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   http://nap.edu/24799
In-Service Performance Evaluation of Guardrail End
Treatments
0 pages | 8.5 x 11 | PAPERBACK
ISBN 978-0-309-46020-0 | DOI 10.17226/24799
Committee on In-Service Performance of Energy-absorbing W-beam Guardrail End
Treatments; Policy Studies; Studies and Special Programs Division;
Transportation Research Board; National Academies of Sciences, Engineering, and
Medicine

Transportation Research Board
Special Report 323
In-Service Performance Evaluation
of Guardrail End Treatments
Prepublication Copy • Uncorrected Proofs
In-Service Performance Evaluation of Guardrail End Treatments
Copyright National Academy of Sciences. All rights reserved.

TRANSPORTATION RESEARCH BOARD
2017 EXECUTIVE COMMITTEE*
Chair: Malcolm Dougherty, Director, California Department of Transportation, Sacramento
Vice Chair: Katherine F. Turnbull, Executive Associate Director and Research Scientist, Texas A&M Transportation Institute, College Station
Executive Director: Neil J. Pedersen, Transportation Research Board
Victoria A. Arroyo, Executive Director, Georgetown Climate Center; Assistant Dean, Centers and Institutes; and Professor and Director,
Environmental Law Program, Georgetown University Law Center, Washington, D.C.
Scott E. Bennett, Director, Arkansas State Highway and Transportation Department, Little Rock
Jennifer Cohan, Secretary, Delaware Department of Transportation, Dover
James M. Crites, Executive Vice President of Operations, Dallas–Fort Worth International Airport, Texas (Past Chair, 2016)
Nathaniel P. Ford, Sr., Executive Director–CEO, Jacksonville Transportation Authority, Jacksonville, Florida
A. Stewart Fotheringham, Professor, School of Geographical Sciences and Urban Planning, Arizona State University, Tempe
John S. Halikowski, Director, Arizona Department of Transportation, Phoenix
Susan Hanson, Distinguished University Professor Emerita, Graduate School of Geography, Clark University, Worcester, Massachusetts
Steve Heminger, Executive Director, Metropolitan Transportation Commission, Oakland, California
Chris T. Hendrickson, Hamerschlag Professor of Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania
Jeffrey D. Holt, Managing Director, Power, Energy, and Infrastructure Group, BMO Capital Markets Corporation, New York
S. Jack Hu, Vice President for Research and J. Reid and Polly Anderson Professor of Manufacturing, University of Michigan, Ann Arbor
Roger B. Huff, President, HGLC, LLC, Farmington Hills, Michigan
Geraldine Knatz, Professor, Sol Price School of Public Policy, Viterbi School of Engineering, University of Southern California, Los Angeles
Melinda McGrath, Executive Director, Mississippi Department of Transportation, Jackson
Patrick K. McKenna, Director, Missouri Department of Transportation, Jefferson City
James P. Redeker, Commissioner, Connecticut Department of Transportation, Newington
Mark L. Rosenberg, Executive Director, The Task Force for Global Health, Inc., Decatur, Georgia
Daniel Sperling, Professor of Civil Engineering and Environmental Science and Policy; Director, Institute of Transportation Studies, University
of California, Davis (Past Chair, 2015)
Gary C. Thomas, President and Executive Director, Dallas Area Rapid Transit, Dallas, Texas
Pat Thomas, Senior Vice President of State Government Affairs, United Parcel Service, Washington, D.C.
James M. Tien, Distinguished Professor and Dean Emeritus, College of Engineering, University of Miami, Coral Gables, Florida
Dean H. Wise, Vice President of Network Strategy, Burlington Northern Santa Fe Railway, Fort Worth, Texas
Charles A. Zelle, Commissioner, Minnesota Department of Transportation, Saint Paul
Alberto Ayala, Deputy Executive Officer, California Air Resources Board, Sacramento (ex officio)
Mary R. Brooks, Professor Emerita, Dalhousie University, Halifax, Nova Scotia, Canada, and Chair, TRB Marine Board (ex officio)
Jack Danielson, Executive Director, National Highway Traffic Safety Administration, U.S. Department of Transportation (ex officio)
Audrey Farley, Executive Director, Office of the Assistant Secretary for Research and Technology, U.S. Department of Transportation (ex
officio)
LeRoy Gishi, Chief, Division of Transportation, Bureau of Indian Affairs, U.S. Department of the Interior, Washington, D.C. (ex officio)
John T. Gray II, Senior Vice President, Policy and Economics, Association of American Railroads, Washington, D.C. (ex officio)
Michael P. Huerta, Administrator, Federal Aviation Administration, U.S. Department of Transportation (ex officio)
Daphne Y. Jefferson, Deputy Administrator, Federal Motor Carrier Safety Administration, U.S. Department of Transportation (ex officio)
Bevan B. Kirley, Research Associate, University of North Carolina Highway Safety Research Center, Chapel Hill, and Chair, TRB Young
Members Council (ex officio)
Howard McMillan, Acting Administrator, Pipeline and Hazardous Materials Safety Administration, U.S. Department of Transportation (ex
officio)
Wayne Nastri, Acting Executive Officer, South Coast Air Quality Management District, Diamond Bar, California (ex officio)
Craig A. Rutland, U.S. Air Force Pavement Engineer, U.S. Air Force Civil Engineer Center, Tyndall Air Force Base, Florida (ex officio)
Reuben Sarkar, Deputy Assistant Secretary for Transportation, U.S. Department of Energy (ex officio)
Todd T. Semonite (Lieutenant General, U.S. Army), Chief of Engineers and Commanding General, U.S. Army Corps of Engineers, Washington,
D.C. (ex officio)
Karl Simon, Director, Transportation and Climate Division, U.S. Environmental Protection Agency (ex officio)
Joel Szabat, Executive Director, Maritime Administration, U.S. Department of Transportation (ex officio)
Walter C. Waidelich, Jr., Acting Deputy Administrator, Federal Highway Administration, U.S. Department of Transportation (ex officio)
Patrick T. Warren, Executive Director, Federal Railroad Administration, U.S. Department of Transportation (ex officio)
Matthew Welbes, Executive Director, Federal Transit Administration, U.S. Department of Transportation (ex officio)
Richard A. White, Acting President and CEO, American Public Transportation Association, Washington, D.C. (ex officio)
Frederick G. (Bud) Wright, Executive Director, American Association of State Highway and Transportation Officials, Washington, D.C. (ex
officio)
Paul F. Zukunft (Admiral, U.S. Coast Guard), Commandant, U.S. Coast Guard, U.S. Department of Homeland Security (ex officio)
* Membership as of May 2017.
I n - S e r v i c e P e r f o r m a n c e E v a l u
C o p y r i g h t N a t i o n a l A

In-Service Performance Evaluation
of Guardrail End Treatments
Committee for the Study of In-Service Performance of W-Beam Guardrail Treatments, Phase 1
Washington, D.C. 20001
www.TRB.org
2017
Special Report 323

Transportation Research Board Special Report 323
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I n - S e r v i c e P e r f o r m a n c e E v a l u a t i o n o f
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Committee for the Study of In-Service Performance of W-Beam
Guardrail End Treatments, Phase 1

Hugh W. McGee, Annandale, Virginia, Chair
Linda Ng Boyle, University of Washington, Seattle
James E. Bryden, Delmar, New York
Douglas J. Gabauer, Bucknell University, Lewisburg, Pennsylvania
Shauna Hallmark, Iowa State University, Ames
David Harkey, University of North Carolina, Chapel Hill
Douglas W. Harwood, MRIGlobal, Kansas City, Missouri
Thomas Hicks, Century Engineering, Inc., Hunt Valley, Maryland
Cing-Dao Kan, George Mason University, Fairfax, Virginia
Susan Martinovich, CH2M Hill, Carson City, Nevada
Priyaranjan Prasad, Plymouth, Michigan
C. Shane Reese, Brigham Young University, Provo, Utah
Omar Smadi, Iowa State University, Ames
Transportation Research Board Staff
Stephen Godwin, Director, Studies and Special Programs
Joseph R. Morris, Study Director

