Roundabouts Design and Safety Analysis

Module 2: Roundabouts
Geometric Design Assessment
Capacity Analysis
Level of Services
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TTE 422 Traffic Operations - Copyright © 2021 Wael ElDessouki Spring 2021

Roundabouts:
1-Geometric Design Safety
Assessment
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Basics of Roundabouts
TTE 422 Traffic Operations - Copyright © 2021 Wael ElDessouki
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Roundabouts: Background
 Circular Intersection Islands are the early versions of current
Modern Roundabouts . It dates back to early 1900.
Columbus Circle NYC, 1920’s
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 Circular Intersection Islands are the early versions of current
Modern Roundabouts . It dates back to early 1900.
Talat Harb Circle, Cairo, 1930’s
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 Rotaries are old-style circular intersections common to the United States
prior to the 1960’s. Rotaries are characterized by a large diameter,
often in excess of 100 m (300 ft). This large diameter typically results in
travel speeds within the circulatory roadway that exceed 50 km/h (30
mph). They typically provide little or no horizontal deflection of the paths
of through traffic and may even operate according to the traditional
“yield-to-the-right” rule, i.e., circulating traffic yields to entering traffic.
 Neighborhood traffic circles are typically built at the intersections of
local streets for reasons of traffic calming and/or aesthetics. The
intersection approaches may be uncontrolled or stop-controlled. They
do not typically include raised channelization to guide the approaching
driver onto the circulatory roadway. At some traffic circles, left-turning
movements can occur to the left of (clockwise around) the central
island, potentially conflicting with other circulating traffic.
 Roundabouts are circular intersections with specific design and traffic
control features. These features include yield control of all entering
traffic, channelized approaches, and appropriate geometric curvature
to ensure that travel speeds on the circulatory roadway are typically
less than 50 km/h (30 mph). Thus, roundabouts are a subset of a wide
range of circular intersection forms.
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 In mid 1950’s, Traffic Circles and Rotaries started to evolve.
Traffic Circles, Small Diameter, Low Speed, Urban Areas
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 In mid 1950’s, Traffic Circles and Rotaries started to evolve.
Highway Rotary, Large Diameter, High Speed
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 Evolution to modern roundabout
Kingston, NY
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 Evolution to modern roundabouts
Columbus Circle NYC, 2012
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 Evolution to modern roundabouts
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Roundabouts Benefits
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Roundabouts: Benefits
 Traffic Safety – Numerous studies have shown significant
safety improvements at intersections converted from
conventional forms to roundabouts. The physical shape of
roundabouts eliminates crossing conflicts that are present at
conventional intersections, thus reducing the total number of
potential conflict points and the most severe of those
conflict points.
 Operational Performance – When operating within their
capacity, roundabouts typically have lower overall delay
than signalized and all-way stop-controlled intersections. The
delay reduction is often most significant during non-peak
traffic periods. These performance benefits can often result
in reduced lane requirements between intersections
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Roundabouts: Benefits (cont.)
 Environmental Factors – Roundabouts often provide environmental
benefits by reducing vehicle delay and the number and duration of
stops compared with signalized or all-way stop-controlled
alternatives. Even when there are heavy volumes, vehicles continue
to advance slowly in moving queues rather than coming to a
complete stop.
 Access Management – Because roundabouts can facilitate U-turns,
they can be a key element of a comprehensive access
management strategy to reduce or eliminate left-turn movements
at driveways between major intersections.
 Traffic Calming – Roundabouts can have traffic calming effects on
streets by reducing vehicle speeds using geometric design rather
than relying solely on traffic control devices.
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 Pedestrian Safety – Due to the reduction of vehicle speeds in and
around the intersection, roundabouts can improve pedestrian crossing
opportunities. Additionally, the splitter island refuge area provides the
ability for pedestrians to focus on one traffic stream at a time while
crossing.
 Aesthetics – The central island and splitter islands offer the opportunity to
provide attractive entries or centerpieces to communities through use of
