H & 1V:8H both require the same clear zone width of 9.0 to 10.0m,
there is no need to prioritise one distance over the other. If different clear zone distances
were required for the two side slopes, the one with the longest width should have been
selected as the deciding distance.
• The non-recoverable foreslope 1V:3.5H falls within the recommended clear zone distance of
the 1V:10H foreslope. Because an errant vehicle would continue travelling until it reaches
the bottom, the clear zone distance cannot logically end on a non-recoverable slope. In such
cases, an additional clear zone space is needed at the bottom of the non- recoverable slope.
This additional clear zone is referred to as the Clear Runout Area. The width of the Clear
Runout Area should be equal to that portion of the clear zone distance that is located on the
non-recoverable slope. In this example, the clear zone distance based on the recoverable
slope (9.0 to 10.0 meters) overlaps on to the non-recoverable slope by
2.0 to 3.0 meters. Therefore, a Clear Runout Area of 2 to 3m is added from the bottom of the non-recoverable
slope, as shown in the figure below.
• Alternatively, the full width of the non-recoverable slope (3.0m) can be added to the
suggested clear zone distance based on recoverable slope (9.0 to 10.0m). Therefore, for the
combination of all three slopes;
Cz = 9.0+3.0 = 12.0m to 10.0+3.0 = 13.0m.
As shown in the figure below, this would give the same result, as the previous method.
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3.5.4 Example 4 – High Risk Hazard
Calculate the clear zone and evaluate the location of the high speed railway.
Road Type: Multilane Divided Design
ADT: 25,000 per direction Design Speed:
120km/h Horizontal Alignment: Straight
Solution:
Step 1 - Determine Base Clear Zone Width (Bcw)
Using Table 3.2 for:
• A foreslope of 1V:8H.(1V:6H or flatter)
• Design speed: 120km/h
• ADT: 25,000 (Over 6,000)
Bcw = 11.0 to 12.5m
Step 2 – Modifier for Outside of Curves
This is a straight section of road; a curve modification is not necessary. Mc = 1.0
Step 3 – Calculate the Clear Zone Distance (Cz)
Cz = Bcw x Mc
Cz = (11.0 x 1.0) to (12.5 x 1.0)
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Cz = 11.0 to 12.5m
Step 4 – Check for High Risk Hazards
• There is a railway line located 3.0m further away from the minimum recommended clear
zone and 1.5m further away from the maximum recommended clear zone.
Step 5 – Comment on the location of the railway line
As shown in the figure above, the railway line does not lie within the normally recommended clear zone
distances. However, as explained in Section 3.2, a small percentage of errant vehicles may still travel further
than the recommended clear zone distance. Considering the high consequences of an errant vehicle reaching
a high-speed railway line, and the relatively close distance of the railway line to the minimum required clear
zone width (3.0m), it can be said that the level of risk posed by the current layout is unacceptable.
Designer/ Engineer should evaluate the following treatment options:
• Remove the railway line
• Relocate the railway line further away from the road
• Shield the railway line with an appropriate roadside barrier
3.6 References
[1] AASHTO, Roadside Design Guide, 4th Edition, Washington D.C.: American Association of State
Highway and Transportation Officials, 2011.
[2] J. Graham and D. Hardwood, “NCHRP Report 247: Effectiveness of Clear Recovery Zones,” NCHRP,
Transportation Research Board, Washington D.C., 1982.
[3] AASHTO, Guide for Selecting, Locating and Designing Traffic Barriers., Washington D.C.: American
Association of State Highway and Transportation Officials, 1977.
[4] CEDR, “SAVERS (Selection of Appropriate Vehicle Restraint Systems) - WP1: Defining the Different
Parameters which can Influence the Need and Selection of VRS (Unpublished Report),” Conference of
European Directors of Roads, 2014.
[5] P. Cooper, “Analysis of Roadside Encroachments - Single Vehicle Run-off-road Accident Data Analysis
for Five Provinces,” B.C. Research, Vancouver, British Columbia, Canada,
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March 1980.
[6] M. King and D. Sicking, “NCHRP 492 - Roadside Safety Analysis Program Engineer's
Manual,” Transportation Research Board, Washington, D.C., 2003.
[7] Norwegian Public Roads Administration, “Manual 231E, Vehicle Restraint Systems and
Roadside Areas,” Norwegian Directorate of Public Roads, Oslo, 2011.
[8] FGSV, Traffic Management Work Group, “Guidlines for passive protection on roads by vehicle
restraint systems,” FGSV Verlag GmbH, Koln, 2009.
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4 IDENTIFICATION OF HAZARDS
4.1 Introduction
A hazard is a roadside feature or object that can cause physical, economic, time-based or strategic harm or
loss, if reached by an errant vehicle (see Section 2.1).
Identification of roadside hazards is an integral part of the risk mitigation process. Hazards that lie within the
recommended clear zone area (see Chapter 3) are under the risk of being reached by errant vehicles, which
may lead to injury or even fatal accidents. Therefore, it is very important to identify and evaluate all hazards
within the clear zone, so that suitable countermeasures can be applied.
However, it may not always be clear to the engineer/designer, whether a specific roadside feature should be
considered a hazard. For example, a roadside ditch may be considered a hazard or a traversable roadside
feature, depending on its geometry. A tree may be considered a hazard, depending on the diameter of its
trunk.
This chapter provides guidance to designer/engineers in the identification of roadside hazards. The chapter
presents a general overview of the types and properties of roadside features that may be considered as
hazards. Each type of feature is presented with local photos and physical properties to help designers
determine if the feature should be considered a hazard.
Information presented in this chapter does not cover every single possible roadside hazard and therefore
should be taken as a general guidance, rather than a definitive check-list. Engineering judgment should be
used to assess possible hazards on a case by case basis, prior to a final decision.
4.2 Overview of Roadside Hazards
Table 4.1 presents an overview of the common roadside features that may be considered as a hazard. As can
be seen from the table, roadside hazards can be grouped into two main categories:
• Hazards with consequences to vehicle occupants;
• Hazards with consequences to vehicle occupants and third parties.
The first group of hazards are the roadside objects and terrain features, which may cause harm or loss to the
occupants of the errant vehicles.
The second group of hazards are the roadside features and areas, which may also have consequences for
third parties. The term “third parties” refers to a group or collection of people in a public space, such as school,
hospital or railway that might be injured in numbers by an errant vehicle or by a hazard that is hit by an errant
vehicle or a high value asset or facility that might be adversely affected by such an event. These hazards are
often referred to as “high risk hazards”. A more detailed description for each of the hazards is presented in the
following sections.
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Table 4.1 – Guidance towards potential roadside hazards [1]
Type of Hazard Examples of Parameters to Evaluate
A - Hazards with Consequences to Vehicle Occupants
Roadside
Topography
Foreslopes (Embankments)
Backslopes (Cutting Slopes)
Gradient
Height
Compaction
Surface Condition
Ditches
Ditch Type
Gradient of Foreslope
Gradient of Backslope
Bottom Width
Compaction
Surface Condition
Transverse Slopes
Gradient Road
Type Design
Speed
Compaction
Surface Condition
Edges of Bridges & Retaining Walls Drop Height
Non-deformable
Single
Objects
Trees
Trunk Diameter
Stump Height
Overhead Gantries and Cantilevers Passively Safe or Not
Roadside Sign Supports
Support Structure
Diameter & Thickness of Support Posts
Passively Safe or Not
CCTV Masts and Luminaire Supports Passively Safe or Not
Concrete Foundations Protruding from the Ground Height from the Ground
Bridge Piers, abutments, tunnel portals Always considered as a hazard
Bridge Railing Ends & Ends of Concrete Barriers Always considered as a hazard
Above Ground Equipment Passively Safe or Not
Drainage Pipes & Culverts
Level with Ground Profile?
Pipe Diameter
Traversable or not?
Non-deformable
Continuous
Objects
Non-deformable extensive obstacles parallel to direction of
travel
Passively Safe or Not
Retaining walls Surface Roughness
Noise barriers Barrier Incorporated or not?
Fencing (Stone wall, wooden fence, concrete wall, etc.) Passively Safe or Not
Roadside Barriers Is it Necessary or not?
Permanent bodies or streams of water
Water Depth
Flooding Expected or not?
B - Hazards with Consequences to 3rd Parties
Adjacent Roads / Opposite Side of Dual Carriageways
Volume of Adjacent Road
Speed of Adjacent Road
Locations of Hazardous Material Storage such as Chemical Plants Always considered as a hazard
Structures at risk of collapse, support and load bearing Always considered as a hazard
Places of public gathering and heavy pedestrian activity
Average Number of People
Volume of Pedestrian Traffic
Average Time People are Exposed
Heavily used bicycle paths Volume of Bicycle Traffic
Structures at risk of collapse Always considered as a hazard
Adjacent Rail lines
Number of Trains per day
Average Speed of Trains
Area of environmental concern such as source of drinking water Always considered as a hazard
ROADSIDE DESIGN GUIDE
PAGE 47
04 IDENTIFICATION OF HAZARDS FIRST EDITION -DECEMBER 2016
4.3 Hazards with Risk to Vehicle Occupants
4.3.1 Roadside Topography
4.3.1.1 Fill Slopes / Embankments
Fill slopes / embankments are the front slopes extending outward and downward from the shoulder or verge
hinge point to intersect with the natural ground line. Consequences of reaching a fill slope for an errant vehicle
may be more serious depending on height, gradient, surface condition and compactness of the slope. A steep
embankment with a very high drop, such as the one shown in Figure 4.1, is an obvious hazard for errant
motorists. However, for embankments with less steep side slopes and lower drops, it may not always be
obvious if a foreslope should be considered as a hazard.
Figure 4.1 – A hazardous embankment
A foreslope of 1V:3H or steeper, such as the one shown in Figure 4.2, is considered as critical (see Section
3.3.3.1) and a hazard by many countries around the world (such as US [2], UK [3], Germany [4], etc.). This is
because experience shows that the likelihood of a rollover increases significantly when the slope is steeper
than the 1V:3H level.