vii
Preface
he Transportation Research Board formed the Committee for Study of In-Service
Performance of W-Beam Guardrail End Treatments, Phase 1 to develop methods for in-
service evaluation of the performance of guardrail end treatments in crashes. Guardrail end
treatments are intended to reduce the risk of injury to vehicle occupants in a highway crash in
which the vehicle strikes the end of a length of guardrail. The performance of guardrail end
treatments and other roadside devices is evaluated by laboratory crash testing. The purpose of the
committee’s study was to aid highway agencies in supplementing crash testing with evaluation
of the devices in use on roads to ensure their safety.
The committee included members with expertise in highway safety research, research
design, statistics, highway safety programs, structural engineering, and simulation modeling. The
study was sponsored by the National Cooperative Highway Research Program.
The committee received presentations at its meetings from King Gee, American
Association of State Highway and Transportation Officials; Monique Evans, Federal Highway
Administration; Michael Griffith, Federal Highway Administration; Dean Sicking, University of
Alabama at Birmingham; Malcolm Ray, RoadSafe LLC; Brelend Gowan; Scott King, Kansas
Department of Transportation; Maria Ruppe, City of Columbus, Ohio; Kevin Lee, Nevada
Department of Transportation; and Mark Burkhead, Pennsylvania Department of Transportation.
In addition, the committee commissioned three resource papers from the following authors: Chad
Heimbecker and Eric Lohrey, Brian Wolshon and Anurag Pande, and Bhagwant Persaud. The
papers are discussed in Appendix A, where links to the papers are provided.
This report has been reviewed in draft form by individuals chosen for their diverse
perspectives and technical expertise, in accordance with procedures approved by the Report
Review Committee. The purpose of this independent review is to provide candid and critical
comments that assist the authors and the National Academies of Sciences, Engineering, and
Medicine in making the published report as sound as possible and to ensure that the report meets
institutional standards for objectivity, evidence, and responsiveness to the study charge. The
contents of the review comments and draft manuscript remain confidential to protect the integrity
of the deliberative process. The following individuals participated in the review of this report:
John Milton, Washington State Department of Transportation; Sandra Larson, Iowa Department
of Transportation; Keith Cota, New Hampshire Department of Transportation; Kathryn
Zimmerman, Applied Pavement Technology, Inc.; Sue McNeil, University of Delaware; H. Clay
Gabler, Virginia Polytechnic Institute and State University; Fred Mannering, University of South
Florida; Chandra Bhat, University of Texas at Austin; William Thomas Hollowell, WTH
Consulting LLC; and E. Dean Carlson, Carlson Associates. Although the reviewers provided
many constructive comments and suggestions, they were not asked to endorse the committee’s
conclusions or recommendations, nor did they see the final draft of the report before its release.
The review of this report was overseen by Susan Hanson, Clark University, and by Paul
Jovanis, Pennsylvania State University. They were responsible for making certain that an
independent examination of the report was carried out in accordance with institutional
procedures and that all review comments were carefully considered. Responsibility for the final
content of this report rests entirely with the authoring committee and the institution.
T

viii Special Report 323: In-Service Performance Evaluation of Guardrail End Treatments

Joseph R. Morris managed the study and drafted the report under the guidance of the
committee and the supervision of Stephen R. Godwin, Director, Studies and Special Programs.
Karen Febey managed the report review process. Janet McNaughton edited the report, and
Jennifer J. Weeks prepared the prepublication manuscript and background papers for web
posting, all under the supervision of Javy Awan, Director of Publications. Timothy Devlin and
Claudia Sauls assisted with meeting arrangements and communications with committee
members.

Contents
Summary.........................................................................................................................................1
1 Study Charge and Origin..........................................................................................................7
Committee Task...................................................................................................................8
Objectives of In-Service Evaluation..................................................................................10
Evaluation Methods ...........................................................................................................17
Organization of the Report.................................................................................................19
2 Methods of Measuring Performance......................................................................................21
Comparative Evaluations...................................................................................................21
Descriptive Evaluations .....................................................................................................41
3 Nationally Coordinated Evaluation Research.......................................................................45
Evaluation Objectives........................................................................................................45
Sources for Evaluation Procedures....................................................................................46
Validating Crash Test Procedures......................................................................................47
Evaluation Methods for Routine Highway Agency Use....................................................53
Impact of Design, Installation, and Maintenance Practices on Performance ....................55
Planning and Organization.................................................................................................57
Annex 3-1: Summary of Procedural Guides to In-Service Evaluation of
Roadside Safety Devices .............................................................................................63
4 Routine In-Service Evaluation of Roadside Devices.............................................................75
Objectives of Routine Evaluations.....................................................................................75
Obstacles to Evaluation......................................................................................................76
Strengthening Highway Agency Capacity to Conduct Evaluations ..................................76
5 Conclusions and Recommendation ........................................................................................83
Conclusions........................................................................................................................83
Recommendations..............................................................................................................87
Appendixes
A Commissioned Papers .......................................................................................................93
B Glossary and Illustrations of Guardrail End Treatments and
Other Roadside Safety Devices ...................................................................................95
Study Committee Biographical Information...........................................................................103

1
Summary
oadside safety devices are designed to reduce the risk of occupant injuries when a vehicle
runs off the road. New devices are evaluated by crash testing: the device is installed at a test
facility, a vehicle collides with it, and engineers assess the consequences of the crash. Guardrails
are roadside safety devices installed at locations that do not provide a clear zone in which a
vehicle can decelerate without striking an object or encountering unsafe terrain. The end of a
length of guardrail must be designed so that it is not a hazard to occupants of a vehicle striking it.
Highway agencies install end treatments in a variety of forms intended to absorb energy in a
crash and to redirect the vehicle into a safe trajectory.
The Transportation Research Board formed a committee with the sponsorship of the
National Cooperative Highway Research Program (NCHRP) to develop a research design for
evaluating the in-service performance of guardrail end treatments, determine the data required
for the analysis, examine state data systems to determine whether the required data would be
available, and identify next steps for carrying out evaluations. The committee’s conclusions
about the objectives of in-service evaluation of end treatments and other roadside safety devices
and about current capabilities and evaluation methods for conducting in-service evaluations are
summarized next, followed by recommendations for proceeding with evaluations.
CONCLUSIONS
Need for In-Service Evaluation
In-service evaluation can help to ensure that roadside safety devices effectively reduce the risk of
injuries and fatalities. Crash testing cannot reproduce the variety of characteristics of crash
dynamics, sites, and installations that affect crash outcomes. In-service data are necessary to
determine the frequencies of various crash, installation, and site characteristics so as to determine
the crash conditions that should be included in crash tests. In-service evaluation also is necessary
to verify that devices in the field perform as they do in testing. Periodic evaluation is needed
because vehicle, traffic, and road characteristics change over time and new device designs
frequently come into use. Validation and refinement of crash testing would be appropriate
objectives of a nationally coordinated program of evaluation research.
Highway Agency Readiness for Conducting Evaluations
Highway agencies are not prepared to commit resources to systematic in-service evaluation of
roadside safety devices. Reasons for not undertaking evaluations are the limitations of data
systems, lack of arrangements with police for timely receipt of crash reports, lack of funding and
staff, and lack of perceived benefit. Agencies are unlikely to invest in evaluation capabilities
without evidence that the results can be useful for guiding decisions on selection, maintenance,
and replacement of devices.
R

2 Special Report 323: In-Service Performance Evaluation of Guardrail End Treatments

Initial Steps Toward Conduct of Evaluations
Trials to demonstrate methods and applications are a necessary first step toward establishing
capabilities for in-service evaluations. More information about the benefits and practicality of
routine in-service evaluation is needed before new data collection and analysis programs can be
launched. Also, comprehensive comparative evaluation of the effects on injury risk of alternative
device designs and of installation and maintenance practices would have a greater chance of
success if it were approached in stages. In the initial stage, studies of modest scale would
develop data collection and modeling methods. The initial results would indicate whether the
potential benefits justified further research investment.
Evaluation of Roadside Safety Devices Other than Guardrail End Treatments
In-service evaluation is equally justified for all roadside safety devices (including barriers, crash
cushions, end treatments, and support structures). Uncertainty about the reliability of crash
testing as an indicator of performance in service applies equally to all such devices. Data systems
for evaluation are more likely to be cost-effective if they have applications for a range of needs.
Methods suitable for evaluating guardrail end treatments also would be applicable for evaluating
the performance of other roadside devices.
Evaluation Objectives
There is a need for the capability to conduct two kinds of in-service evaluation of roadside
devices, each with distinct objectives: (a) a nationally coordinated evaluation research program
to meet common needs of the states and (b) routine in-service evaluation of roadside devices by
state highway agencies. Objectives that would be appropriate within the scope of a nationally
coordinated program include the following:
 Validating and refining crash test procedures to improve the reliability of testing as an
indicator of the performance of the device in use;
 Demonstrating cost-effective methods for routine in-service evaluation by a highway
agency; and
 Evaluating how design, installation, maintenance, and deterioration in use affect
performance, to help highway agencies define cost-effective practices for selection, inspection,
and replacement.
Routine in-service evaluation by highway agencies could have at least three applications:
 Providing notice that a device type is not performing as expected,
 Long-term monitoring to identify cost-effective practices for device selection and
maintenance, and
 Ensuring that devices are appropriate for their locations and are properly installed and
maintained.
Because of the lack of highway agency experience in conducting evaluations, the practicality,
appropriate scale, and utility of such routine evaluation activities have yet to be determined.