landscaping, monuments, and art, provided that they are appropriate
for the speed environment in which the roundabout is located.
 Land Use – Roundabouts can provide a transition area between high-
speed rural and low-speed urban environments. They can also be used
to demarcate commercial areas from residential areas.
 Ongoing Operations and Maintenance – A roundabout typically has
lower operating and maintenance costs than a traffic signal due to the
lack of technical hardware, signal timing equipment, and electricity
needs.TTE 422 Traffic Operations - Copyright © 2021 Wael ElDessouki
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 Approach Roadway Width – A roundabout may reduce the
amount of widening needed on the approach roadways in
comparison to alternative intersection forms. While signalized
or stop-controlled intersections can require adding lengthy
left-turn and/or right-turn lanes, a roundabout may enable
maintaining a narrower cross section in advance of the
intersection.
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Roundabouts Vs Traffic Signals
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Roundabouts: Comparison with Traffic Signals
Conflict Points for Vehicles
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Conflict Points with Pedestrians
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Crash Types
Typical 4-leg intersection
Angle Left turn
Roundabout
Sideswipe
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Cost of Operation & Maintenance:
 No signal equipment to install, power, and maintain
 May require less right-of-way
 Less pavement may be needed
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Other Advantages for Roundabouts:
 Aesthetically pleasing
 Quieter
 Functional
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Roundabouts Capacity
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Roundabouts Design: Capacity
 A basic question that needs to be answered at the
planning level is how many entering and circulating
lanes a roundabout would require to serve the
traffic demand.
 The number of lanes affects not only the capacity
of the roundabout, but also the size of the
roundabout footprint.
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 he roundabout, but also the size of the roundabout
footprint.
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Roundabouts:
Geometric Design Safety
Assessment
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Geometric Design Elements of
Modern Roundabout
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Roundabouts: Elements of Modern Roundabout
A modern roundabout has the following distinguishing
characteristics and design features:
 Channelized approaches;
 Yield control on all entries;
 Counterclockwise circulation of all vehicles around the
central island; and
 Appropriate geometric curvature to encourage slow
travel speeds through the intersection.
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Key Roundabout Characteristics
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Roundabout Design Features
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Roundabouts: Description of Design Features.
 Central island: The central island is the raised area in the center of a
roundabout around which traffic circulates.
 Splitter island: A splitter island is a raised or painted area on an approach
used to separate entering from exiting traffic, deflect and slow entering
traffic, and provide storage space for pedestrians crossing the road in two
stages.
 Circulatory roadway: The circulatory roadway is the curved path used by
vehicles to travel in a counterclockwise fashion around the central island
 Apron: If required on smaller roundabouts to accommodate the wheel
tracking of large vehicles, an apron is the mountable portion of the central
island adjacent to the circulatory roadway.
 Yield line: A yield line is a pavement marking used to mark the point of
entry from an approach into the circulatory roadway and is generally
marked along the inscribed circle. Entering vehicles must yield to any
circulating traffic coming from the left before crossing this line into the
circulatory roadway..
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 Accessible pedestrian crossings Accessible pedestrian crossings
should be provided at all roundabouts. The crossing location is set
back from the yield line, and the splitter island is cut to allow
pedestrians, wheelchairs, strollers, and bicycles to pass through.
 Bicycle treatments Bicycle treatments at roundabouts provide
bicyclists the option of traveling through the roundabout either as a
vehicle or as a pedestrian, depending on the bicyclist’s level of
comfort.
 Landscaping buffer Landscaping buffers are provided at most
roundabouts to separate vehicular and pedestrian traffic and to
encourage pedestrians to cross only at the designated crossing
locations. Landscaping buffers can also significantly improve the
aesthetics of the intersection.
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Roundabouts: Description of Design Features.