Figure 4.2 – Critical side slope by an interchange ramp
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Documento 50 - 1-120 - 51+52 borrador.pdf

  • 1. ROADSIDE DESIGN GUIDE PAGE 1 01 INTRODUCTION FIRST EDITION -DECEMBER 2016 1 INTRODUCTION 1.1 Overview In 2010, the Abu Dhabi Department of Municipal Affairs and Transport commenced with the “Unifying and Standardizing of Road Engineering Practices” Project. The objective of the project was to enhance the management, planning, design, construction, maintenance and operation of all roads and related infrastructures in the Emirate and ensure a safe and uniform operational and structural capacity throughout the road network. To achieve this objective a set of standards, specifications, guidelines and manuals were developed in consultation with all relevant authorities in the Abu Dhabi Emirate, including the Department of Municipal Affairs (DMA) and Urban Planning Council (UPC). In future, all authorities or agencies involved in roads and road infrastructures in the Emirate shall exercise their functions and responsibilities in accordance with these documents. The purpose, scope and applicability of each document are clearly indicated in each document. It is recognized that there are already published documents with similar objectives and contents prepared by other authorities. Such related publications are mentioned in each new document and are being superseded by the publication of the new document, except in cases where previously published documents are recognized and referenced in the new document. 1.2 Purpose and scope The guidance supplied in this document, the Abu Dhabi Roadside Design Guide (ADRSDG), is based on established international practices. This especially includes the American Association of State Highway and Transportation Officials (AASHTO) Roadside Design Guide [1] and is supplemented by recent research and documents prepared by the CEDR funded ‘SAVERS’ [2] project, the United Kingdom, Austroads [3] and Dubai [4]. This Guide has been prepared to provide uniform practices for government transport agencies within the Abu Dhabi Emirate and consultant personnel preparing studies, reports and contract road plans for these agencies. The designer should use this document to develop roadside designs that meet the operational and safety requirements while preserving the aesthetic, historic, and cultural resources of an area. This Guide will be updated regularly as new data and experience with best practices become available. Roadside design is defined as the design of the area between the outside shoulder edge and the right-of-way limits. The Abu Dhabi Emirate is reducing the number of fatalities year on year, however, roadside crashes account for a significant portion of the total fatal highway crashes. Approximately 25 percent are the result of a single vehicle run-off-the-road crash. This emphasizes the importance of providing as safe a roadside design as practical. 1.3 The application of the Abu Dhabi Roadside Design Guide The guidance presented in this document is intended for the roadside design of new construction and major reconstruction of roads and streets located within the Abu Dhabi road network. Local municipalities are encouraged to adopt these guidelines to ensure uniformity of roadside designs
  • 2. ROADSIDE DESIGN GUIDE PAGE 2 01 INTRODUCTION FIRST EDITION -DECEMBER 2016 within the Emirate. Design guidance is provided, and is considered applicable, for all facility types in both urban and rural locations. The designer should attempt to meet all criteria and practices presented in this Guide; however, it should not be considered a standard that must be met regardless of cost and impacts. Designers must exercise good judgment on individual projects and, frequently, they must be innovative in their approach to roadside design. Designers should review the listed references to gain an understanding of the basis for the selected roadside design criteria. The guidance provided in this document should not be a substitute for good engineering knowledge, experience, or sound judgment. The concepts, designs and philosophies presented in this Guide may not be applicable to every project. Each project is unique and offers an individual opportunity to enhance that particular roadside environment. The amount of monetary resources available for all roadside safety enhancements is limited. The objective of designers is to maximize roadside safety on a system-wide basis with the given funds. Accomplishing this objective means addressing those specific roadside features that can contribute the most to the safety enhancement of that individual project. Given that objects and slope changes must be introduced at varying points off the pavement edge, the enhancement of roadside safety involves selecting the “best” choice among several acceptable design alternatives. This document is intended to represent the spectrum of commonly available roadside design alternatives. 1.3.1 Background to Some of the Key Approaches Adopted One of the main aims of this Guide is to provide guidance on roadside design based on international best practice which fits best to reflect the local conditions, requirements and applications. To help achieve this, a team of internationally recognised, independent road safety specialists have carried out site visits around the Abu Dhabi Road Network, reviewed the existing processes and assessed the local conditions in collaboration with the Abu Dhabi Department of Municipal Affairs and Transport. A summary of these assessments were presented in the “Abu Dhabi Roadside Design Guide – Inception Report” (See Appendix B). An important outcome of the Inception Report was recommendations for some key approaches, which helped form the basis of this Guide. An overview of these key approaches is presented in the following sections. For more detail, please refer to Appendix B. 1.3.2 A Performance Based Approach Rather Than a Prescriptive One During the development of Vehicle Restraint Systems (VRS), the number and type of systems available to the roadside designer/engineer were limited. The limited number of designs would have distinct performance characteristics and therefore the designer/engineer could prescriptively refer to these systems. For example a designer/engineer could expect a concrete or a cable barrier to perform at a certain performance level. As alternative systems were developed, the available types of different systems increased substantially, and now there are many different systems available on the market together with variants of the same original design, all with slightly different performance characteristics. As the number of the alternative designs increased, the gaps between the performance levels of the original VRS designs were filled by new products. Therefore it is no longer valid to use a product-specific prescriptive approach, where reference is only made to a number of specific designs.
  • 3. ROADSIDE DESIGN GUIDE PAGE 3 01 INTRODUCTION FIRST EDITION -DECEMBER 2016 A prescriptive approach, which focuses on the use of a limited number and type of systems, is not necessarily the best one. The lack of choice can force a designer/engineer to use systems which may not be the optimal solution for a specific site in terms of performance and cost. This sort of prescriptive approach not only limits the ability of the designer to use engineering judgment but it also makes it difficult for new systems to enter the market. It furthermore leads to less of an understanding from the engineer, which may lead to inappropriate systems and arrangements being used in certain locations. In order to overcome these issues, current international best practice is to identify the performance requirements and physical constraints of a site under consideration and then let the engineer/designer chose the most appropriate solution based on proven properties of impact tested and approved systems. Therefore, this version of the manual is based on the required performance specifications, rather than prescriptive descriptions of certain products. In order to achieve this, rather than listing the design details of certain products available on the market, the proposed layout of the roadside design guide was organised to explain, for each VRS type, in basic terms: • The main types of systems available on the worldwide market; • The performance classifications of the products based on impact testing; • The selection criteria based on the performance classifications of the product. 1.3.3 The Use of Proprietary over Non-Proprietary Systems The term ‘proprietary’ means that the system has been independently designed and successfully tested by a VRS manufacturer, who is ultimately responsible for the design of the system. Conversely, non-proprietary systems have often been developed by National Road Authorities and/or Universities, who have subsequently published the drawings of the system and, as such, these can be manufactured by anyone. Whilst many of the non-proprietary systems have historically performed well, many of these systems were developed a number of years ago, and were tested to older versions of the testing standards. Due to budgetary constraints within the original developers of the systems, these systems are often not updated, nor are they supported in terms of future development and product refinement. This is not the case in the commercial domain of the proprietary systems, where development of systems continues on a frequent basis, with product designers keen to develop more effective products than their rivals, testing and certifying products to the latest standards. This competition and the continuous development have led to proprietary products which perform better (higher containment levels with less deflection and lower acceleration severity) than their non-proprietary predecessors. For these reasons, it is now common practice for many countries to install proprietary systems, with a number of countries, such as the UK, USA (FHWA) and Ireland maintaining a list of ‘accepted’ vehicle restraint systems products. Therefore the use of proprietary systems is recommended over non-proprietary ones.
  • 4. ROADSIDE DESIGN GUIDE PAGE 4 01 INTRODUCTION FIRST EDITION -DECEMBER 2016 1.3.4 Use of a Single Testing Standard In the past, barriers successfully tested to the requirements of USA NCHRP350 [5]/MASH [6], European EN1317 [7], and non-proprietary systems were permitted for use on the Abu Dhabi road Network, and in many other countries. However, the testing parameters for these two types of international systems are very different. As a result, no comparison or equivalence can be assumed between systems tested to the different standards. This can lead to, for example, inappropriate transitions between systems, and unknown levels of performance where the performance of one system depends on that of another (for example where the performance of a safety barrier relies on the performance of a terminal). For this reason it is recommended that systems which are tested only to a single standard are permitted for future use on the Abu Dhabi road network. 1.3.5 The Use of NCHRP350 and MASH Tested Products Instead of EN1317 The road network and the vehicle fleet in Abu Dhabi are more similar to an American one rather than a European one. The adoption of the American testing standards NCHRP350 and MASH are felt to be more suitable for the Abu Dhabi road network, over the European test standard EN1317 for the reasons explained in the following sub-sections. However, it is felt that the knowledge of the European test standard EN1317 [7] can still be useful to designers/engineers, especially when comparing VRS products tested to different standards. The Appendix A provides a brief overview of the EN1317. The information provided includes the test types and performance classes and test criteria for barriers, bridge parapets, terminals, transitions and crash cushions. 1.3.5.1 Weight of Vehicles used in NCHRP350, MASH & EN1317 Vehicles used in NCHRP350 and more recently MASH, represent larger and heavier vehicles, such as pickup trucks, which are more common in the USA; whilst EN1317 uses smaller and lighter vehicles which are more common in Europe. This can be seen by comparing Test Level-3 of MASH and NCHRP350 to containment level N2 of EN1317 as these are the most commonly used base performance criteria for America and Europe, respectively. In MASH, a product should perform satisfactorily during impact testing to achieve the basic test level TL-3 using a 1,100 kg passenger car and a 2,270kg pickup truck. While in EN1317 to achieve an N2 containment level, a barrier should perform satisfactorily using a 900kg passenger car and a 1,500kg passenger car (see Appendix A). There is currently no vehicle type in EN1317 which represents an SUV or a pickup truck, which are common in the Abu Dhabi vehicle fleet. As a result, it can be said that American testing standards MASH and NCHRP350, offer a better representation of the vehicle fleet, more specifically enabling vehicle restraint systems to prove their ability to safely contain and redirect (under standard testing conditions) the high percentage of SUVs in Abu Dhabi; while the vehicle classes in EN1317 do not represent the SUVs and therefore are not guaranteed to work on impact with these types of vehicles. 1.3.5.2 Height of Vehicles used in NCHRP350, MASH & EN1317 The weight of the vehicles used in NCHRP350 and MASH are a better representation of the SUVs which represent a high percentage of the vehicles used in Abu Dhabi. However, it is not only the
  • 5. ROADSIDE DESIGN GUIDE PAGE 5 01 INTRODUCTION FIRST EDITION -DECEMBER 2016 weight of the vehicles that make American standards a better fit for Abu Dhabi, but it is also the height, more specifically the height of the centre of mass, required for the test vehicles. Vehicle rollover is one of the more common types of incidents observed in Abu Dhabi. Vehicles with a higher centre of mass, such as SUVs are more likely to roll over. The minima required centre of mass locations for a pickup truck for testing to the basic TL-3 containment levels in MASH & NCHRP350 are 710mm and 700±50mm, respectively. In EN1317 the highest vehicle used to test a N2 normal containment level, requires to have a centre of mass location 530mm above the ground. As can be seen from these numbers, EN1317 test vehicles are far from representing the higher centre of mass seen with SUVs, which are common in Abu Dhabi. Therefore, there is no guarantee that an SUV, which is likely to have a centre of mass higher than 530mm, will be contained by an N2 containment level barrier. 1.3.5.3 Impact Angles used in NCHRP350, MASH & EN1317 Another important difference between the American and European testing standards is the angle of impact. A good comparison can be made between the base performance levels of TL-3 for MASH/NCHRP350 and N2 for EN1317. MASH uses an impact angle of 25˚ for car and pickup truck impacts, to demonstrate a successful TL-3 classification. EN1317 on the other hand, uses an impact angle of 20˚ for TB11 and TB32 tests, which are required to demonstrate a successful classification of N2. The roads in Abu Dhabi, in general, appear to be wider than the European ones, with expressways regularly featuring up to 4 lanes in each direction with generous shoulder areas provided on each side of the road. Research shows that in similar conditions, impacts with roadside barriers are more likely to occur with higher angles of incidence on wider carriageways. For this reason it is believed that the larger impact angles used in MASH and NCHRP350 may provide a better representation of the actual impacts that are likely to occur on the Abu Dhabi road network. 1.4 Scope of the ADRSDG 1.4.1 Overview This Guide has been developed to enable practitioners to follow a step-by-step risk-based process to understand and mitigate the risks posed by hazards, integrating International best practice. This has been achieved by first introducing the user of the Guide to the Risk Mitigation Approach, explaining the concept of the clear zone, and assisting users in the identification of hazards. The following chapters provide information on experience-based advice and internationally recognised practices, to assist Engineers in dealing with the hazards present. This includes details of the various product types available on the market to mitigate road user risk. An overview of the ways in which these solutions can be assessed on an economic basis is then presented. The final chapter of this Guide deals exclusively with the risks existing within the urban environment, and ways in which such risks should be assessed. An overview of the content of each chapter is presented below: 1.4.2 Chapter 2 “Risk Mitigation Approach” This chapter discusses the hazard mitigation process and the forgiving roadside approach to road safety. The Chapter also introduces the clear roadside concept and its application to roadside design.