Summary 3

Evaluation Methods
A program of in-service evaluation of roadside safety devices (whether a national program of
evaluation research or routine evaluation by a state highway agency) will have three essential
components:
 An administrative and planning structure that defines evaluation objectives, the scope
of evaluation, and responsibilities for evaluation and that oversees the application of results;
 General-purpose data systems for recording roadway and traffic characteristics,
maintenance activities, and crashes that are adequate to support the objectives of the evaluations;
and
 An evaluation methodology that specifies performance measures and methods for
computing them.
Past activities have laid the groundwork for a methodology, especially a 2003 NCHRP
report on evaluation, the 2015 investigations of end treatment crashes conducted by a federal–
state task force, and the Federal Highway Administration (FHWA) pilot end treatment evaluation
begun in 2016.
The experience of these past studies has shown that the principal technical obstacles to
evaluation are
 Obtaining prompt notification of crashes so that data may be obtained at the site,
 Obtaining notification of crashes not reported to police,
 Obtaining a sample of crashes large enough to reveal infrequent failures and to allow
inferences about determinants of severity, and
 Ensuring the reliability of data.
An evaluation methodology must include procedures for overcoming these obstacles.
The past studies will be especially useful models in future evaluations for their
definitions of required data elements and data coding methods and for their demonstrations of
notification and communication arrangements. The past studies have been less successful at
demonstrating statistical methods of controlling for confounding factors (i.e., the influence on
crash severity of factors other than the design and condition of the roadside device, such as road
and traffic characteristics), methods of selecting scientifically valid samples, and auditing data
quality.
RECOMMENDATIONS
Validation of Crash Test Procedures
The U.S. Department of Transportation (U.S. DOT) and the state departments of transportation
should cooperate in undertaking a research program to validate and refine crash testing of
guardrail end treatments and other roadside safety devices through in-service evaluation and
simulation modeling. Evaluation to validate or improve crash testing will require assembling a


database of crashes involving the device being evaluated. The database should have the
following properties:
 A sample of crashes that is representative of the population of crashes,
 A sample size that allows the frequency of rare events and of crash characteristics to
be estimated, and
 Information about the crash and the roadside device sufficient to compare the
circumstances of the crash with the characteristics of tests and the outcome with the outcomes of
test crashes.
The procedures of the FHWA pilot in-service evaluation of end treatments should be the
model for data collection. The pilot results may indicate opportunities for simplifying data
collection. The recommended evaluation research program should improve on two aspects of the
methods of the FHWA pilot:
 The research program should assemble a scientifically valid sample of crashes for
analysis.
 The program should apply quantitative performance measures, including the
severity distribution of crash outcomes.
The analysis for validating crash test procedures should have two components: comparison of
actual crash circumstances with circumstances in the tests and comparison of the outcomes of
actual crashes that match test circumstances with test outcomes.
The evaluation should estimate the frequency of each defined category of crash
conditions (e.g., crash conditions that correspond to prescribed test conditions) and the increase
in injury risk associated with each condition. If crashes outside the range of test conditions are
found to cause significant numbers of casualties, or if outcomes of actual crashes that match test
circumstances differ from test outcomes, then the research should identify changes in testing to
more accurately predict the performance of devices.
Assessment of Simulation Modeling
The crash test validation research program should include assessment of the usefulness of
simulation models in conjunction with crash testing to certify new device designs. In-service
evaluation findings and crash test results can support the specification and validation of
simulation models of vehicle collisions with roadside safety devices. The research should
examine the application of models to evaluate (a) device performance in crash circumstances
that would be impractical to test physically and (b) the effects of site details such as placement of
the device, slopes, and soil conditions.
Demonstration of Evaluation Methods for Routine Highway Agency Use
The state departments of transportation, or the U.S. DOT and states acting cooperatively, should
conduct a demonstration of methods suitable for highway agencies to use for routine in-service
evaluation of roadside devices. The demonstration should test methods and determine whether
evaluation can be useful for improving the safety and cost-effectiveness of highway programs.

Summary 5

The results would be used to revise guidance on evaluation and to develop training materials.
The nationally coordinated program would provide technical support and coordination.
Participating agencies would independently test methods with their own resources and assess the
utility of results. Participants would be agencies with maintenance management systems, and
data and analysis processes would be integrated with these existing systems.
Participating agencies would be encouraged to include trials of new technology for data
capture and trials of contractual innovations that give the manufacturer or installer responsibility
for providing inventory and performance data. The demonstration should include analysis of
agencies’ costs and staff time requirements for data collection and evaluation. Benefits should be
characterized in terms of the impact of information from the evaluation on agency decisions and
practices.
Evaluating Effects of Design, Installation, and Maintenance Practices on Performance
The U.S. DOT and the state departments of transportation should begin exploratory data analysis
toward development of a statistical modeling approach to measurement of the effects of device
design, installation, maintenance, and site characteristics on the performance of guardrail end
treatments and other roadside safety devices. The exploratory analysis should begin with the data
collected in the crash test validation study recommended above. The results could support
highway agency decisions regarding the selection of device types to be installed at particular
locations and the priorities for maintenance and replacement of the devices. The recommended
exploratory analysis would provide a basis for deciding the appropriate scale and direction of
future research on development and application of crash severity models.
Organization of the Nationally Coordinated Evaluation Research Program
The U.S. DOT and the states should consider at least two alternative organizational forms for
planning and oversight of the nationally coordinated evaluation research program:
 Extension of the charge and term of the American Association of State Highway and
Transportation Officials–Federal Highway Administration (AASHTO-FHWA) Task Force on
Guardrail Terminal Crash Analysis, as a means of coordinating state and federal interests and
resources and
 An AASHTO-led effort conducted through the National Cooperative Highway
Research Program.
The entity overseeing the evaluation program should first develop a plan that defines the
objectives of evaluations (i.e., how the results will be applied in the management of the highway
system), scope (e.g., which devices are to be evaluated), funding needs, and schedules. The
entity also will be responsible for obtaining cooperation of the federal, state, and local agencies
and offices that would be involved and for monitoring the conduct of evaluations and
applications of results.

7
1
Study Charge and Origin
oadside safety devices (including guardrails, break-away supports for signposts and
roadway lights, median barriers, crash cushions, and other devices) are designed to reduce
the risk of occupant injuries when a vehicle runs off the road. The safety of new devices is
evaluated by crash testing: the device is installed at a test facility; a vehicle collides with it, and
engineers assess the consequences of the crash according to criteria related to the risk of
occupant injury (e.g., whether the occupant compartment is deformed or penetrated). The
American Association of State Highway and Transportation Official’s (AASHTO’s) Manual for
Assessing Safety Hardware (MASH) (AASHTO 2009) defines methods for conducting crash
tests of roadside devices and roadside design features (Box 1-1) and the criteria for determining
whether the test results demonstrate that the device is crashworthy. On federal-aid highway
projects, the federal government will reimburse the state for federal share of the expense of
installing a roadside device only if the device has been shown to be crashworthy according to
MASH criteria (or according to earlier guidelines if the device was developed before the MASH
was published). A manufacturer who wishes to produce a new device arranges for the conduct of
the test at an independent testing facility and submits the results for review to the Federal
Highway Administration (FHWA). FHWA maintains lists of devices approved for federal aid
reimbursement (FHWA 2015).
Box 1-1
Safety Devices and Roadside Features with
Test Procedures Specified in the MASH
(AASHTO 2009, 4–5, 13–36)
 Longitudinal barriers
 Crash cushions
 Terminals (including guardrail end treatments)
 Support structures (including sign supports, mailbox supports, and luminaire
supports)
 Breakaway utility poles
 Work zone traffic control devices
 Truck- and trailer-mounted attenuators
 Variable message sign and arrow board trailers
 Work zones longitudinal channelizers
Roadside geometric features (including drainage structures, curbs, driveways, and
embankments)
R