Roundabouts: Key Design Dimensions

Roundabouts: Key Design Dimensions.
 Inscribed circle diameter The inscribed circle diameter is the basic
parameter used to define the size of a roundabout. It is measured
between the outer edges of the circulatory roadway.
 Circulatory roadway width The circulatory roadway width defines the
roadway width for vehicle circulation around the central island. It is
measured as the width between the outer edge of this roadway and
the central island. It does not include the width of any mountable
apron, which is defined to be part of the central island.
 Approach width The approach width is the width of the roadway used
by approaching traffic upstream of any changes in width associated
with the roundabout. The approach width is typically no more than half
of the total width of the roadway.
 Departure width The departure width is the width of the roadway used
by departing traffic downstream of any changes in width associated
with the roundabout. The departure width is typically less than or equal
to half of the total width of the roadway.
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Roundabouts: Design Elements Comparison
 FHWA/NCHRP Informational Guide
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Roundabouts: Design Objectives
The Roundabout should be designed such that it will achieve
the following objectives:
1. Provide slow entry speeds and consistent speeds through the
roundabout by using deflection.
2. Provide the appropriate number of lanes and lane
assignment to achieve adequate capacity, lane volume
balance, and lane continuity.
3. Provide smooth channelization that is intuitive to drivers and
results in vehicles naturally using the intended lanes.
4. Provide adequate accommodation for the design vehicles.
5. Design to meet the needs of pedestrians and cyclists.
6. Provide appropriate sight distance and visibility for driver
recognition of the intersection and conflicting users.
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Roundabouts:
Design Elements Safety Checks
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Roundabouts Safety: Speed Check
 A well-designed roundabout reduces vehicle speeds upon entry
and achieves consistency in the relative speeds between
conflicting traffic streams by requiring vehicles to negotiate the
roundabout along a curved path.
 Careful attention to the design speed of a roundabout is
fundamental to attaining good safety performance.
 The recommended design speed of a roundabout is primarily a
function of the number of lanes rather than the design speed of
the intersecting roadways.
 The design speed of a roundabout is defined by the theoretical
speed that drivers could achieve through the roundabout if taking
the fastest path through the roundabout without regard to lane
line striping, if present.
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Roundabouts Safety: Speed Check (cont.)
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Roundabouts Design: Pathway Alignment
 With multilane roundabouts, the designer should
also consider the alignment of vehicles, or the
natural path, to ensure the proposed geometry
directs vehicles to stay within the proper lanes
through the circulatory roadway and exits.
 Path overlap occurs when the natural paths of
vehicles in adjacent lanes overlap or cross one
another.
 The entry design should align vehicles into the
appropriate lane within the circulatory roadway.
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Roundabouts Design: Pathway Alignment (cont.)
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Roundabouts Design: Design Vehicle Check
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Roundabouts Design: Design Vehicle Check (Cont.)
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Roundabouts Design: Size (Inscribed Diameter)
The inscribed circle diameter is determined by a
number of design objectives, including
accommodation of the design vehicle and
providing speed control, and it may require
iterative experimentation.
Once a sketch-level design concept has been
completed, the engineer is encouraged to look
critically at the design to identify whether the initial
assumed diameter produces a desired outcome
(e.g., acceptable speeds, adequately serving the
design vehicle, appropriate visibility for the central
island) or whether a larger or smaller diameter
would be beneficial.
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Roundabouts Design: Size (Inscribed Diameter) (cont.)
Recommended Inscribed Diameter for different configurations
and design vehicles
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Roundabouts Design: Splitter Island
Their purpose is:
 To provide refuge for pedestrians,
 assist in controlling speeds,
 guide traffic into the roundabout,
 physically separate entering and exiting traffic streams, and
 deter wrong-way movements.
 a place for mounting signs.
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Roundabouts Design: Pedestrian Design Treatment
 Wherever possible, sidewalks at roundabouts should be set
back from the edge of the circulatory roadway by a
landscape buffer. The buffer discourages pedestrians from
crossing to the central island or cutting across the circulatory
roadway of the roundabout, and it helps guide pedestrians
with vision impairments to the designated crosswalks.
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Roundabouts Design: Bicycle Design Treatment
 Bicycle lanes are not recommended within the circulatory
roadway of roundabouts, as it has been demonstrated
internationally to have adverse safety effects (see the
Roundabout Guide). Where bicycle lanes or shoulders are
used on approach roadways, they should be terminated in
advance of roundabouts.
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Roundabouts Design: Bicycle Design Treatment (cont.)
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Roundabouts Design: Bicycle Design Treatment (cont.)
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Roundabouts Design: Sight Distance & Visibility
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Roundabouts Design: Sight Distance & Visibility (cont.)
As can be seen in the exhibit, the distance between the
entering vehicle and the circulatory roadway is fixed.
The other legs of the sight distance “triangle” are based
on two conflicting approaches that are typically
checked independently:
1. Entering stream, comprised of vehicles from the
immediate upstream entry. The speed for this
movement can be approximated using the
average of the entering speed and circulating
speed.
2. Circulating stream, comprised of vehicles that
entered the roundabout prior to the immediate
upstream entry. This speed can be approximated
using the speed of left turning vehicles.
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Roundabouts Design: Sight Distance & Visibility (cont.)
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Roundabouts Design: Pavement Markings & Signs
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Roundabouts Design: Pavement Markings & Signs (cont.)
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Roundabouts Design and Safety Analysis

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