  • 6. ROADSIDE DESIGN GUIDE PAGE 6 01 INTRODUCTION FIRST EDITION -DECEMBER 2016 1.4.3 Chapter 3 “Concept and Calculation of Clear Zone” This chapter gives further details of the clear zone concept, providing advice on how the clear zone distance should be calculated for straight sections, foreslopes, backslopes and for curved sections of road. Consideration is also given for determining the clear zone for medians and high risk hazards. To aid understanding, examples of clear zone calculations are also detailed. 1.4.4 Chapter 4 “Identification of Roadside Hazards” Once the user of this Guide has followed the procedures within Chapter 3 to determine the clear zone for a particular road scheme, location or hazard, Chapter 4 assists the designer in determining whether objects and/or features within the clear zone are hazards. This includes the identification of risks to third parties. 1.4.5 Chapter 5 “Passively Safe Support Structure and Traversable Objects” This chapter investigates ways in which hazards can be made passively safe or traversable (i.e. less hazardous to road users in the event of an impact). Advice is given on the types of commercially available systems and technologies which exist, and how these should be selected and applied. 1.4.6 Chapter 6 “Description of Roadside, Median and Bridge Barriers” If a hazard cannot be made passively safe or traversable, one of the most common road safety devices is the barrier. However, there are many different types available and this chapter gives an overview of the three different types available (flexible, semi-rigid and rigid). Details of the testing procedures for barrier systems are then explained. 1.4.7 Chapter 7 “Selection and Application of Roadside, Median, and Bridge Barriers” This chapter firstly gives an overview of the way in which roadside, median and bridge barriers should be specified, in terms of their containment level, deflection characteristics, impact severity level and both maintenance and inspection requirements. Secondly, the chapter provides guidance on how the barrier systems should be used on the roadside in terms of the length required and where this should be sited, how to place the barrier laterally at the side of the road, and how to flare back the end of the barrier to reduce road user risk. Guidance on barrier foundations, such as compaction requirements, push-and-pull test and common foundation mistakes to avoid, are also given. Specific guidance is also presented for roadside barriers (in terms of barriers which are installed on sharp horizontal curves), median barriers (in terms of emergency and maintenance crossings and glare screens), and bridge barriers (in terms of minimum height requirements and fixation to bridge decks). 1.4.8 Chapter 8 “Motorcyclist Protection Systems” This chapter gives details of the different types of motorcyclist protection systems (MPS) available (continuous and discontinuous), and gives details of the impact testing requirements for such systems. An explanation of the performance classifications resulting from this testing is also given, together with a description of how to use the performance classifications for the application of the
  • 7. ROADSIDE DESIGN GUIDE PAGE 7 01 INTRODUCTION FIRST EDITION -DECEMBER 2016 products. Specific guidance is given with regard to locations where a positive cost/benefit could result from the application of an MPS. 1.4.9 Chapter 9 “Terminals” This chapter gives details of the different types of terminals available (ramped down end, full height, flared and buried), and gives details of the impact testing requirements for such systems. An explanation of the performance classifications resulting from this testing (for example gating and non-gating terminals) is also given, together with an explanation of how to use the performance classifications for the application of the products. Specific guidance is provided with regard to the site grading for terminals. 1.4.10 Chapter 10 “Crash Cushions” This chapter gives details of the different types of crash cushions available (redirective/ non- redirective, sacrificial/reusable and both low maintenance and self-restoring crash cushions). The chapter gives details of the impact testing requirements for such systems, and an explanation of the performance classifications resulting from this testing (for example gating and non-gating terminals). An explanation of how to use the performance classifications for the application of the products is also outlined. Specific guidance is provided with regard to the site grading for crash cushions. 1.4.11 Chapter 11 “Transitions” This chapter gives details of the different types of terminals available and gives details of the impact testing requirements for such systems. An explanation of the performance classifications resulting from this testing is also provided, together with an explanation of how to use the performance classifications for the application of the products. Specific guidance is provided with regard to the design of terminals, and examples of good and bad practice are outlined. 1.4.12 Chapter 12 “Economic Assessment” The chapter discusses the use of economic analysis to make roadside safety decisions, and provides an overview of the economic assessment process which should be undertaken. The first part of the chapter focuses on the assessment of economic feasibility for alternative roadside treatment options. A step by step guide into benefit/cost ratio (BCR) analysis is presented. Guidance is provided into the prediction of expected number of crashes at a site, prediction of decrease in the number of crashes due to a safety treatment and the estimation of monetary benefits associated with roadside crashes prevented. Example calculations are included for each step of the BCR analysis. A wide range of crash modification factors is provided to help designer/engineers estimate the reduction in the number and/or severity of crashes due to specific roadside safety treatments. The second part of the chapter focuses on treatment prioritization methods. Guidance is given on treatment assessment and ranking based on cost-effectiveness, risk reduction and non-monetary considerations. 1.4.13 Chapter 13 “Urban Roadside Design” This final chapter of this Guide identifies risk relating to the specific case of urban areas, and provides overarching guidance as to how the risks in these areas differ and, hence, which additional measures should be considered when mitigating risk to road users, and to vulnerable users of the road corridor (such as pedestrians).
  • 8. ROADSIDE DESIGN GUIDE PAGE 8 01 INTRODUCTION FIRST EDITION -DECEMBER 2016 Guidance is provided on the lateral offset required between the roadway and the roadside hazards, for different road configurations, such as curves, merge locations and junctions. Information is also provided on the specific applications for pedestrian and bicyclist facilities. Finally, specific guidance is provided into the application of common urban roadside features, such as curbs, pedestrian barriers, street furniture, etc. 1.5 References [1]AASHTO, Roadside Design Guide, 4th Edition, Washington D.C.: American Association of State Highway and Transportation Officials, 2011. [2]CEDR, “SAVERS (Selection of Appropriate Vehicle Restraint Systems) - WP1: Defining the Different Parameters which can Influence the Need and Selection of VRS (Unpublished Report),” Conference of European Directors of Roads, 2014. [3]Austroads, Guide to Road Design Part 6: Roadside Design, Safety and Barriers, Sydney, NSW: Austroads, 2010. [4]Roads & Transport Authority, Roadside Design Guide for Dubai, First Edition, Dubai: RTA, 2008. [5]NCHRP, “NCHRP Report 350, Recommended Procedures for the Safety Performance Evaluation of Highway Features,” Transportation Research Board, National Research Council, Washington DC, 1993. [6]AASHTO, “Manual for Assessing Safety Hardware,” Ammerican Association of State Highway and Transportation Officials, Washington DC, 2009. [7]CEN , “EN 1317 Road Restraint Systems - Part 2: Performance classes, impact test acceptance criteria and test methods for safety barriers including vehicle parapets,” CEN (European Committee for Standardization), Brussels, 2010.
  • 9. ROADSIDE DESIGN GUIDE PAGE 9 02 ROADSIDE RISK MITIGATION FIRST EDITION -DECEMBER 2016 2 ROADSIDE RISK MITIGATION 2.1 Introduction There are many reasons why vehicles may leave the road and potentially encroach on the roadside. These include: • Driver fatigue or inattention; • Excessive speed; • Crash avoidance; • Roadway conditions (e.g. pavement deterioration); • Vehicle component failure; • Poor visibility; and • Driver impairment. When a vehicle runs off the road, it may reach a hazard, collide or overturn; all of which may result in injuries or even fatalities. These casualties may be reduced by making every roadside flat, traversable and free of obstacles; therefore giving enough space to errant vehicles to regain control and return to the road with a reduced likelihood of injury. However, in reality this is not always possible due to physical and economic constraints. Engineers and designers often have to find an optimal solution; one that finds a balance between the maximum amount of safety and economic feasibility. A good way of achieving this is to evaluate each roadside on a case by case basis with a risk based approach. Not every road has the same probability of a vehicle running off the road, nor does every roadside hazard have the same level of consequences, if reached by an errant vehicle. It is important to identify and prioritise the sites with a higher level of risk and apply the necessary countermeasures to keep the risk to a reasonable level. This approach forms the basis for the roadside design guides and standards of many countries around the world [1]. In the UK the decision for roadside treatments is based on The Road Restraint Risk Assessment Process (RRRAP); a software based tool which aims to decrease the level of risk for the evaluated area to “As Low as Reasonably Practicable” [2]. In Germany, the decision on whether to implement a roadside barrier is based on the probability of a vehicle running off the road and the level of risk posed by different type of hazards [3]. In the United States, a recommended practice is the evaluation of different roadside treatment options with Roadside Safety Analysis Program (RSAP); a risk based benefit/cost analysis tool [4]. Following International best practice, the Abu Dhabi Roadside Design Guide is structured as an easy to use and understand roadside risk mitigation tool. This chapter presents an explanation of the concept of risk from a roadside safety perspective and the recommended risk mitigation approach of this Guide.
  • 10. ROADSIDE DESIGN GUIDE PAGE 10 02 ROADSIDE RISK MITIGATION FIRST EDITION -DECEMBER 2016 2.2 Definition of Risk from a Roadside Safety Perspective There are many definitions of hazard and risk, but for the purpose of roadside safety design a hazard can be described as a roadside feature or object that can cause physical, economic, time- based or strategic harm or loss. Risk is the chance, high or low, that somebody or something will be harmed by the roadside hazard. Risk, as shown in Figure 2.1, is directly related to the likelihood of the hazard being reached by a vehicle and the resulting consequences if the hazard is reached. Figure 2.1 - Risk from a roadside safety perspective As shown in Figure 2.1, the likelihood of a roadside accident depends on the probability of a vehicle running off the road and the probability of the errant vehicle subsequently reaching the hazard if it does leave the carriageway. Run-off-the road probability is related to parameters such as traffic volume and horizontal curve radius; while the probability of an errant vehicle reaching a hazard depends on factors such as the distance of the hazard from the edge of the travelled way, speed of the errant vehicle and roadside topography. When a vehicle reaches a hazard, the most obvious consequences are the ones to the occupants of the vehicle, in the form of physical harm and economic loss. But some hazards, if reached by errant vehicles can have consequences for third parties as well. For example, an errant vehicle reaching the opposite side of a dual carriageway can cause serious harm to the people travelling on the other side. An errant vehicle entering a water reservoir may contaminate the drinking water needed by many others. Engineers and designers can mitigate the level of risk by controlling either or both likelihood and/or consequences of a roadside accident. The following sections present the methodology to achieve this. 2.3 Abu Dhabi Roadside Design Guide Risk Mitigation Approach 2.3.1 Outline of the Risk Mitigation Approach Figure 2.2 presents the risk mitigation approach adopted for this Design Guide. As can be seen from the figure, the corresponding chapter/s for each step is also presented.
  • 11. ROADSIDE DESIGN GUIDE PAGE 11 02 ROADSIDE RISK MITIGATION FIRST EDITION -DECEMBER 2016 Figure 2.2 – Abu Dhabi Roadside Design Guide risk mitigation approach The following sections present further explanation on each step of the hazard mitigation approach. 2.3.2 Step 1 - Understand the Area under Evaluation The first action the designer/engineer needs to take is to gather information and understand the conditions at the site being evaluated. The area under evaluation may be a section of roadside / median, or it may even be a specific hazard with a known history of accidents. As defined in Section 1.3.1, the area of evaluation may be a part of the design of a new construction or a major reconstruction of an existing road. Information gathered at this stage will not only be necessary in the following steps of the risk mitigation process, but it will also enable the designer/engineer to establish an overall Design the Optimal roadside treatment Step 7 Choose the Optimal treatment option Step 6 Chapter 12 Assess & Rank treatment options Step 5 Shield Delineate Safe Roadside Provide Recoverable Make Passively Relocate Remove Chapters 5 to 11 Identify applicable treatment options Step 4 Identify the hazards within clear zone Step 3 Calculate the clear zone Step 2 Chapter 4 Understand the area under evaluation Step 1
  • 12. ROADSIDE DESIGN GUIDE PAGE 12 02 ROADSIDE RISK MITIGATION FIRST EDITION -DECEMBER 2016 understanding of the site and hence will enable them to make more informed decisions in each of the following steps of the risk mitigation process. The following are some example questions which can guide a designer/engineer into a better understanding of the area: • What are the traffic characteristics? Traffic information such as volume and speed are not only necessary for the clear zone calculations (See Section 2.2.3), but they are also useful during the assessment of treatment options (See Section 2.2.5). For example the designer/engineer may consider a motorcyclist protection system (MPS) installation depending on the volume of motorcyclist traffic; subject to engineering judgment. However, depending on the traffic speed, they may not always be able to gain the full benefits from these systems, as they are designed to work up to certain impact speeds. (See Chapter 8). • What are the physical characteristics of the road and the roadside? Geometric characteristics such as horizontal curve radius and the gradient of the side slope are not only necessary information for the calculation of the clear zone (See Section 2.2.3), but they may also become physical boundary conditions; limiting the type of countermeasures, which may be applicable to the site. For example, relocating a hazard (See Section 2.2.5.2) may not be a possible option if there isn’t enough physical space for this, and all mitigation measures, such as crash cushions and terminals have a defined road space requirement, both for their installation, and their operation. • What is the history of accidents on the site? This document is designed to give guidance to designers/engineers on the mitigation of most common roadside risks, based on international best practice. However, every site is different and should be evaluated individually. Studying the accident history can reveal local problems, which would help in a more focused risk mitigation approach. • What and why are the potential hazards located on the roadside? Identifying the potential hazards on the roadside is one of the required steps of the risk mitigation approach (see Section 2.2.4). But understanding the nature and reasons why they are located on the roadside is also important for identifying applicable countermeasures. For example, a roadside sign may need to be within a certain distance to the roadside, and therefore relocating it beyond the clear zone may not even be an applicable countermeasure. Figure 2.3 introduces an example of a typical roadside hazard in Abu Dhabi. Examination of this example case will be followed through in the subsequent step-based analysis. The figure represents some of the information that needs to be gathered at Step 1. 2.3.3 Step 2 – Calculate the Clear Zone The second step of the hazard mitigation process is the calculation of the clear zone. The clear zone is the area beside the road, along which the majority of the errant vehicles are expected to regain control. This is the area which should ideally be kept clear of any hazards; hence leading to the title “Clear Zone”.