Guardrails are installed at locations that do not provide a safe clear zone in which a
vehicle can decelerate without striking a fixed object or encountering unsafe terrain such as a
drop-off or precipice. The end of a length of guardrail must be designed so that it does not pose a
hazard to the occupants of a vehicle that strikes it. Highway agencies install end treatments of
various designs. The end treatment is intended to allow a vehicle striking it to maintain a safe
trajectory beyond the device, to redirect the vehicle into a safe trajectory, or (in the case of
energy-absorbing end treatment types) to bring the vehicle to a stop at a safe deceleration rate,
depending on the impact conditions. The upstream end of a length of guardrail, that is, the end
that a vehicle in the adjacent lane encounters first at the beginning of the length of guardrail, is
the end more likely to be struck head-on. The more complex end treatment types (such as energy
absorbing types) usually are installed only at the upstream end. Simpler end treatments are used
at the downstream end of the length of guardrail if the end treatment is positioned so that it is
unlikely to be struck by vehicles crossing the center of the road from the opposing direction.
Appendix B contains illustrations of guardrail end treatments and other roadside safety devices
and a glossary of terms relating to these.
COMMITTEE TASK
The Transportation Research Board formed a committee with the sponsorship of the National
Cooperative Highway Research Program (NCHRP). The committee’s task (Box 1-2), as
requested by NCHRP, is to
 Develop a research design for evaluating the in-service crash performance of
guardrail end treatments currently installed;
 Determine the data required to carry out the analysis and examine data systems in
selected states to determine whether the required data would be available; and
 Identify appropriate next steps, which may include new data collection for evaluation
or use of existing state data resources.
The statement of task specifies that the committee is to develop a method for evaluating
“the more common energy-absorbing W-beam guardrail end treatments currently installed.” The
committee anticipated that methods suitable for this specific category of devices also would be
applicable for evaluating the safety performance of other kinds of roadside safety devices. Also,
any special data collection or data systems established for evaluation purposes are more likely to
be cost-effective if the data have applications for a range of evaluation and management needs.
The statement of task is not specific about some of the considerations that will be most
important in selecting appropriate evaluation methods. It does not identify the objective of the
evaluation, that is, the decisions about highway safety practices that will be guided by the
evaluation results; it does not define a measure for performance; and it does not stipulate any
administrative requirements or constraints (e.g., the parties who are to perform the evaluation or
budget or duration constraints). To fill in these gaps in the statement of the problem, the
committee examined the context of the study request, in particular, the past difficulties highway

Study Charge and Origin 9

Box 1-2
In-Service Performance of Energy-Absorbing
W-Beam Guardrail End Treatments, Phase 1
Statement of Task
An ad hoc committee will develop an appropriate research design for evaluating the in-
service crash performance of the more common energy-absorbing W-beam guardrail end
treatments currently installed throughout the United States. The study will also determine
the data required to carry out the analysis and include exploratory work in selected states
to determine whether the required data, either retrospective or prospective, would be
available in sufficient quantity and quality to allow meaningful inferences to be drawn
about in-service performance of end treatments collectively and individually. On the
basis of the results from the exploratory work, the committee will either identify
appropriate next steps for gathering data or, should it appear unlikely that the necessary
data would become available, advise states on the existing process of conducting in-
service evaluations of guardrail end treatments.
agencies have encountered in performing in-service evaluations of road features and concerns
over the safety of certain guardrail end treatment designs. The committee also reviewed past
evaluation studies as sources of examples of objectives, methodologies, and important practical
constraints.
In this report, unless otherwise defined, the performance of a roadside safety device
means the distribution of severities of the outcomes of crashes—that is, the fraction of all crashes
that result in property damage only, minor injuries only, incapacitating injuries, and fatalities.
This definition applies to devices whose intended function is to mitigate crash outcomes. Some
road design features—for example, rumble strips—are intended to prevent collisions rather than
mitigate the consequences of collisions. For such features, the measure of performance would be
the difference in frequency of collisions at locations with and without the feature in place.
Collision frequency also is a relevant supplemental measure for the evaluation of end treatments
because differences in the design of two alternative types of end treatments—for example,
location of the installation with respect to the edge of the roadway—may affect collision
frequency. Cost-effectiveness is an additional performance measure important to highway
agencies: if two alternative types of devices are equally effective at reducing the risk of injury in
a collision, the highway agency will prefer the device with the lowest life-cycle cost.
The next section of this chapter describes objectives that have been proposed for in-
service evaluation of roadside devices. The subsequent section describes methods used in past
evaluations. The final section outlines the remainder of the report.


OBJECTIVES OF IN-SERVICE EVALUATION
The motivation for the committee’s study came from two sources: NCHRP’s recognition of the
need for in-service evaluation and the concern about the performance of a commonly used end
treatment type, the ET-Plus.
Need for In-Service Evaluation
NCHRP recognized that the states historically have not been successful in following the
guidance in the MASH (Box 1-3) and its predecessor documents regarding the need for in-
service evaluation. The MASH explains the need for in-service evaluation as follows (AASHTO
2009, 111):
Although the crash testing guidelines set forth in this report assure that safety
devices function well for the specified test conditions, there are many unknowns
and concerns about the impact performance of roadside features under real-world
conditions. Differences between field performance and crash test results can arise
due to many factors, including:
 Field impact conditions that are not included in crash test guidelines, such as
nontracking and side impacts;
 Site conditions, such as roadside slopes and ditches, that adversely affect
vehicle kinematics before, during, or after impact with the safety device; and
 Sensitivity to installation details, such as soil resistance or barrier flare
configuration.
Therefore, if necessary, conduct an in-service performance evaluation to assess
and monitor field performance of roadside safety features. In-service performance
evaluation allows user agencies to identify the overall impact performance of a
feature as well as identify potential weaknesses or problems with the design.
In spite of the advice in the MASH and its predecessor guidelines, the states have carried
out few in-service evaluations of roadside features, and most such studies have been hampered
by serious gaps in data. State databases of asset inventory, crashes, and traffic have not provided
adequate data to support evaluation of the relative safety performance of alternative roadside
device designs. States have not perceived malfunctioning roadside devices to be a safety problem
of sufficient magnitude to justify the cost of obtaining the data on crashes, devices, traffic, and
other road characteristics with the timeliness, accuracy, and detail required for evaluation
(Heimbecker and Lohrey 2016, 41–45; AASHTO 2009, 154–155; Ray et al. 2003, 3).1

1
Presentation of King Gee to the committee, April 14, 2015.


Box 1-3
In-Service Performance Evaluation Procedures Proposed in the MASH
(AASHTO 2009, 111–117)
The MASH presents a conceptual framework for in-service evaluation and refers to
NCHRP Report 490: In-Service Performance of Traffic Barriers (Ray et al. 2003), which
is described in Chapter 2, as a source for a detailed procedure. Two kinds of evaluation
are outlined: new feature evaluation and continuous monitoring.
The new feature evaluation would collect information on a limited number of devices of
a new design installed as a trial. The information to be collected would include the
following:
 Installation and maintenance checklist and inventory. The design type and location of
each device would be recorded, along with the procedures followed for installation and
subsequent maintenance of each device
 Crash monitoring. All crashes with the new devices in the test period would be
recorded. Information on reported crashes would come from police reports and
information on unreported crashes would come from maintenance records.
 Crash investigation. In-depth investigations of all fatal and serious injury crashes
would include site inspection, examination of the involved vehicle, and reconstruction of
the crash in terms of impact configuration and conditions.
The evaluation based on this data would include the following:
 Judgment of “whether the device performed as designed, and if not, whether there are
extenuating circumstances”;
 Identification of “problems identified with the construction, installation, and
maintenance of the device and subsequent remedies”; and
 “Recommended changes or modifications to the design and application of the
feature.”
The term of the evaluation is suggested to be a maximum of 3 to 5 years, and pooling of
efforts among several states is suggested to increase the number of devices and observed
crashes.
The continuous monitoring system would be a set of four spatially linked databases:
 Highway geometric and traffic data,
 Roadside feature inventory,
 Maintenance records for roadside features, including records of crashes not reported
to police, and
 Crash data from police reports, with indication of the roadside devices involved.


Concern Regarding the ET-Plus Guardrail End Treatment
The second motivation for the committee’s study was a controversy that arose in 2012 about a
particular guardrail end treatment, the ET-Plus, manufactured by Trinity Industries. The ET-Plus
has been installed in large numbers across the United States. Concerns have been raised that
occupants of vehicles in collisions with this device were at greater risk of serious injury than
occupants in collisions with alternative end treatment designs (Box 1-4). FHWA and the states
were faced with an immediate need to verify the safety of this device and the validity of their
crash-test-based evaluation procedures. A task force formed by FHWA and the state departments
of transportation conducted investigations of 161 serious crashes involving collisions with
extruding W-beam guardrail end treatments, including 131 involving the ET-Plus, 20 involving
other types, and 10 involving unidentified types (Joint AASHTO-FHWA Task Force 2015, 23).
Also, FHWA led an effort with several states to begin to collect detailed data on end treatment
crashes immediately after their occurrence as a step toward a more definitive evaluation of the
performance of these devices (FHWA n.d.).
Quantitative comparison of the safety performance of alternative types of end treatments
requires data on crashes for an extensive road network over an extended time period, because
crashes involving collisions with end treatments are infrequent events. In the period 2010–2015,
0.3 percent of U.S. motor vehicle occupant fatalities were in crashes in which collision with a
guardrail end treatment was recorded as the most harmful event (Table 1-1). Fatalities were more
frequent in crashes in which the most harmful event was a collision with a guardrail or other
longitudinal barrier or with a support structure, roadside devices for which the MASH also
specifies test procedures. Three percent of all 2010–2015 occupant fatalities were in crashes in
which a collision with one of the seven kinds of roadside devices listed in Box 1-1 was recorded
as the most harmful event (Table 1-1). (MASH does not specify a crash test for utility poles other
Box 1-4
The ET-Plus Guardrail End Treatment Controversy
In 2012, a controversy developed about the safety of a particular guardrail end treatment,
the ET-Plus, manufactured by Trinity Industries. As a result of media reports and
statements by a private party, highway officials in some states began to analyze crashes
involving collisions with that device that resulted in injuries and fatalities and raised their
concerns with FHWA. In 2014, a federal jury found Trinity Industries liable for damages
under the False Claims Act on the basis of allegations of failure to notify FHWA about a
change in the design of the device. Trinity has appealed the verdict.
The states recognized that these issues with a widely used roadside device (estimated
by FHWA to be installed in more than 200,000 locations) was a serious public safety
concern and could necessitate a costly replacement program. The case called into
question the system of crash testing that the states and FHWA have relied on to ensure
the safety of all roadside devices.
(continued)


Box 1-4 (continued)
AASHTO and FHWA formed a task force in 2014 to examine the safety of the ET-
Plus. FHWA published a Federal Register notice seeking information (Federal Register
2014):
The purpose of this notice is to request data and information regarding the ET-Plus
guardrail end terminal (ET-Plus) manufactured by Trinity Industries, Inc. (Trinity). In
2005, the FHWA determined that [the] ET-Plus guardrail end terminal met the relevant
crash test criteria and therefore was eligible for Federal-aid highway funding. This fall, a
jury issued a verdict that Trinity made a false or fraudulent claim to FHWA when it
sought the eligibility determination for the ET-Plus. Additionally, a number of parties
have raised concerns about the in-service performance of the ET-Plus and the potential
variability in the dimensions of installed units of the ET-Plus. As a result, FHWA is
undertaking a number of efforts to assess these issues. The FHWA is seeking technical
information and data to assist in this work.
In 2016, FHWA published the following summary of the work of the AASHTO-
FHWA Task Force (FHWA 2016):a
FHWA, in concert with the American Association of State Highway and Transportation
Officials (AASHTO) and the States, has acted on multiple fronts to assess the
performance of roadside safety hardware and, specifically, the ET-Plus guardrail end
terminal.
In particular, questions had been raised about whether the ET-Plus guardrail end
terminal satisfies applicable safety criteria and performs as intended in the field. FHWA
conducted a comprehensive evaluation of the safety performance of the ET-Plus to
answer these questions. . . .
FHWA and AASHTO formed two joint task forces to examine and address many of
these issues.
 The first joint task force examined measurements of more than 1,000 ET-Plus devices
installed throughout the country to reach two important conclusions:
– First, the devices tested are representative of the devices on the road.
– And, second, the measurements do not support allegations that Trinity
manufactured a second version of the 4-inch ET-Plus.
 The second joint task force, comprised of Federal and State safety experts, analyzed
the data collected by FHWA to determine whether there is any evidence of unique
performance limitations of the ET-Plus 4-inch guardrail terminal and the degree to which
any such performance limitations extend to other extruding W-beam guardrail terminals.
. . . FHWA has concluded that the device meets the appropriate NCHRP Report 350
criteria [Ross et al. 1993] and it continues to be eligible for Federal aid reimbursement.
_______________
a
The 4-inch ET-Plus is a design version of the ET-Plus end treatment. The ET-Plus was manufactured with
feeder channels that were either 4 inches or 5 inches wide. The feeder channel is a component of the head
assembly of the end treatment that aligns the end treatment with the guardrail behind it (Joint AASHTO-
FHWA Task Force 2015, 121.).


than “breakaway” poles. Breakaway poles are not in general use in the United States.) Collisions
with end treatments or with other roadside devices account for similarly small percentages of the
first harmful events in crashes leading to fatalities (Table 1-2) and of the most harmful events in
all police-reported crashes, as estimated by the U.S. DOT’s National Automotive Sampling
System–General Estimate System (NASS-GES) (Table 1-3).
TABLE 1-1 U.S. Motor Vehicle Occupant Fatalities in Crashes in Which Striking a
Roadside Object Was the Most Harmful Event, Selected Roadside Objects, 2010–2015
Year
End
Terminal Guardrail
Concrete
Barrier
Cable
Barrier
Bridge
Rail
Impact
Attenuator
Sign
Support
Utility
Pole/Light
Support Tree
All
Occupant
Fatalities
2010 71 436 154 21 80 11 104 1,019 3,602 27,889
2011 96 402 154 21 78 14 132 913 3,567 27,140
2012 92 407 176 27 61 22 97 1,013 3,687 28,003
2013 104 393 197 21 55 21 118 921 3,616 27,175
2014 110 372 203 17 82 21 127 957 3,508 26,901
2015 99 405 189 34 68 21 117 926 3,605 28,671
NOTE: End terminal fatalities are vehicle occupant fatalities in crashes in which the U.S. Department of Transportation’s
(U.S. DOT’s) Fatality Analysis Reporting System (FARS) identifies the most harmful event as striking an end terminal.
Fatalities involving the other struck objects shown in the table are defined analogously. Guardrail fatalities exclude end
terminal fatalities.
SOURCE: FARS (https://www.nhtsa.gov/research-data/fatality-analysis-reporting-system-fars).
TABLE 1-2 U.S. Motor Vehicle Occupant Fatalities in Crashes in Which Striking a
Roadside Object Was the First Harmful Event, Selected Roadside Objects, 2010–2015
Year End Terminal Guardrail
Concrete
Barrier
Cable
Barrier
Bridge
Rail
Impact
Attenuator
Sign
Support
2010 153 877 257 54 111 27 384
2011 182 800 256 48 105 23 433
2012 168 780 279 70 91 38 421
2013 195 774 290 66 81 34 402
2014 178 767 306 62 96 29 387
2015 184 799 283 79 106 37 445
NOTE: Some fatalities were in crashes in which FARS records the collision with the roadside object as being both
the first harmful event and the most harmful event. For example, in 2015, of the 99 occupant fatalities in which
collision with an end terminal was the most harmful event (Table 1.1), in 83 the end terminal collision was also the
first harmful event. Thus the number of fatalities in crashes in which collision with an end terminal was the first
harmful or most harmful event, or both, was 200.
SOURCE: FARS (https://www.nhtsa.gov/research-data/fatality-analysis-reporting-system-fars ).


TABLE 1-3 Numbers of Crashes in Which Striking a Roadside Object Was the Most
Harmful Event, Selected Roadside Objects, 2010–2015
Year
End
Terminal Guardrail
Concrete
Barrier
Cable
Barrier
Bridge
Rail
Impact
Attenuator
Sign
Support
All
Crashes
2010 12,100 84,900 67,100 1,000 7,800 2,200 43,700 5,419,000
2011 8,900 83,700 72,400 5,300 9,800 3,500 37,400 5,338,000
2012 11,200 83,200 68,200 5,800 8,300 3,000 43,100 5,615,000
2013 9,200 83,700 74,900 7,600 9,200 2,900 55,600 5,687,000
2014 10,400 92,100 67,200 10,900 6,800 2,800 48,800 6,064,000
2015 11,600 88,200 67,100 9,900 7,300 2,800 51,900 6,296,000
NOTE: NASS estimates of numbers of crashes are derived from a sample of all crashes in which a police accident
report was completed and which resulted in property damage, injury, or death.
SOURCE: NASS-GES (https://www.nhtsa.gov/research-data/national-automotive-sampling-system-nass).
FARS and NASS provide limited information relevant to assessing the performance of
roadside safety devices. If a device performs as intended (i.e., preventing or reducing the severity
of injuries in a crash in which the vehicle leaves the roadway), its involvement may not be
indicated as a harmful event, or the crash may not be severe enough to be included in the
database. Fatal and severe injury crashes in which collision with a roadside device is recorded as
the most harmful event may be instances of failure of the device to perform as intended, or may
be events beyond the design capabilities of the device (e.g., a collision at very high speed or a
collision of a tractor–semitrailer). However, crashes in which collision with a device is not coded
as a harmful event may also represent failure of the device. (A hypothetical example would be a
crash in which a vehicle struck another vehicle, then overrode a barrier and struck a tree. Each
NASS and FARS crash record contains a crash events table, a detailed sequence of events during
the crash that should indicate any contact with roadside devices.)
This background to the committee’s study suggests a need for capabilities to conduct two
kinds of evaluations with distinct objectives:
 A nationally coordinated evaluation research program to respond to special problems, such as
the concern about the adequacy of designs now in use, and to meet common needs of the
states and the federal government such as validation of crash testing, and
 The routine in-service evaluation of end treatments and other roadside devices that MASH
recommends for applications such as
– Monitoring the highway agency’s inventory of roadside devices to ensure proper
condition,
– Planning cost-effective maintenance and replacement, and
– Checking that newly adopted devices are performing as expected.