  • 13. ROADSIDE DESIGN GUIDE PAGE 13 02 ROADSIDE RISK MITIGATION FIRST EDITION -DECEMBER 2016 Figure 2.3 – Understanding the area under evaluation Clear Zone effectively represents the “likelihood” element of the risk formula shown in Figure 2.1, as it is a measure of how far errant vehicles are likely to travel along the roadside. It therefore gives an idea of the likelihood of a hazard being reached, depending on parameters such as traffic volume, design speed, horizontal curve radius, gradient of side slope and the distance of the hazard from the edge of the travelled way. By controlling one or more of these parameters, the designer/engineer may be able to decrease the likelihood of a hazard being reached; hence decreasing the risk posed by the hazard. Figure 2.4 shows the required clear zone area for the example roadside and it can be seen that the sign post lies within it. Hazards lying within the clear zone are more likely to be reached by errant vehicles than the hazards lying beyond it. Chapter 3 presents a detailed explanation of the concept and the calculation of recommended clear zones.
  • 14. ROADSIDE DESIGN GUIDE PAGE 14 02 ROADSIDE RISK MITIGATION FIRST EDITION -DECEMBER 2016 Figure 2.4 – Clear zone area 2.3.4 Step 3 – Identify Hazards within Clear Zone Step 3 in hazard mitigation process involves the identification of all roadside hazards within the clear zone and consideration of high-risk hazards beyond the clear zone, for example a railway line. The road designer should identify all roadside hazards within the area of interest (based on clear zone widths). However, it is not always straightforward to understand if and when a roadside feature becomes a hazard. For example, a roadside ditch may be considered a hazard or a traversable roadside feature, depending on its geometry. A tree may be considered a hazard or not, depending on the diameter of its trunk. A shallow pool of water may become a hazard with floods following seasonal rain. To help the designer/engineer with identifying hazards within the clear zone, Chapter 4 presents a detailed explanation of the types and properties of the most common types of roadside hazards. The following types of hazards are explained in detail within Chapter 4: • Foreslopes (Embankments); • Backslopes(Cutting Slopes); • Ditches; • Transverse Slopes; • Trees; • Overhead Gantries and Cantilevers; • Other Sign Supports; • CCTV Masts and Luminaire Supports;
  • 15. ROADSIDE DESIGN GUIDE PAGE 15 02 ROADSIDE RISK MITIGATION FIRST EDITION -DECEMBER 2016 • Concrete Foundations Protruding from the Ground; • Bridge Piers, Abutments and Portals; • Bridge Railing Ends & Ends of Concrete Barriers; • Above Ground Equipment; • Culverts, Pipes, Headwalls; • Pedestrian Fences and Walls; • Retaining Walls; • Noise Barriers; • Bodies of Water; • Adjacent Roads and Carriageways; • Storage of Hazardous Material; • Places of Frequent Pedestrian Activity / Places of Public Gathering; • Cycle Lanes; • Structures at Risk of Collapse; • Rail Lines • Speed cameras. Hazard identification effectively relates to the “consequences” element of the risk formula shown in Figure 2.1, as it is a measure of how severe the consequences would be if an errant vehicle reaches a hazard. Hazards that lie within the clear zone, as previously shown in Figure 2.4, pose a risk to road users and this risk should be mitigated through one of the treatment options explained in the following step. 2.3.5 Step 4 – Identify Applicable Treatment Options Where hazards exist within the clear zone (or outside the clear zone in the case of high- consequence hazards), potential treatment options should be identified so that their effectiveness in reducing the risk associated with the hazard can be assessed. The following are the basic treatment options that should be considered: • Remove the hazard; • Relocate the hazard; • Make the hazard passively safe or traversable; • Shield the hazard with a longitudinal barrier or crash cushion; • Delineate the hazard; • Design roadside safety (refer Austroads Sections 4.4 bullet point no: 3). These options are listed in an order of decreasing desirability from a safety perspective; i.e. it is more desirable to completely remove a hazard than shielding it with a barrier, if costs and physical attributes are not constraints. Details of each of these approaches are given in the subsequent sections.
  • 16. ROADSIDE DESIGN GUIDE PAGE 16 02 ROADSIDE RISK MITIGATION FIRST EDITION -DECEMBER 2016 2.3.5.1 Remove the hazard The first approach in decreasing roadside risk is to remove the hazard. This is the most desirable treatment option from a safety perspective as it completely eliminates the risk of a roadside accident through eliminating the consequences if a vehicle were to run-off-the road. This option, although desirable from a safety perspective, may not always be physically possible, as the hazard may be an item of essential roadside infrastructure, or it may not be cost effective, as the costs of completely removing the hazard may not be justified by the benefits. See Chapter 12 for more information on Economic Assessment. Figure 2.5 follows the previous example and shows the hazard removed from the clear zone, therefore eliminating the risk. Figure 2.5 – Hazard removed from the area 2.3.5.2 Relocate the hazard If removing a hazard is not physically possible or cost effective, the second option to be considered is the relocation of the hazard beyond the clear zone, where it is less likely to be reached by an errant vehicle. Relocating a hazard may not always be physically possible due to right-of-way or other physical constraints. For example a roadside sign may have to be within a certain distance from the travelled way, so that it is clearly visible by all travelling vehicles. Relocating some of the roadside features may be too expensive to justify the benefits. Figure 2.6 follows the previous example and shows it with the hazard relocated beyond the clear zone.
  • 17. ROADSIDE DESIGN GUIDE PAGE 17 02 ROADSIDE RISK MITIGATION FIRST EDITION -DECEMBER 2016 Figure 2.6 – Hazard relocated beyond the clear zone 2.3.5.3 Provide recoverable roadside This treatment option refers to the provision of a recoverable roadside between the road and the hazard, which would assist the driver to regain the control of the vehicle once it has left the road, before reaching the hazard. This type of treatment is related to the likelihood part of the risk model, more specifically to the probability of an errant vehicle reaching the hazard, once it runs off the road. An example to this kind of treatment is flattening of a foreslope between the edge of the travelled way and a hazard. Errant vehicles travel further on steeper foreslopes and therefore a wider clear zone is required. By flattening the foreslope, designer/engineer may decrease the required clear zone to a point where the hazard is left outside. Therefore the risk of a vehicle reaching the hazard would be considerably less. Section 3.3.3 provides further guidance on the effects of roadside topography on the required clear zone. 2.3.5.4 Make the hazard passively safe or traversable Another treatment option is to make the hazard either passively safe or traversable. For example, it may be possible to replace a fixed sign post with a crash tested passively safe alternative, such as slip-base system (see Figure 5.1 in Chapter 5). A cross-drainage culvert may be made traversable by installing an appropriate grate to cover the opening (see Section 4.3.2.9). Unfortunately, not every roadside hazard can be made passively safe or traversable; therefore this alternative may not always be applicable. Chapter 5 provides guidance on the use of available passively safe roadside hardware and the ways of making roadside features traversable. Figure 2.7 shows the example roadside with the sign post still within the clear zone, but with the post made passively safe through the use of a slip base system.
  • 18. ROADSIDE DESIGN GUIDE PAGE 18 02 ROADSIDE RISK MITIGATION FIRST EDITION -DECEMBER 2016 Figure 2.7 – Hazard made passively safe 2.3.5.5 Shield the hazard with a longitudinal barrier or crash cushion If none of the previous alternatives is physically possible or cost effective, the alternative of shielding the hazard with an appropriate VRS (or a longitudinal barrier or crash cushion) should be considered. At this point the designer should remember that vehicle restraint systems, although designed to provide controlled impact, are a hazard themselves and they should ideally be used only if the consequences of hitting the VRS is likely to be less than the consequences of reaching and/or impacting the hazard behind. VRS systems have their own physical requirements, which they need in order to perform as designed, such as enough clear space behind, minimum length of installation, etc. The roadside under consideration may not always have the required physical space or it may not always be cost effective to install VRS for hazards that are less likely to be reached. Chapters 6 to 11 provide comprehensive information on vehicle restraint systems; i.e. roadside and median barriers, parapets, terminals, transitions and crash cushions. Information on the general principals of these systems, testing requirements and detailed design properties are also provided. Figure 2.8 shows the example roadside with the sign post being shielded by a roadside barrier.
  • 19. ROADSIDE DESIGN GUIDE PAGE 19 02 ROADSIDE RISK MITIGATION FIRST EDITION -DECEMBER 2016 Figure 2.8 – Hazard shielded with a roadside barrier 2.3.5.6 Delineate the hazard The final treatment option is to delineate the hazard; i.e. make it more visible to the motorists and make them aware of the danger. This may be achieved by using reflective material, as shown in Figure 2.9, and/or by warning signs. This treatment option does not provide any physical protection but it may still be better than doing nothing, where other options are either physically impossible or not cost effective. Figure 2.9 – Hazard delineated
  • 20. ROADSIDE DESIGN GUIDE PAGE 20 02 ROADSIDE RISK MITIGATION FIRST EDITION -DECEMBER 2016 2.3.6 Step 5 – Assessment & Ranking of Treatment Options 2.3.6.1 Overview of Treatment Assessment & Ranking Process Once the potential treatment options are identified, each option should be assessed from a perspective of physical applicability and economic feasibility. The options which are both physically applicable and economically feasible should then be ranked based on the amount of risk reduction they provide, their benefit cost ratio and other non-monetary considerations should be determined for a final decision. An overview of the treatment assessment and ranking process is shown in Figure 2.10. Further detail on each individual step of the process is presented in the following sections. Figure 2.10 – Overview of treatment assessment & ranking process
  • 21. ROADSIDE DESIGN GUIDE PAGE 21 02 ROADSIDE RISK MITIGATION FIRST EDITION -DECEMBER 2016 2.3.6.2 Assess Physical Practicability After a hazard and the possible safety treatments are identified, an initial evaluation to the physical practicability of the treatment should be carried out. A treatment option may not be reasonably practicable for reasons such as: • Constructability of the treatment; • Right of Way Limitations; • Insufficient physical space for the treatment to function as intended; • Intended function of the hazardous object. Some of the more obvious of the physically impracticable or extremely difficult to apply treatment options may be related to the removal of a hazard. For example for coastal roads, the sea may be considered as a continuous hazard, if the water is located within the clear zone distance, removal of such a hazard would be considered extremely difficult, if not entirely impossible. Some treatment options may not be possible due to the right of way limitations. For example a hazard may not be able to be relocated beyond the clear zone, if the end of the clear zone area lies outside the right-of-way boundaries. Some treatment options may not be practicable due to restrictions in physical space available. For example “shielding a hazard with a barrier” is only possible if the distance between the back face of the barrier and the hazard is less than the deflection distance of the barrier (eee Section 7.3.5). If not, the impacting vehicle would still reach the object behind as the barrier deflects. If the hazard is simply too close to the travelled way, designer/engineer may not be able use certain types of barriers, due to physical limitations of the product and the physical space available on the site. Sometimes the intended function of a roadside hazard may be a reason for a treatment to be impracticable. For example “relocation of the hazard beyond clear zone” may not be possible for a road sign as the sign should be within a certain distance from the road to fulfil its intended function. Similarly “removal of the hazard” may also not be an option for the same hazard. For some treatment options, assessing the physical practicability is relatively straightforward. Treatment options, which are either extremely difficult to apply or simply impracticable may be discarded from the risk mitigation process, or the treatment may be modified and reassessed again. Treatment options which are physically practicable should be carried over to the next phase of the process. 2.3.6.3 Assess Economic Feasibility Once the physically impracticable treatment options are eliminated, the remaining should be assessed for their economic feasibility. The expected benefits of a treatment (i.e. the anticipated reduction in the frequency and severity of injuries, reduction in economic loss, reduction in losses due to traffic disruptions, etc.) should be more than the associated costs (i.e. construction costs, maintenance costs, etc.) for its application. In other words, the Benefit/Cost Ratio (BCR) of any safety treatment should be more than 1 (see Chapter 12 for Economic Analysis).