Two expert groups—the AASHTO-FHWA Task Force that investigated guardrail end
treatment crashes and the NCHRP project panel that prepared the project statement that was the
origin of the present study—recently identified a need for a research effort with national
applicability to evaluate the in-service performance of roadside devices. The report of the
AASHTO-FHWA Task Force included the following recommendation:
The Task Force recommends that comprehensive in-service performance
evaluations of guardrail terminals be conducted at the national and State levels.
As previously highlighted in this report, the Task Force’s assessment did not
involve a complete in-service evaluation and concentrated on a limited group of
mostly higher severity crashes, specifically focused on crashes with the ET-Plus
terminal. The findings of this report should be considered by the National
Academies’ National Research Council (NRC) committee that is conducting a
project entitled “In-Service Performance of Energy-Absorbing W-Beam Guardrail
End Treatments.” (Joint AASHTO-FHWA Task Force 2015, 8)
In the AASHTO-FHWA Task Force’s conception, a “complete in-service evaluation” would
 Be “a well-designed and executed research study to collect all of the necessary data
and account for exposure and confounding factors such as proper installation” (Joint AASHTO-
FHWA Task Force 2015, 12);
 “Provide relative comparisons of the in-service safety performance of individual
terminal types” and “an indication of the frequency of occurrence of the individual performance
limitations” (Joint AASHTO-FHWA Task Force 2015, 5); and
 “Determine whether crash-tested hardware [has] performance limitations that are not
detected by the testing process,” providing guidance for amending testing criteria (Joint
AASHTO-FHWA Task Force 2015, 8).
The NCHRP project statement (NCHRP 2013) describes the evaluation envisioned as a
research project to improve understanding of the performance of end treatments. The objectives
would be to
 Allow highway agencies to select the most cost-effective device types for new
construction,
 Allow agencies to maximize the cost-effectiveness of projects to upgrade end
treatments (i.e., projects to selectively replace existing end treatments with better-performing
devices), and
 Allow agencies to avoid installation and maintenance practices that degrade end
treatment performance.
The NCHRP project statement specified that the evaluation was to take into account the
sensitivity of the performance of each device type to environmental conditions, site
characteristics, impact conditions, and the quality of installation and maintenance of the device.
In summary, evaluation objectives that have been proposed and that would be within the
scope of a nationally coordinated program of evaluation research include the following:


 Comparison of the effectiveness of the various designs of guardrail end treatments
and other roadside safety devices now in use, taking into account factors aside from design that
may affect injury severity in a crash, such as roadway and roadside characteristics, vehicle type,
and crash dynamics. The results would aid highway agencies in selecting designs to be installed
at particular locations and in setting priorities for replacement of devices.
 Evaluation of the influence of installation and maintenance practices on device
performance.
 Validation and refinement of crash test procedures.
 Design of more effective roadside safety devices on the basis of the measured in-
service performance of existing designs.
Routine, state-level evaluation, as recommended by the MASH, could have the following
objectives:
 Assurance to the highway agency that newly adopted roadside device designs provide
the safety performance expected on the basis of crash testing results;
 Identification of local factors that affect performance (e.g., local installation and
maintenance practices or local roadside environment conditions);
 Evaluation of the long-term performance of roadside safety devices with respect to
safety and cost-effectiveness, as affected over time by maintenance and repair, normal
deterioration, and changes in vehicle characteristics; and
 Periodic evaluation of the agency’s inventory of roadside safety devices to ensure that
the devices installed are appropriate for their locations and that instances of improper installation
or maintenance that can compromise performance are minimized.
As noted above, the state highway agency interviews conducted for the committee, as well as
earlier studies, found that highway agencies lack resources for the conduct of routine in-service
evaluations. They would be unlikely to be willing to invest in developing evaluation capabilities
without clear evidence that the results of evaluations can be useful for guiding decisions on
selection, maintenance, and replacement of roadside safety devices.
EVALUATION METHODS
The committee reviewed past evaluations of guardrail end treatments, evaluations of other
highway risk mitigation measures, and analogous evaluation studies in other fields to learn about
the methods used and also about the motivations and objectives of the studies and the uses of
their results. Methods of evaluation studies can be grouped into three general categories: true
experiment, nonexperimental comparative evaluation, and descriptive evaluation.
1. True experiment. In an evaluation with an experimental study design, a pool of test
subjects is assembled and each subject is randomly assigned one of two or more alternative
treatments. For example, an experimental evaluation of a new type of some category of roadside
device would first select from the highway system a group of sites warranting installation of that
category of device. Either the new type or the current type would be randomly selected for
installation at each site, and measures of safety at the group of sites with the new design would


be compared with those at the group of control sites for a set period of time. Any difference in
safety outcomes between the two groups is then attributed to the difference in the performance of
the two types of devices because the two groups of test sites are expected to be similar with
respect to other characteristics affecting safety.
True experiments are almost never conducted in highway research—in part because
random assignment is logistically difficult in the highway construction environment—and are
not considered further in this report.
2. Nonexperimental comparative evaluation. In a nonexperimental study, the
treatments are not assigned randomly. Instead, statistical methods are used to control for factors
that may affect safety other than the treatments being evaluated. For example, a comparative
evaluation of two alternative types of roadside devices might test the hypothesis that a motorist
has the same risk of injury in a crash when traveling on a road segment where a roadside device
of type X has been installed as on a road segment where a device of type Y has been installed.
The evaluation would use data on the types of devices installed, traffic, and other characteristics
that affect safety on a collection of road segments to control statistically for other risk factors
(e.g., through multiple regression). An evaluation of this kind may be retrospective—that is, an
examination of historical experience that uses existing data that were not necessarily collected
for the purpose of the evaluation—or prospective—a study that uses data on crashes and on
factors related to crash and casualty risks collected according to a plan designed to meet the
requirements of the evaluation.
3. Descriptive evaluation. An evaluation of this kind consists of a program of regular
observation of the performance of the subject treatment for a period of time, after which a
qualitative judgment is made regarding the adequacy of the performance. The new feature
evaluation proposed in the MASH (Box 1-3) is an example of a descriptive or qualitative
evaluation: Trial installations of a new type of device are regularly monitored for a period of
time and all collisions involving the device type are investigated. At the end of the trial period,
the evaluators determine by judgment whether the new device type performs as intended. No
quantitative estimate is made of the effect of the device on casualty risk as compared with
alternative treatments.
For any particular evaluation need, the three methods will differ in cost, practicality, and
data resources required and in the questions that each is capable of answering. Nonexperimental
comparative research designs that seek causal relationships are the predominant method in
highway safety research. This method has been used to demonstrate the safety benefits of a wide
range of road design and vehicle design features and traffic control measures and the risks of
specific driver behaviors. However, the new feature evaluation outlined in the MASH and the
2016 FHWA end treatment pilot evaluation (described in Chapter 2) are descriptive evaluations,
and highway agencies probably have used this method more often than comparative evaluations.
Practical considerations will dictate the choice of evaluation methods that highway
agencies will be willing to adopt. The past studies summarized in Chapters 2 and 3 illustrate the
difficulties of assembling timely and reliable data on the occurrence, circumstances, and outcome
of crashes involving roadside safety devices; the locations, designs, and installation and
maintenance history of the devices; and traffic and roadway characteristics in the study area. The
scale and complexity of the study design for an evaluation should be consistent with the potential
value of the information the evaluation will produce. Undertaking an evaluation is reasonable
only if the expected safety benefits justify the cost and the likely diversion of resources from