  • 22. ROADSIDE DESIGN GUIDE PAGE 22 02 ROADSIDE RISK MITIGATION FIRST EDITION -DECEMBER 2016 Any treatment option with a BCR less than or equal to 1.0 should be discarded from the risk assessment process and the following phase should continue for options with a BCR greater than 1.0. 2.3.6.4 Rank Treatment Options After economically feasible treatment options are identified, they should be compared and ranked from a risk reduction and an economic perspective. Ranking is useful to identify the optimal treatment option. Rank by Risk Reduction: Different safety treatments provide different levels of reduction in overall risk. For example, as explained in Section 0.1, removing a hazard usually provides a higher risk reduction than shielding the hazard with a VRS. For this reason, alternative treatment options should be ranked by the amount of risk reduction they are expected to provide. The amount of risk reduction can be quantified in terms of the expected reduction in crash frequency, injuries and property damage. Example methods of carrying out such a ranking are presented in Chapter 12. At this point the road authority or the designer/engineer may choose to set a certain level of risk reduction as a minimum and eliminate any treatment options which do not provide the minimum desired level of risk reduction. Once the treatment options are ranked in risk reduction order, options that provide a higher level of risk reduction should be given greater consideration. However the amount of risk reduction is not the only factor to be used to determine the optimal solution. The treatment which provides the highest level of risk reduction may also be the most expensive. In such a case, a second alternative may provide an acceptable level of risk reduction for a better economic value. For this reason, the treatment options should also be assessed from an economic perspective: Rank by Benefit Cost Ratio As explained above, the amount of risk reduction is not the only important factor when deciding upon the optimal safety treatment. Treatment options which provide an acceptable level of risk reduction should also be evaluated and ranked from a cost effectiveness perspective. BCR is a good indicator of project value. However, simply comparing the BCR of different treatment options to each other may be misleading. This is because BCR is a ratio of the benefits of a specific treatment to its costs and it does not necessarily provide a meaningful comparison between the benefits of different treatment options. For example, “delineation of a hazard” is often the cheapest treatment option and due to its low cost compared to its potential benefits, it is usually the option with the highest BCR. However, this high BCR does not necessarily mean that the benefits gained from delineation is more than the benefits gained from a more expensive option, for example shielding the hazard with a barrier. A barrier would cost more, but its benefits would also be greater than for delineation. Therefore, a solution can provide higher benefits, but with a lower BCR. In such a case an incremental BCR analysis may be applied to rank the treatment options economically. For incremental BCR and more on Economic Analysis, see Chapter 12.
  • 23. ROADSIDE DESIGN GUIDE PAGE 23 02 ROADSIDE RISK MITIGATION FIRST EDITION -DECEMBER 2016 The economic ranking of treatment options is essential, but is not the only consideration for deciding upon an optimal solution. The decision should also be based on the assessment of non- monetary considerations. 2.3.6.5 Assess Non-Monetary Considerations In most cases, the main benefits of applying a roadside safety treatment can be quantified in monetary terms; i.e. the monetary gains expected due to a reduction in crash frequency, severity, and the associated repair costs. However, there are some factors which may be influential in the decision about which safety treatment to adopt which cannot be quantified in monetary terms. Examples of these considerations include: • Aesthetics; • Public demands and perception of road safety improvements; • Air quality, noise, visual intrusion or other environmental considerations; • Road user needs. As these considerations cannot be quantified, their effect in the final decision should be evaluated on a case by case basis through engineering judgment. For example, aesthetics may be of a significant importance in the selection of a certain type of treatment over the other alternatives around areas of natural beauty, land marks, major tourist attractions, etc. while it may not be an important factor in a remote rural area of no special importance. Environmental concerns may be of significant importance in areas such as natural reserves, natural protection areas such as source of drinking water, in conservation areas for a certain species of animal, etc. In some cases the designer/engineer may choose to apply a certain treatment option to satisfy the public demand, although it may not be the best option from an economic perspective. For example, one may choose to install a motorcyclist protection system on a certain location to satisfy the demands of a motorcyclist action group. Non-monetary considerations can be of significant importance in the final decision; however the designer/engineer should always ensure that an adequate level of safety is provided. 2.3.7 Step 6 – Choose the Optimal Treatment Option Once the risk reduction and BCR ranking, and the non-monetary assessment of treatment options are complete, the designer/engineer should rank the preferred treatment options based upon the risk reduction, cost effectiveness and non-monetary considerations. As each site is different, engineering judgment should be used to find the optimal solution that fits the needs of the particular site, as shown in Figure 2.11. 2.3.8 Step 7 – Design the Optimal Roadside Treatment The final step in the risk mitigation process is the detailed design of the appropriate (optimal) roadside treatment. This step may include the design of just a single treatment or the whole safety improvement scheme along a section of road which addresses different types of hazards in one
  • 24. ROADSIDE DESIGN GUIDE PAGE 24 02 ROADSIDE RISK MITIGATION FIRST EDITION -DECEMBER 2016 go. In some cases Abu Dhabi DoT Standard Drawings will provide the necessary detail, whereas the road design layout will show the location of the treatment as well as information not covered by the standard drawings. Necessary guidance for the design of individual roadside safety treatments is provided between Chapters 3 to 11 of this Guide. Figure 2.11 – Selection of optimal treatment option through engineering judgment The designer/engineer of a specific treatment should consider that the final design of the roadside treatment can affect the other responsible bodies which provide roadside infrastructure items, such as signage and lighting. Lateral extensions of the clear zone or hazard corridor should be shown on the final plans so the other responsible bodies can assess the final design from their own perspective. The final design should include, but not be limited to the following information: • All hazards for which a treatment warrant has been identified; • The treatment options chosen for those hazards; • The priority of the treatment options.
  • 25. ROADSIDE DESIGN GUIDE PAGE 25 02 ROADSIDE RISK MITIGATION FIRST EDITION -DECEMBER 2016 2.4 Abu Dhabi DoT Product Approval Process Prior to the use of any proprietary vehicle restraint system on the Abu Dhabi road network, the system must have been accepted for use by the Abu Dhabi Department of Municipal Affairs and Transport, or their representatives. This is to ensure that the product has been successfully tested to appropriate standards (NCHRP350 and MASH), and that sufficient consideration has been given to the local conditions and demands of the Abu Dhabi road environment. Whilst a large number of systems currently exist on the market, these have often been developed with the local requirements of the USA, Australasia and (in some cases) European road conditions in mind, not necessarily those of Abu Dhabi. A list of those products deemed to be acceptable for use on such roads is available. In order for a product to be listed, an application must first be filed with the Abu Dhabi Department of Municipal Affairs and Transport, or its representatives. The Department will then issue a proforma detailing questions regarding the suitability of product for the local conditions in Abu Dhabi. This must be completed in a satisfactory way, supported by evidence where possible. For each product, the manufacturer will also be required to supply a set of full (i.e. not summary) impact test reports and videos of their system to the Department of Municipal Affairs and Transport, or its representatives, for assessment against the relevant testing standard (either NCHRP350 or MASH). This should be accompanied by any other relevant supporting evidence which may include: • Inspection, maintenance and repair requirements; • Installation manual; • Restrictions on the use of the product, and compatibility with other products; • Details of any modifications made to the product since it was tested, and any supporting evidence/independent approval of the modifications made; • Details of agreements with local distributors; • Drawings and specifications for the system; • Details of any in-service performance evaluation (including impacts with higher and faster vehicles than specified in the testing standards); • Details of any durability/environmental testing; • Promotional literature; • Any other information supporting the application. Note that whilst the acceptance of a product in another territory (e.g. by Federal Highways in the USA or by the award of a CE mark within Europe) may be considered as part of the approval process, this will be no guarantee of acceptance for use by the Abu Dhabi Department of Municipal Affairs and Transport due to its local needs and road conditions. 2.5 Summary and Conclusions This chapter explains the recommended risk mitigation approach for Abu Dhabi. It also provides an overview of how the manual should be used in general, by referencing the related chapters of the manual for each step of the risk mitigation procedure.
  • 26. ROADSIDE DESIGN GUIDE PAGE 26 02 ROADSIDE RISK MITIGATION FIRST EDITION -DECEMBER 2016 Risk, from a roadside safety perspective, is defined as the probability, high or low, that somebody or something will be harmed by a roadside hazard. Roadside risk is directly related to both the likelihood of a roadside accident and the consequences of it. Designers/engineers can mitigate the risk by controlling either or both the likelihood or consequences of a run-off-the road accident, through the recommended treatment options. The recommended risk mitigation approach consists of the following stages: 1. Understand the area under evaluation; 2. Calculate the clear zone; 3. Identify the hazards located within the clear zone; 4. Identify applicable treatment options to mitigate risks from the hazards located within the clear zone. Recommended treatment options include: o Remove the hazard; o Relocate the hazard; o Provide recoverable roadside; o Replace the hazard with a passively safe system; o Shield the hazard with a VRS; o Delineate the hazard; 5. Assess and rank applicable treatment options: o Assess physical applicability; o Assess economic feasibility; o Rank treatment options economically; o Assess non-monetary considerations; 6. Choose the optimal treatment option through engineering judgment; 7. Design the appropriate (optimal) roadside treatment. An overview of the DoT product approval process is also given in this chapter. Prior to the use of any proprietary vehicle restraint system on the Abu Dhabi road network, the system must have been accepted for use by the Abu Dhabi Department of Municipal Affairs and Transport, or its representatives. 2.6 References [1]CEDR, “SAVERS (Selection of Appropriate Vehicle Restraint Systems) - WP1: Defining the Different Parameters which can Influence the Need and Selection of VRS (Unpublished Report),” Conference of European Directors of Roads, 2014. [2]TD19/06 Design Manual for Roads and Bridges, Volume2 Highway Structures: Design, Section 2 Special Structures, Part 8, The Highways Agency, Transport Scotland, Welsh Assembly Government, The Department for Regional Development Norther Ireland, 2006. [3]FGSV, Traffic Management Work Group, “Guidlines for passive protection on roads by vehicle
  • 27. ROADSIDE DESIGN GUIDE PAGE 27 02 ROADSIDE RISK MITIGATION FIRST EDITION -DECEMBER 2016 restraint systems,” FGSV Verlag GmbH, Koln, 2009. [4] AASHTO, Roadside Design Guide, 4th Edition, Washington D.C.: American Association of State Highway and Transportation Officials, 2011.
  • 28. ROADSIDE DESIGN GUIDE PAGE 28 03 CONCEPT AND CALCULATION OF CLEAR ZONE FIRST EDITION -DECEMBER 2016 3 CONCEPT AND CALCULATION OF CLEAR ZONE 3.1 Introduction The “Clear Zone Concept” is a key part of this Guide’s risk mitigation process, as it provides engineers and designers an easy to use tool for the assessment of risk of a roadside accident for selected road environments. The chapter starts with a brief look at the concept of clear zone, its origins and its evolution over time. This is followed by a look at the factors which may affect the required clear zone distance for different roads. The Clear Zone Calculation Model for this Guide is presented in detail and followed by examples on clear zone distance calculations. 3.2 The Clear Zone Concept “The clear zone is the unobstructed, traversable area provided beyond the edge of the through travelled way for the recovery of errant vehicles”. [1] In an ideal world, providing unlimited, flat and obstacle-free areas along every road would completely eliminate the problem of roadside accidents. However, in reality this is neither economically viable, nor physically possible. For this reason, engineers and designers should assess the level of risk along each roadside and find an optimal design solution; one which provides a balance between the amount of forgiving roadside provided and the economic feasibility of the selected application. Accordingly, AASHTO became the first organization to promote the idea of providing clear recovery areas along highways. In 1974, the AASHTO document known as the “Yellow Book” stated that, “for adequate safety, it is desirable to provide an unencumbered roadside recovery area that is as wide as practical on a specific highway section. Studies have indicated that on high- speed highways, a width of 9 m or more from the edge of the through travelled way permits about 80 percent of the errant vehicles leaving the roadway to recover.” [2]. The idea of providing 9m wide clear zones was trialled by several highways agencies, and it was understood that a constant width of recovery space is not always the optimal solution for roads with different characteristics. Errant vehicles travel further along the roadside with increased traffic speeds, sharper horizontal curves and steeper side slopes. Therefore 9m clear zone was not always enough for errant vehicles to recover safely. On the other hand it was understood that 9m is too wide and economically not viable for roads with lower speeds and traffic volumes. For these reasons, in 1977 AASHTO modified its earlier clear zone concept by introducing variable clear-zone distances based on traffic volumes, speeds and roadside geometry [3]. This new approach was well received and became widespread. Many countries around the world today either use the exact AASHTO variable clear zone model or modified versions of it to fit the specific needs of their road networks. [4] The clear zone distances recommended by AASHTO Roadside Design Guide [1] are based on empirical research data, which was later extrapolated to fill in the gaps of the database. Therefore, they are intended as reference points rather than definitive values. It should be noted that these
  • 29. ROADSIDE DESIGN GUIDE PAGE 29 03 CONCEPT AND CALCULATION OF CLEAR ZONE FIRST EDITION -DECEMBER 2016 clear zone values, although considered sufficient for majority of errant vehicles to safely regain control, are not enough to stop all 100% of errant vehicles. For example, the recommended clear zone widths may not be enough for an over-speeding errant vehicle to safely regain control. A clear zone, where even the over- speeding errant vehicles would regain control before reaching the end, would require a considerably wider space. However, considering the low probability of such incidents, providing this extra space is not always economically viable. However, The Abu Dhabi DoT lately extended the speed range of AASHTO Table, shown in Table 3.2, up to 140 km/h. The values given were only extracted by interpolating the trend of previous speed ranges given by AASHTO. The relationship between the distance from the edge of the travelled way to a hazard and the proportion of errant vehicles that will be able to reach the hazard has been the subject of several researches. One of the most comprehensive datasets of run-off-road accidents was collected during late 1970s in Canada by P. Cooper [5]. Cooper’s research involved weekly observations of wheel tracks on grass-covered roadsides of rural highways of various functional classes, where he looked at the distance travelled by errant vehicles. Cooper’s encroachment data was later re-analysed for development of the RSAP [6], and the relationship shown in Figure 3.1 was derived. Figure 3.1 shows the proportion of errant vehicles which are expected to travel over a certain distance from the edge of the travelled way during a run-off-road accident. The relationship is presented for two-lane undivided roads, multi-lane divided roads and for the combination of both undivided and divided roads. It can be seen from the figure that, as the distance from the edge of the travelled way increases, the proportion of errant vehicles which are likely to reach the distance decreases. Figure 3.1 shows that clear zone distances recommended by AASHTO are likely to allow enough space for approximately 85% of the errant vehicles to stop or regain control. In other words, roughly 15% of errant vehicles could still reach a hazard beyond these distances. A considerably wider space is required to ensure that more than 85% of the errant vehicles will stop within the clear zone. For example for divided road, increasing the proportion of vehicles which would stop within the clear zone from 85% to 95%, would require the clear zone distance to increase from roughly around 12m to 20m. To take the same ratio from around 85% to around 100%, the clear zone distance should almost be tripled from 12m to 30m. The considerable increase in the required clear zone to cater for every single possible incident may not always be justified due to economic reasons. Therefore, the designer should always use engineering judgment on a site-by- site basis when deciding the acceptable clear zone distance. However, some hazards, especially the ones where third parties may be affected, would yield more severe consequences if reached by an errant vehicle. Examples of these are chemical plants, school playgrounds, areas of public gathering, source of drinking water, etc. The consequences of a vehicle reaching these hazards would be so high, that even a low likelihood of a vehicle reaching them would pose a significant risk (see Section 2.2). For such high consequence hazards, the extra width required to increase the 85% mark to 95- 100% may be justified. This approach of providing wider clear zones for higher consequence hazards is adopted by several countries around the world. For example, the Norwegian VRS Manual [7] recommends up to doubling the clear zone distance for higher risk hazards, while the German guidelines [8] recommend 8, 4 and 3m of extra clear zone space for high risk hazards located close to highways of 100, 80 and 60km/h posted speed limit, respectively. This approach forms the basis of the clear zone calculation model for the ADRSDG.
  • 30. ROADSIDE DESIGN GUIDE PAGE 30 03 CONCEPT AND CALCULATION OF CLEAR ZONE FIRST EDITION -DECEMBER 2016 Distance from the edge of travelled way (m) Figure 3.1 – Distance from the edge of travelled way vs proportion of errant vehicles which may reach it by highway type [6] 3.3 Factors Affecting the Clear Zone Distance The required clear zone distance along a particular road is related to several factors. These factors are explained briefly to provide an insight into how the clear zone calculation model works. 3.3.1 Traffic Volume Traffic Volume is an important factor which affects the required clear zone distance, as it is directly related to the exposure level of a roadside. Probability of a vehicle running off the road increases as the number of vehicles passing through the area increases. Traffic volume does not directly affect the distance travelled by an errant vehicle. However, there is an indirect effect, as the probability of a faster vehicle running off the road increases with increased traffic volume. For this reason, the required clear zone distance goes up with increased volume of traffic. 3.3.2 Design Speed Design speed is another important factor, which affects the required clear zone distance. Design speed determines the distance up to which an errant vehicle will travel and therefore the likelihood of the vehicle reaching a hazard within a certain distance. For this reason, the required clear zone distance goes up with increased design speed. er c e nt of v e hi cl e s e x c e e di 10
  • 31. ROADSIDE DESIGN GUIDE PAGE 31 03 CONCEPT AND CALCULATION OF CLEAR ZONE FIRST EDITION -DECEMBER 2016 3.3.3 Roadside Topography Roadside topography has an important effect on how far an errant vehicle would travel; i.e. the probability of an errant vehicle reaching a hazard within a certain distance. Due to the change in its potential energy, an errant vehicle is likely to travel further distance along steeper downhill slopes, i.e. foreslopes, and is likely to travel less distance along steeper uphill slopes, i.e. backslopes. 3.3.3.1 Foreslopes According to their gradient, foreslopes are usually divided into three categories: Recoverable Slopes: Recoverable slopes are those on which a motorist can retain or regain control of a vehicle. Slopes equal to or flatter than 1V:4H, as shown in Figure 3.2, are considered recoverable. Smooth, compact slopes with no significant discontinuities and no protruding fixed objects are required from a safety standpoint. Slope should be rounded so that an encroaching vehicle remains in contact with the ground. Also the toe of the slope should be rounded to improve reversibility by an errant vehicle. Figure 3.2 – Recoverable slope Non-recoverable Slopes: A non-recoverable foreslope is defined as one that is traversable but from which most vehicles will not be able to stop or return to the roadway easily. A foreslope with gradient between 1V:4H and 1V:3H, as shown in Figure 3.3, is considered as non-recoverable, as long as it has firm compacted surface or if it is treated with concrete, rip-rap, etc. Conversely if it has a loose sandy surface which may cause a vehicle to overturn, a foreslope with gradient between 1V:4H and 1V:3H should be considered as a hazard rather than a non- recoverable slope. See Chapter 4 for more information on hazard assessment of foreslopes. An errant vehicle reaching a non-recoverable slope would continue at least until it reaches the end of it. For this reason, fixed obstacles along such slopes should be avoided and a clear runout area should be provided at the base. This is also why, for the calculations, the total width of a non- recoverable slope is added to the clear zone distance.
  • 32. ROADSIDE DESIGN GUIDE PAGE 32 03 CONCEPT AND CALCULATION OF CLEAR ZONE FIRST EDITION -DECEMBER 2016 Figure 3.3 – Non-recoverable slope Critical Slopes: A foreslope with gradient steeper than 1V:3H, as shown in Figure 3.4, is considered as a critical slope; one on which an errant vehicle has a higher probability to overturn. For this reason a foreslope is considered as a hazard by itself. Critical slopes which are located within the clear zone should ideally be flattened. If this is not possible, or economically viable, a barrier is typically used. 3.3.3.2 Backslopes Figure 3.4 – Critical slope A backslope in a cut section may be traversable depending on its relative smoothness and the presence of fixed obstacles. It may not be a significant obstacle if the front slope between the roadway and the base of the backslope is traversable (1V:3H or flatter) and the backslope is obstacle-free. However, a steep, rough-sided rock cut normally should begin outside the clear zone or be shielded. A rock cut normally is considered to be rough-sided when the face will cause excessive vehicle snagging rather than provide relatively smooth redirection. 3.3.4 High Risk Hazards The type of a hazard does not affect the probability of an errant vehicle reaching it. However, it is still an important factor for the determination of the required clear zone distance as it affects the overall risk. As explained in Section 3.2, some hazards yield higher consequences than others in case of an errant vehicle reaching them. Since risk is a product of likelihood and consequences (see Chapter 2), these high consequence hazards can still pose a considerable risk for even low
  • 33. ROADSIDE DESIGN GUIDE PAGE 33 03 CONCEPT AND CALCULATION OF CLEAR ZONE FIRST EDITION -DECEMBER 2016 probability of a vehicle reaching them. Therefore, high risk hazards affect the required clear zone distance, as they can economically justify the provision of wider clear zones. 3.4 Calculation of Clear Zone Distance 3.4.1 Clear Zone Model The clear zone is calculated by using the following formula, as detailed in Table 3.1: Where: Table 3.1 – Calculation of clear zone distance, Cz Calculation of Clear Zone Distance, Cz Cz=(Bcw x Mc) Bcw, Base Clear Zone Width See Table 3.2 Mc, Modification Factor for Outside of Horizontal Curves Straight Sections & Inside of Curves Mc=1.0 Outside of Curves See Table 3.3 3.4.2 Base Clear Zone Width The base clear zone width is the recommended clear zone width from the edge of the travelled way for straight road sections, and it is determined on the basis of ADT, Design Speed and Side Slope, by using Table 3.2. The clear zone distances presented in Table 3.2 are based on the clear zone distances suggested by AASHTO Roadside Design Guide [1]. To cater for the needs of the Abu Dhabi Road Network the AASHTO clear zone values are extrapolated for design speeds up to 140km/h. The clear zone values presented in Table 3.2 are based on empirical research data, which was later extrapolated to fill in the gaps of the database. Therefore, they are intended as guidance values rather than definitive values. It should be remembered that, as explained in Section 3.2, some vehicles can still travel further than these recommended clear zone distances. Therefore, designer/engineer should assess the risk for each site on a case-by-case basis, using engineering judgment. In Table 3.2, a two-way ADT should be used for single carriageways and one-way ADT should be used per direction of dual carriageways. The traffic volumes should be based on a 20-year projection from the anticipated date of construction. For recommended clear zone widths over 15m, an additional assessment should be carried out to justify the required empty space, as these are likely to cross the right-of-way limitations.