other safety-related activities. Therefore the committee took care to identify practical challenges
and potential safety benefits in its comparison of alternative evaluation methods.
Merely specifying data collection and analysis techniques would be insufficient guidance
to highway agency administrators considering undertaking evaluation. A program of in-service
performance evaluation of road safety features (whether a national program of evaluation
research or routine evaluation by a state highway agency) will have three essential components:
 Administrative framework. This framework includes definition of the objectives of
evaluations and of how the agency will use the results, protocols determining which features are
to be evaluated and the schedule for evaluations, organizational arrangements for cooperation
among the agencies and offices that are to be involved (e.g., police agencies and highway agency
offices of maintenance, traffic and safety, and design), and funding provisions.
 Data systems and procedures. The content, reliability, and timeliness of general-
purpose databases recording roadway geometry, traffic, roadside features, maintenance activities,
and crashes must be adequate to support the objectives of the evaluations. Some evaluations may
require special data collection, but routine evaluation will be most cost-effective if the required
data collection and management are integrated with the agency’s safety management,
maintenance management, and asset management systems.
 Evaluation methodologies. How safety performance will be measured, including the
data elements required and the methods for analyzing the data, must be specified.
The statement of task refers to each of these components: the study is to propose a research
design and identify data needs, and also to advise the states on their evaluation processes. The
committee’s recommendations address each component.
ORGANIZATION OF THE REPORT
The remainder of this report is organized as follows:
 Chapter 2 describes the methods used in past in-service evaluations of guardrail end
treatments and other roadside safety devices.
 Chapter 3 proposes objectives and procedures for a nationally coordinated research
program for evaluation of roadside safety devices.
 Chapter 4 outlines methods for highway agencies to conduct routine in-service
evaluations of roadside safety devices.
 Chapter 5 presents the committee’s conclusions about the need for in-service
evaluation and recommendations regarding methods and procedures.
 Appendix A describes the papers commissioned by the committee. One paper surveys
highway agency practices regarding the selection, installation, and maintenance of end
treatments and other roadside safety devices. Two commissioned papers are literature reviews of
studies that provide examples of methods applicable to the evaluation of roadside safety devices.
 Appendix B contains a glossary and illustrations relating to the design, testing, and
use of guardrail end treatments and other roadside safety devices.


REFERENCES
Abbreviations
AASHTO American Association of State Highway and Transportation Officials
FHWA Federal Highway Administration
GAO U.S. Government Accountability Office
NCHRP National Cooperative Highway Research Program
AASHTO. 2009. Manual for Assessing Safety Hardware 2009. AASHTO, Washington, D.C.
Federal Register. 2014. ET-Plus Guardrail End Terminal. Vol. 79, No. 247, Dec. 24, p. 77595.
https://www.federalregister.gov/documents/2014/12/24/2014-30081/et-plus-guardrail-end-terminal.
FHWA. 2015. Guardrail Basics. https://www.fhwa.dot.gov/guardrailsafety/guardrailbasics.cfm.
FHWA. 2016. FHWA Review of ET-Plus. https://www.fhwa.dot.gov/guardrailsafety/.
FHWA. n.d. Data Collection: In-Service Performance Evaluation of Guardrail End Terminals.
Heimbecker, C., and E. C. Lohrey. 2016. Examples of State Highway Agency Practices Regarding
Design, Installation, Maintenance, and Evaluation of Guardrail End Treatments. Background Paper
for Special Report 323: In-Service Performance Evaluation of Guardrail End Treatments: Summary.
Transportation Research Board, Washington, D.C.
Joint AASHTO-FHWA Task Force on Guardrail Terminal Crash Analysis. 2015. Safety Analysis of
Extruding W-Beam Guardrail Terminal Crashes.
https://www.fhwa.dot.gov/guardrailsafety/safetyanalysis/.
NCHRP. 2013. NCHRP 22-30 [Pending] In-Service Performance Evaluation of W-Beam End Terminals.
http://apps.trb.org/cmsfeed/TRBNetProjectDisplay.asp?ProjectID=3669.
Ray, M. H., J. Weir, and J. Hopp. 2003. NCHRP Report 490: In-Service Performance Evaluation of
Traffic Barriers. Transportation Research Board of the National Academies, Washington, D.C.
Ross, H. E., Jr., D. L. Sicking; R. A. Zimmer; and J. D. Michie. 1993. NCHRP Report 350:
Recommended Procedures for the Safety Performance Evaluation of Highway Features.
Transportation Research Board of the National Academies, Washington, D.C.

21
2
Methods of Measuring Performance
hapter 1 identified three components essential to a program of in-service performance
evaluation: the administrative and planning framework that determines the scope and
objectives of evaluation, responsibilities for evaluation, and the applications of results; data
systems, which constitute the infrastructure of evaluation; and evaluation methods, that is,
specifications for analysis of crash and roadway data to measure performance. This chapter
presents examples of evaluation methods. The examples also illustrate challenges regarding
administration and planning of in-service evaluations.
Alternative methods of measuring performance are illustrated with examples from past
road safety evaluations. The alternatives include the categories identified in Chapter 1,
retrospective and prospective comparative evaluation and descriptive evaluation. Chapter 3
proposes applications of these generic methods in special evaluation research studies, and
Chapter 4 describes applications for routine evaluation needs of state highway agencies as part of
their safety management and asset management programs.
COMPARATIVE EVALUATIONS
The objective of a comparative evaluation of two alternative types of roadside safety features is
to determine which type provides greater protection (that is, the lower risk of injury or death) for
road users. The same evaluation methods may be applied to determine whether risk is reduced by
the use of a particular roadside safety treatment as compared with risk if no action is taken.
Statistical techniques are used to control for factors other than the treatments being evaluated that
may affect safety. The methods are standard practice in research on the safety effects of roadway
and vehicle design features and the benefits of safety interventions.
In the examples of methods of evaluation of guardrail end treatments given below, the
primary measure of performance usually is the distribution of the severity of crash outcomes (for
example, the fraction of all collisions with an end treatment that result in a severe injury or
fatality). The methods are applicable to evaluations of other kinds of roadside features, although
for some features, such as rumble strips or traffic roundabouts, the measure of performance will
be the frequency of collisions rather than (or in addition to) the consequences of collisions.
Prospective Evaluation
In a prospective study, procedures are put in place for identifying and documenting all crashes of
interest (e.g., collisions with guardrail end treatments) on specified sections of road within a
specified future time period. Such a study proceeds in the following steps:
1. Definition of the hypotheses concerning safety performance to be tested;
2. Selection of the roadways and future time period for which crashes are to be
observed;
3. Collection of collision data;
C


4. Assembly of a road features inventory, that is, data on the characteristics of the
features being evaluated and on other characteristics of the environment that may influence the
risk of crashes and casualties; and
5. Analysis by means of statistical methods to determine whether the observed
frequency and characteristics of crashes are consistent with the hypotheses. Meeting the study
objectives also may require comparison of costs of alternative safety treatments and engineering
assessment of the sources of poor performance.
These steps are described below. National Cooperative Highway Research Program (NCHRP)
Report 490, In-Service Performance Evaluation of Traffic Barriers (Ray et al. 2003) and a report
by the Texas Transportation Institute (TTI) for the Texas Department of Transportation (DOT)
(van Schalkwyk et al. 2004) propose detailed procedures for each of the steps. (Both reports are
summarized in the annex to Chapter 3.) Box 2-1 presents an example of a prospective
comparative evaluation of end treatments.
Box 2-1
Prospective Evaluation Example
Washington State Evaluation of Guardrail End Treatment
The Washington State Department of Transportation undertook an in-service evaluation
of guardrail end treatments in use in the state to help determine state policy regarding
replacement of terminals of older designs (Igharo et al. 2004). The project also served as
a demonstration of the methods developed in NCHRP Report 490 (Ray et al. 2003). The
same data collection and analysis effort also evaluated the performance of unrestrained
precast concrete barriers and illustrated that cost-effectiveness can be increased by
combining evaluations of multiple features.
Procedure
The steps in the end treatment evaluation were as follows:
1. Definition of hypotheses to be tested. Two hypotheses regarding end treatments were
tested statistically:
– The probability distribution of outcome severity, given that an end treatment is
struck, is independent of the device type.
– The probability of being struck is independent of the design type. (This can be
regarded as a test of whether the end treatment design installed at a location depended
on road characteristics.)
2. Selection of roadways and time period. Data were collected for 1 year for 752 miles
of state roads in three highway agency maintenance districts with 6 billion annual vehicle
miles of travel.
(continued)

Methods of Measuring Performance 23

Box 2-1 (continued)
3. Road features inventory. As the state had no inventory of end treatments, a survey of
the location and design type of end treatments on the study roads was conducted. The
survey recorded 2,318 end treatments of the six designs to be evaluated. Geometric and
traffic data for the study roads were obtained from existing state databases.
4. Collection of collision data. The department’s maintenance staff identified and
inspected the site of each end treatment collision on the study roads within several days
of the collision to determine device installation characteristics and damage to the device.
A standard data form was used. These maintenance reports were matched with police
accident reports. Collision scenarios were determined on the basis of the maintenance
staff inspections and police reports. Thirty collisions were identified, of which 20 had
accident reports. No crashes resulted in fatalities. Four crashes resulted in disabling
injuries, eight resulted in lesser injuries, and 18 resulted in property damage only.
5. Analysis. Of the six end treatment types in the road inventory data, two—the
breakaway cable terminal (BCT) and the slotted rail terminal (SRT)—experienced a
sufficient number of collisions to support statistical comparison of severity frequencies.
The number of collisions by severity for the two types was as follows:
Type Disabling Injury Other Injury Property Damage Only
BCT 3 4 11
SRT 1 2 6
A test of statistical significance of the difference in severity distribution between the
two types shows that the hypothesis (that the probability of crash outcome is independent
of end treatment type) cannot be rejected. The similarity of the distributions is illustrated
in Figure 1, which shows the percentage of crashes in each severity category for the two
types of devices.
FIGURE 1 Percentage of BRT and SRT crashes by severity of injury.
(continued)
0
20
40
60
80
incapacitating
injury
other injury property
damage only
percent of
crashes BCT
SRT