  • 34. ROADSIDE DESIGN GUIDE PAGE 34 03 CONCEPT AND CALCULATION OF CLEAR ZONE FIRST EDITION -DECEMBER 2016 Table 3.2 - Recommended base clear zone width, Bcw (in metres) Design Year ADT Design Speed (km/h) Under 750 750-1500 1500-6000 Over 6000 Foreslopes 60 2.0 - 3.0 3.0 - 3.5 3.5 - 4.5 4.5 - 5.0 80 3.0 - 3.5 4.5 - 5.0 5.0 - 5.5 6.0 - 6.5 1V:6H 90 3.5 - 4.5 5.0 - 5.5 6.0 - 6.5 6.5 - 7.5 100 5.0 - 5.5 6.0 - 7.5 8.0 - 9.0 9.0 - 10.0 or flatter 110 5.5 - 6.0 7.5 - 8.0 8.5 - 10.0 9.0 - 10.5 120 6.5 - 7.0 8.5 - 10.0 10.5 - 11.5 11.0 - 12.5 140 9.5 - 10.5 12.5 - 14.0 15.5* - 16.5* 16.0* - 17.5* 60 2.0 - 3.0 3.5 - 4.5 4.5 - 5.0 5.0 - 5.5 Steeper 80 3.5 - 4.5 5.0 - 6.0 6 - 8.0 7.5 - 8.5 90 4.5 - 5.5 6.0 - 7.5 7.5 - 9.0 8.0 - 10.0 than 1V:6H to 100 6.0 - 7.5 8.0 - 10.0 10.0 - 12.0 11.0 - 13.5 Flatter 110 6.0 - 8.0 8.5 - 11.0 10.5 - 13.0 11.5 - 14.0 than 1V:3H 120 8.5 - 10.5 10.5 - 13.5 13.0 - 16.5* 14.5 - 18.0* 140 13.5 - 15.5* 15.5* - 20.0* 19.0* - 25.0* 20.0* - 27.0* Backslopes 60 2.0 - 3.0 3.0 - 3.5 3.5 - 4.5 4.5 - 5.0 80 2.5 - 3.0 3.0- 3.5 3.5 - 4.5 4.5 - 5.0 1V:3H 90 2.5 - 3.0 3.0 - 3.5 4.5 - 5.0 5.0 - 5.5 100 3.0 - 3.5 3.5 - 4.5 4.5 - 5.5 6.0 - 6.5 or steeper 110 3.0 - 3.5 3.5 - 5.0 5.0 - 6.0 6.5 - 7.5 120 3.5 - 3.5 3.5 - 5.0 5.5 - 6.0 6.5 - 7.5 140 4.0 - 4.5 4.0 - 6.0 6.0 - 7.0 8.0 - 9.0 60 2.0 - 3.0 3.0 - 3.5 3.5 - 4.5 4.5 - 5.0 Steeper 80 2.5 - 3.0 3.5 - 4.5 4.5 - 5.0 5.5 - 6.0 90 3.0 - 3.5 4.5 - 5.0 5.0 - 5.5 6.0 - 6.5 than 1V:6H 100 3.5 - 4.5 5.0 - 5.5 5.5 - 6.5 7.5 - 8.0 Flatter 110 4.5 - 5.0 5.5 - 6.0 6.5 - 7.5 8.0 - 9.0 than 1V:3H 120 5.0 - 5.5 6.5 - 7.0 7.0 - 8.0 9.0 - 10.0 140 6.5 - 7.0 8.0 - 8.5 9.0 - 10.0 11.5 - 12.5 60 2.0 - 3.0 3.0 - 3.5 3.5 - 4.5 4.5 - 5.0 80 3.0 - 3.5 4.5 - 5.0 5.0 - 5.5 6.0 - 6.5 1V:6H 90 3.0 - 3.5 5.0 - 5.5 6.0 - 6.5 6.5 - 7.5 100 4.5 - 5.0 6.0 - 6.5 7.5 - 8.0 8.0 - 8.5 or flatter 110 4.5 - 5.0 6.0 - 6.5 8.0 - 8.5 8.5 - 9.0 120 5.5 - 5.5 7.5 - 8.0 9.0 - 10.0 10.0 - 10.5 140 7.0 - 8.0 10.0 - 10.5 11.5 - 13.0 13.0 - 13.5 * Clear Zone distances over 15m should be evaluated on a site-by-site basis to justify the extra space
  • 35. ROADSIDE DESIGN GUIDE PAGE 35 03 CONCEPT AND CALCULATION OF CLEAR ZONE FIRST EDITION -DECEMBER 2016 3.4.3 Modification for the Outside of Curves The base clear zone width (Bcw) values in Section 3.4.2 assume a tangent alignment. However, horizontal curves may increase the angle of departure from the roadway and thus increase the distance the vehicle will need to recover. The designer should adjust the tangent values to provide wider clear zones on the outside of horizontal curves. Table 3.3 provides recommended modification factors for the outside of curves (Mc). A value of Mc=1.0 should be used for straight sections and insides of curves, as a modification is not necessary for these. 3.4.4 Clear Zone on Combination of Slopes 3.4.4.1 Variable Slopes Sometimes a combination of two different side slopes may be used on a roadside, as shown in Figure 3.5. A common application is a relatively flat recovery area immediately adjacent to the roadway followed by a steeper foreslope. This type of application, often called as ‘barn roof’, can provide the flat recovery area by using less material and space than a single continuous slope would require. Figure 3.5 – Variable Slope If the clear zone width required for the first slope does not reach the second one, no extra consideration is necessary, as an errant vehicle would stop before reaching the second slope. If the clear zone width required for the first slope extends beyond the first and overlaps into the second slope, the clear zone width for the steeper slope should be used. Table 3.3 – Modification factor for the outside of curves, Mc [1] Radius (m) Design Speed km/h 60 70 80 90 100 110 120 900 1.1 1.1 1.1 1.2 1.2 1.2 1.3 700 1.1 1.1 1.2 1.2 1.2 1.3 - 600 1.1 1.2 1.2 1.2 1.3 1.4 - 500 1.1 1.2 1.2 1.3 1.3 1.4 - 400 1.2 1.2 1.3 1.3 1.4 - - 300 1.2 1.3 1.4 1.5 1.5 - - 200 1.3 1.4 1.5 - - - - 100 1.5 - - - - - -
  • 36. ROADSIDE DESIGN GUIDE PAGE 36 03 CONCEPT AND CALCULATION OF CLEAR ZONE FIRST EDITION -DECEMBER 2016 3.4.4.2 Drainage Channels Drainage channels are typically variable slopes with at least one foreslope and one backslope, as shown in Figure 3.6. Drainage channels, depending on their geometry, can become hazards by themselves. This can be checked from Figure 4.8 and Figure 4.9 in Chapter 4. Drainage channels, which are not considered to be hazards, should be treated as variable slopes, and the clear zone should be calculated accordingly, as explained in Section 3.4.4.1. 3.4.5 High Risk Hazards Figure 3.6 – Drainage channel High risk hazards are the ones where people other than the vehicle occupants (third parties) may be harmed if reached by an errant vehicle. Chapter 4, Section 4.4 presents more detail into the identification of some of the more common high risk hazards. Following are some of the examples to high risk hazards where 3rd parties may be harmed: • Adjacent roads and carriageways (Section 4.4.1) • Storage of hazardous material (Section 4.4.2) • Places of frequent pedestrian activity / places of public gathering (Section 4.4.3) • Cycle Lanes (Section 4.4.4) • Structures at Risk of Collapse (Section 4.4.5) • Rail Lines (Section 4.4.6) As explained in Sections 3.2 & 3.3.4, the recommended clear zone distances in Table 3.2 are expected to provide enough space for approximately %85 of the errant vehicles to stop or regain control. However, roughly %15 of errant vehicles may still travel further than the recommended clear zone distance. It is possible to ensure a safe stop for a higher percentage of errant vehicles by providing even wider clear zones. However, for regular roadside hazards, the costs of providing the extra space may not always be justified by the benefits gained. However, in the case of 3rd party hazards, the consequences can be so high, that even a 15% probability of an errant vehicle reaching the hazard may result in an unacceptable level of risk. Therefore, if a high risk hazard is located in the vicinity of the travelled way, the designer/engineer should check for distances which are even further than the clear zone width recommended in Table 3.2. Based on the information presented in Figure 3.1, it is recommended that the risk posed by any 3rd party hazard located up to 30m to the travelled way should be assessed using engineering judgment. If the assessment suggests that the likelihood of a vehicle reaching the 3rd party hazard is too high, then the hazard should be removed, relocated to a further distance or
  • 37. ROADSIDE DESIGN GUIDE PAGE 37 03 CONCEPT AND CALCULATION OF CLEAR ZONE FIRST EDITION -DECEMBER 2016 shielded by an appropriate barrier. Considering the high consequences of the hazard, a high containment level barrier should be used (see Chapter 7). 3.4.6 Other Considerations 3.4.6.1 The Use of Engineering Judgement As explained in the previous sections, the recommended clear zone widths presented in this chapter are based on empirical research data, which was later extrapolated to fill in the gaps in the database. Therefore, the clear zone distances recommended in this chapter should only be taken as guidance, rather than definitive values. The designer/engineer should assess each site on a case-by-case basis applying engineering judgment and a justification should be provided for the final decision. 3.4.6.2 Departures from Standards A Departure or Departure from Standard can be described as a non-compliance with a Mandatory Requirement of a Declared Standard. The clear zone distances presented in this chapter are given as reference points rather than definitive values. However, the designer/engineer should aim to provide at least the minimum recommended clear zone width for each site. Where special circumstances arise and the straightforward application of the technical requirements cannot be achieved or justified for some reason, such as the environmental impact, exceptional layout situations or cost, users are encouraged to come forward with Departures or to propose additional criteria (for aspects not covered by existing documents) based on a reasoned assessment. Full justification for the grounds of the proposed departure must be forwarded to Abu Dhabi Department of Municipal Affairs and Transport at an early stage in design. The departure must demonstrate that the risk level of the proposed solution is as low as reasonably practicable. Formal approval of the proposed Departure must be received before incorporation into the design and the commencement of construction. Departures from Standard are determined on an individual basis and a decision regarding a Departure for one location must not be assumed to apply to any other site, even for similar situations. 3.5 Example Clear Zone Distance Calculations 3.5.1 Example 1 – Simple Recoverable Side Slope Calculate the clear zone and evaluate the location of the electrical box. Road Type: Multilane Divided ADT: 10,000 per direction Design Speed: 120km/h Horizontal Alignment: Straight
  • 38. ROADSIDE DESIGN GUIDE PAGE 38 03 CONCEPT AND CALCULATION OF CLEAR ZONE FIRST EDITION -DECEMBER 2016 Solution: Step 1 - Determine Base Clear Zone Width (Bcw) Using Table 3.2 for: • A foreslope of 1V:10H (i.e. 1V:6H or flatter) • Design Speed: 120km/h • ADT: 10,000 (i.e. over 6,000) Bcw = 11.0 to 12.5m Step 2 – Modifier for Outside of Curves • This is a straight section of road; a curve modification is not necessary. Mc = 1.0 Step 3 – Calculate the Clear Zone Distance (Cz) Cz = Bcw x Mc Cz = 11.0 to 12.5m Step 4 – Comment on the location of the electrical box As shown in the figure below, the electrical box lies within the minimum recommended clear zone distance of 11.0m from the edge of the travelled way. The following treatment options should be evaluated: • Remove the electrical box • Relocate the electrical box beyond the 11.0 to 12.5m clear zone • Use a passively safe electrical box • Shield the electrical box with a barrier • Delineate the electrical box, for example with reflective material.
  • 39. ROADSIDE DESIGN GUIDE PAGE 39 03 CONCEPT AND CALCULATION OF CLEAR ZONE FIRST EDITION -DECEMBER 2016 3.5.2 Example 2 – Side Slope on the Outside of a Curve Calculate the clear zone and evaluate the location of the tree. Road Type: Two Lane Undivided ADT: 5,000 for both directions Design Speed: 60km/h Radius of Horizontal Curvature: 900m Solution: Step 1 - Determine Base Clear Zone Width (Bcw) Using Table 3.2 for: • A foreslope of 1V:4H.(1V:6H to 1V:4H) • Design speed: 60km/h • ADT: 5,000 (1,500 – 6,000) Bcw = 4.5 to 5.0m Step 2 – Determine Modifier for Outside of Curves (Mc) Using Table 3.3 for: • Radius: 900m • Design speed: 60 km/h, Mc = 1.1 Step 3– Calculate the Clear Zone Distance (Cz) Cz = Bcw x Mc Cz = (4.5 x 1.1) to (5.0 x 1.1) Cz = 4.95m to 5.5m
  • 40. ROADSIDE DESIGN GUIDE PAGE 40 03 CONCEPT AND CALCULATION OF CLEAR ZONE FIRST EDITION -DECEMBER 2016 Step 4 – Comment on the location of the tree • As shown in the figure below, the tree lies outside both minimum and maximum recommended clear zone distance. Therefore no action is required. 3.5.3 Example 3 – Variable Side Slope Calculate the clear zone for the variable side slope. Road Type: Multilane Divided ADT: 8,000 per direction Design Speed: 100km/h Horizontal Alignment: Straight Surface Condition of the Side Slope: Firm Compacted Soil Solution: Step 1 - Determine if the Non-Recoverable Side Slope (1V:3.5H) is a Hazard by Itself: The surface of the side slope is firm and compacted, therefore, it should be considered as a non- recoverable slope rather than a hazard by itself (see Section 3.3.3.1) Step 2 - Determine Base Clear Zone Width (Bcw) for each side slope Using Table 3.2 for both side slopes: • For foreslope 1V:10H Bcw = 9.0 to 10.0m • For foreslope 1V:8H, Bcw = 9.0 to 10.0m • Foreslope 1V:3.5H is a non-recoverable slope. An errant vehicle would continue until the bottom of the slope.