Box 2-1 (continued)
The analysis also tested the hypothesis that the probability of being struck is
independent of the design type. SRT terminals were struck four times more often than
BCT terminals per 100 million vehicles passing, as Figure 2 shows. The difference is
statistically significant. No information in the report explains this difference.
FIGURE 2 Guardrail end treatment collision rate,
by type of device and severity of injury.
Limitations of the Method
 The scale of the study, in terms of the time period and extent of the roads included,
allowed only 30 end treatment collisions to be observed. Most device types experienced
too few collisions to support conclusions about relative performance. Any failure modes
arising from flaws in design or installation of the devices probably would not have been
observed unless they occurred in a high percentage of collisions.
 The comparison of end treatment types with respect to crash severity does not
adequately take into account factors other than the design of the devices that may affect
severity. Road conditions and crash characteristics varied greatly: roads included
Interstates and minor low-volume roads; ramps were the locations of 3 percent of end
treatment installations but 16 of the 30 crashes observed. Such differences are likely to be
important in explaining severity distribution. The comparative analysis made no use of
the device inventory data collected, although the device inventory was used in a
descriptive analysis in the report.
 The study attempted to control for other factors using crash modification factors
(Igharo et al. 2004, 26–28). However, the factors calculated for the SRT and BCT device
types hardly differed from unity (1.008 and 1.012, respectively) and application of the
factors had no effect on the large difference in collision rates between the two device
types.
(continued)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
incapacitating
injury
other injury property
damage only
total
collisions per
100 million
vehicles passing
BCT
SRT


Box 2-1 (continued)
 The difference in collision frequency between the BCT and SRT terminals might arise
if the state’s policies favored installation of either BCT terminals on roads with relatively
low overall run-off-road crash rates or SRT terminals on ramps (where a large share of
collisions occurred) or if one end treatment type presented a larger cross section for
collisions because of differences in design or installation. Any of these circumstances
might invalidate the study’s statistical comparison of crash severity for the two types.
 As the study report noted, the rate of injury collisions per 100 million vehicles
passing was much lower than in other published studies (e.g., 10 percent of the rate
observed in a study that used Iowa data). The difference is further indication that all
factors that influence crash severity require full examination.
Step 1. Safety Performance Hypotheses
The essential first step of the evaluation is to define the hypotheses to be tested to ensure that the
necessary data are obtained and that the planned sample size will be adequate and to avoid effort
in collecting inessential information. In a comparative evaluation of end treatment types, the
hypotheses may include the following:
 For specified alternative roadside feature designs (e.g., two types of end treatment),
the distribution of outcome severity (e.g., the fraction of all collisions that result in a fatality or
serious injury), given that a feature is struck, is independent of the device type.
 The probability of being struck is independent of the design type. Testing this
hypothesis will serve as a check for systematic errors in data collection for the importance of
overlooked variables in the analysis. Evaluations of roadside devices have generally assumed
that the probability of being struck is independent of device type. If this assumption is sound, any
observed difference in the frequency of collisions (per vehicle passage) may arise from a
systematic difference in the characteristics of the roads on which each of the types compared is
installed. Conceivably, the design of some types of devices may render them more likely to be
struck than other types. For example, one type may have greater overall dimensions than others
or may require placement closer to the travel lane.
 The distribution of outcome severity is independent of the quality of installation, age,
and condition of the struck feature.
Comparison of the life-cycle cost or cost-effectiveness of alternative types of devices may be an
additional objective of the evaluation, in which case a hypothesis involving comparison of repair
costs would be included.
Step 2. Selection of Roads and Study Duration
Past evaluations of roadside features have used crash data from road networks ranging from a
segment of a single route (e.g., Bischoff and Battaglia 2007) to the entire state road system (e.g.,
Schrum 2014) or the systems of multiple states (e.g., Gabauer 2014) and have been conducted in


time periods typically ranging from 1 year to several years. Considerations regarding the
selection of the locations and the time period for crash data collection are as follows:
 Sample size requirements. The final section of this chapter discusses the
determination of the number of observations of crashes required to allow the hypotheses to be
tested. The number of observations obtained will depend on the extent of the roads included in
the study, the density of the roadside features of interest, traffic volume, and the frequency of
crashes per vehicle passing the features.
 Representativeness of the sample of roads and crashes. Confining the study to
roads with high traffic volumes will reduce the road mileage and time required to observe the
target number of crashes; however, the study results may not be generalizable to low-volume
roads, which generally differ from high-volume roads in geometric and operational
characteristics that affect crash severity.
 Presence of roadside features to be evaluated (e.g., end treatments of selected
design types). Similarly, selecting roads with a high density of the features being evaluated will
reduce mileage and time requirements, but the performance of the feature on these roads might
differ from the performance on other kinds of roads.
 Cooperation of the necessary participants. A state-conducted evaluation will need
to recruit the cooperation of road maintenance staff and contractors, police agencies, and,
possibly, local road agencies. Some past evaluations have been confined to selected highway
agency maintenance districts to simplify the involvement of maintenance workers. National
evaluation research studies will require the cooperation of multiple state highway agencies.
 Data availability. The timeliness and completeness of crash reports, roadside and
roadway inventory data, and maintenance reports may vary within a state or from state to state in
a national evaluation.
Past evaluations that used data on all crashes within a defined study area and time period have
thereby avoided the problems of statistical sampling; however, as noted above, selection of the
study area or roads included by judgment rather than by a sampling process might bias results.
Step 3. Collection of Collision Data
Past evaluations have relied on three sources of information about crashes:
 Police accident reports,
 Maintenance department reports of damage to roadside devices, and
 Special inspections of crash sites, including inspection of the struck roadside devices
involved.
Obtaining prompt, reliable reports of relevant crashes has been a difficult organizational
challenge in past in-service evaluations. The procedure for a prospective in-service evaluation
generally will call for inspection of crash sites as soon as possible after the event. The normal
procedure for entering police crash reports in state crash data systems may be too slow to provide
notification to the evaluators; in this case, special arrangements with police will be needed to
identify relevant crashes. Consistent notification of crashes and data quality assurance require


cooperative relationships between the agency staff responsible for the evaluation, police, and
maintenance staff.
Box 2-2 lists crash data elements that have been employed in past safety evaluations of
roadside devices or that may be relevant to such evaluations. The data elements required will
depend on the specific objectives of the evaluation. Box 2-3 shows the crash data elements that
the Federal Highway Administration (FHWA) judged to be relevant for its recent investigation of
the in-service performance of the ET-Plus end treatment design. Data collection forms in
NCHRP Report 490 (Ray et al. 2004), the TTI-proposed evaluation method for Texas (van
Schalkwyk et al. 2004), and the FHWA end treatment in-service evaluation pilot study (FHWA
n.d.) are described in the annex to Chapter 3.
Some past evaluations have restricted crashes included in the analysis to those in which
collision with the roadside device to be evaluated was the most harmful event (as judged by the
police officer completing the crash report form), reasoning that, in other crashes involving the
device, the outcome severity was determined by events other than collision with the device.
However, this restriction probably excludes some crashes that could provide information about
the performance of the device and introduces a subjective judgment in the selection of crashes.
Studies also have placed other restrictions on crashes included in the analysis, for example, the
exclusion of motorcycle and truck crashes.
Box 2-2
Crash-Related Data Elements Relevant to
Evaluation of Roadside Devices
Crash Scenario:
 Collision type (e.g., run-off-road)
 Sequence of events (including, but
not limited to, first harmful event
and most harmful event)
 Number and types of vehicles
involved
 Contributing factors (e.g., speeding)
 Driver characteristics (age, sobriety,
gender)
 Injury severity
 Number of occupants involved
 Number of injured occupants
 Restraint usage
 Severity of vehicle damage
Roadside Device Information:
 Characteristics of devices involved
(design type, installation details,
condition, age, maintenance history)
 Postcrash condition
 Repair cost
Road and Environmental Conditions:
 Road surface condition
 Roadway geometry
 Traffic characteristics
 Light condition
 Time of day
 Weather condition

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