  • 41. ROADSIDE DESIGN GUIDE PAGE 41 03 CONCEPT AND CALCULATION OF CLEAR ZONE FIRST EDITION -DECEMBER 2016 Step 3 – Modifier for Outside of Curves • This is a straight section of road; a curve modification is not necessary. Mc = 1.0 Step 4 – Calculate the Clear Zone Distance (Cz) • For side slope 1V:10H, Cz = (9.0 x 1.0) to (10.0 x 1.0) = 9.0 to 10.0m • For side slope 1V:8H, Cz = (9.0 x 1.0) to (10.0 x 1.0) = 9.0 to 10.0m • Because foreslopes 1V:10H & 1V:8H both require the same clear zone width of 9.0 to 10.0m, there is no need to prioritise one distance over the other. If different clear zone distances were required for the two side slopes, the one with the longest width should have been selected as the deciding distance. • The non-recoverable foreslope 1V:3.5H falls within the recommended clear zone distance of the 1V:10H foreslope. Because an errant vehicle would continue travelling until it reaches the bottom, the clear zone distance cannot logically end on a non-recoverable slope. In such cases, an additional clear zone space is needed at the bottom of the non- recoverable slope. This additional clear zone is referred to as the Clear Runout Area. The width of the Clear Runout Area should be equal to that portion of the clear zone distance that is located on the non-recoverable slope. In this example, the clear zone distance based on the recoverable slope (9.0 to 10.0 meters) overlaps on to the non-recoverable slope by 2.0 to 3.0 meters. Therefore, a Clear Runout Area of 2 to 3m is added from the bottom of the non-recoverable slope, as shown in the figure below. • Alternatively, the full width of the non-recoverable slope (3.0m) can be added to the suggested clear zone distance based on recoverable slope (9.0 to 10.0m). Therefore, for the combination of all three slopes; Cz = 9.0+3.0 = 12.0m to 10.0+3.0 = 13.0m. As shown in the figure below, this would give the same result, as the previous method.
  • 42. ROADSIDE DESIGN GUIDE PAGE 42 03 CONCEPT AND CALCULATION OF CLEAR ZONE FIRST EDITION -DECEMBER 2016 3.5.4 Example 4 – High Risk Hazard Calculate the clear zone and evaluate the location of the high speed railway. Road Type: Multilane Divided Design ADT: 25,000 per direction Design Speed: 120km/h Horizontal Alignment: Straight Solution: Step 1 - Determine Base Clear Zone Width (Bcw) Using Table 3.2 for: • A foreslope of 1V:8H.(1V:6H or flatter) • Design speed: 120km/h • ADT: 25,000 (Over 6,000) Bcw = 11.0 to 12.5m Step 2 – Modifier for Outside of Curves This is a straight section of road; a curve modification is not necessary. Mc = 1.0 Step 3 – Calculate the Clear Zone Distance (Cz) Cz = Bcw x Mc Cz = (11.0 x 1.0) to (12.5 x 1.0)
  • 43. ROADSIDE DESIGN GUIDE PAGE 43 03 CONCEPT AND CALCULATION OF CLEAR ZONE FIRST EDITION -DECEMBER 2016 Cz = 11.0 to 12.5m Step 4 – Check for High Risk Hazards • There is a railway line located 3.0m further away from the minimum recommended clear zone and 1.5m further away from the maximum recommended clear zone. Step 5 – Comment on the location of the railway line As shown in the figure above, the railway line does not lie within the normally recommended clear zone distances. However, as explained in Section 3.2, a small percentage of errant vehicles may still travel further than the recommended clear zone distance. Considering the high consequences of an errant vehicle reaching a high-speed railway line, and the relatively close distance of the railway line to the minimum required clear zone width (3.0m), it can be said that the level of risk posed by the current layout is unacceptable. Designer/ Engineer should evaluate the following treatment options: • Remove the railway line • Relocate the railway line further away from the road • Shield the railway line with an appropriate roadside barrier 3.6 References [1] AASHTO, Roadside Design Guide, 4th Edition, Washington D.C.: American Association of State Highway and Transportation Officials, 2011. [2] J. Graham and D. Hardwood, “NCHRP Report 247: Effectiveness of Clear Recovery Zones,” NCHRP, Transportation Research Board, Washington D.C., 1982. [3] AASHTO, Guide for Selecting, Locating and Designing Traffic Barriers., Washington D.C.: American Association of State Highway and Transportation Officials, 1977. [4] CEDR, “SAVERS (Selection of Appropriate Vehicle Restraint Systems) - WP1: Defining the Different Parameters which can Influence the Need and Selection of VRS (Unpublished Report),” Conference of European Directors of Roads, 2014. [5] P. Cooper, “Analysis of Roadside Encroachments - Single Vehicle Run-off-road Accident Data Analysis for Five Provinces,” B.C. Research, Vancouver, British Columbia, Canada,
  • 44. ROADSIDE DESIGN GUIDE PAGE 44 03 CONCEPT AND CALCULATION OF CLEAR ZONE FIRST EDITION -DECEMBER 2016 March 1980. [6] M. King and D. Sicking, “NCHRP 492 - Roadside Safety Analysis Program Engineer's Manual,” Transportation Research Board, Washington, D.C., 2003. [7] Norwegian Public Roads Administration, “Manual 231E, Vehicle Restraint Systems and Roadside Areas,” Norwegian Directorate of Public Roads, Oslo, 2011. [8] FGSV, Traffic Management Work Group, “Guidlines for passive protection on roads by vehicle restraint systems,” FGSV Verlag GmbH, Koln, 2009.
  • 45. ROADSIDE DESIGN GUIDE PAGE 45 04 IDENTIFICATION OF HAZARDS FIRST EDITION -DECEMBER 2016 4 IDENTIFICATION OF HAZARDS 4.1 Introduction A hazard is a roadside feature or object that can cause physical, economic, time-based or strategic harm or loss, if reached by an errant vehicle (see Section 2.1). Identification of roadside hazards is an integral part of the risk mitigation process. Hazards that lie within the recommended clear zone area (see Chapter 3) are under the risk of being reached by errant vehicles, which may lead to injury or even fatal accidents. Therefore, it is very important to identify and evaluate all hazards within the clear zone, so that suitable countermeasures can be applied. However, it may not always be clear to the engineer/designer, whether a specific roadside feature should be considered a hazard. For example, a roadside ditch may be considered a hazard or a traversable roadside feature, depending on its geometry. A tree may be considered a hazard, depending on the diameter of its trunk. This chapter provides guidance to designer/engineers in the identification of roadside hazards. The chapter presents a general overview of the types and properties of roadside features that may be considered as hazards. Each type of feature is presented with local photos and physical properties to help designers determine if the feature should be considered a hazard. Information presented in this chapter does not cover every single possible roadside hazard and therefore should be taken as a general guidance, rather than a definitive check-list. Engineering judgment should be used to assess possible hazards on a case by case basis, prior to a final decision. 4.2 Overview of Roadside Hazards Table 4.1 presents an overview of the common roadside features that may be considered as a hazard. As can be seen from the table, roadside hazards can be grouped into two main categories: • Hazards with consequences to vehicle occupants; • Hazards with consequences to vehicle occupants and third parties. The first group of hazards are the roadside objects and terrain features, which may cause harm or loss to the occupants of the errant vehicles. The second group of hazards are the roadside features and areas, which may also have consequences for third parties. The term “third parties” refers to a group or collection of people in a public space, such as school, hospital or railway that might be injured in numbers by an errant vehicle or by a hazard that is hit by an errant vehicle or a high value asset or facility that might be adversely affected by such an event. These hazards are often referred to as “high risk hazards”. A more detailed description for each of the hazards is presented in the following sections.
  • 46. ROADSIDE DESIGN GUIDE PAGE 46 04 IDENTIFICATION OF HAZARDS FIRST EDITION -DECEMBER 2016 Table 4.1 – Guidance towards potential roadside hazards [1] Type of Hazard Examples of Parameters to Evaluate A - Hazards with Consequences to Vehicle Occupants Roadside Topography Foreslopes (Embankments) Backslopes (Cutting Slopes) Gradient Height Compaction Surface Condition Ditches Ditch Type Gradient of Foreslope Gradient of Backslope Bottom Width Compaction Surface Condition Transverse Slopes Gradient Road Type Design Speed Compaction Surface Condition Edges of Bridges & Retaining Walls Drop Height Non-deformable Single Objects Trees Trunk Diameter Stump Height Overhead Gantries and Cantilevers Passively Safe or Not Roadside Sign Supports Support Structure Diameter & Thickness of Support Posts Passively Safe or Not CCTV Masts and Luminaire Supports Passively Safe or Not Concrete Foundations Protruding from the Ground Height from the Ground Bridge Piers, abutments, tunnel portals Always considered as a hazard Bridge Railing Ends & Ends of Concrete Barriers Always considered as a hazard Above Ground Equipment Passively Safe or Not Drainage Pipes & Culverts Level with Ground Profile? Pipe Diameter Traversable or not? Non-deformable Continuous Objects Non-deformable extensive obstacles parallel to direction of travel Passively Safe or Not Retaining walls Surface Roughness Noise barriers Barrier Incorporated or not? Fencing (Stone wall, wooden fence, concrete wall, etc.) Passively Safe or Not Roadside Barriers Is it Necessary or not? Permanent bodies or streams of water Water Depth Flooding Expected or not? B - Hazards with Consequences to 3rd Parties Adjacent Roads / Opposite Side of Dual Carriageways Volume of Adjacent Road Speed of Adjacent Road Locations of Hazardous Material Storage such as Chemical Plants Always considered as a hazard Structures at risk of collapse, support and load bearing Always considered as a hazard Places of public gathering and heavy pedestrian activity Average Number of People Volume of Pedestrian Traffic Average Time People are Exposed Heavily used bicycle paths Volume of Bicycle Traffic Structures at risk of collapse Always considered as a hazard Adjacent Rail lines Number of Trains per day Average Speed of Trains Area of environmental concern such as source of drinking water Always considered as a hazard
  • 47. ROADSIDE DESIGN GUIDE PAGE 47 04 IDENTIFICATION OF HAZARDS FIRST EDITION -DECEMBER 2016 4.3 Hazards with Risk to Vehicle Occupants 4.3.1 Roadside Topography 4.3.1.1 Fill Slopes / Embankments Fill slopes / embankments are the front slopes extending outward and downward from the shoulder or verge hinge point to intersect with the natural ground line. Consequences of reaching a fill slope for an errant vehicle may be more serious depending on height, gradient, surface condition and compactness of the slope. A steep embankment with a very high drop, such as the one shown in Figure 4.1, is an obvious hazard for errant motorists. However, for embankments with less steep side slopes and lower drops, it may not always be obvious if a foreslope should be considered as a hazard. Figure 4.1 – A hazardous embankment A foreslope of 1V:3H or steeper, such as the one shown in Figure 4.2, is considered as critical (see Section 3.3.3.1) and a hazard by many countries around the world (such as US [2], UK [3], Germany [4], etc.). This is because experience shows that the likelihood of a rollover increases significantly when the slope is steeper than the 1V:3H level. Figure 4.2 – Critical side slope by an interchange ramp