Development of Safety Performance-Based Guidelines for the Roadside Design Guide (2022)

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2022 N A T I O N A L C O O P E R A T I V E H I G H W A Y R E S E A R C H P R O G R A M NCHRP RESEARCH REPORT 972 Development of Safety Performance-Based Guidelines for the Roadside Design Guide Malcolm H. Ray Christine E. Carrigan Ethan M. Ray RoadSafe LLC Canton, ME Subscriber Categories Highways • Design • Safety and Human Factors Research sponsored by the American Association of State Highway and Transportation Ofcials in cooperation with the Federal Highway Administration

NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM Systematic, well-designed, and implementable research is the most effective way to solve many problems facing state departments of transportation (DOTs) administrators and engineers. Often, highway problems are of local or regional interest and can best be studied by state DOTs individually or in cooperation with their state universities and others. However, the accelerating growth of highway transporta- tion results in increasingly complex problems of wide interest to high- way authorities. These problems are best studied through a coordinated program of cooperative research. Recognizing this need, the leadership of the American Association of State Highway and Transportation Officials (AASHTO) in 1962 ini- tiated an objective national highway research program using modern scientific techniques—the National Cooperative Highway Research Program (NCHRP). NCHRP is supported on a continuing basis by funds from participating member states of AASHTO and receives the full cooperation and support of the Federal Highway Administration (FHWA), United States Department of Transportation, under Agree- ment No. 693JJ31950003. The Transportation Research Board (TRB) of the National Academies of Sciences, Engineering, and Medicine was requested by AASHTO to administer the research program because of TRB’s recognized objectivity and understanding of modern research practices. TRB is uniquely suited for this purpose for many reasons: TRB maintains an extensive com- mittee structure from which authorities on any highway transportation subject may be drawn; TRB possesses avenues of communications and cooperation with federal, state, and local governmental agencies, univer- sities, and industry; TRB’s relationship to the National Academies is an insurance of objectivity; and TRB maintains a full-time staff of special- ists in highway transportation matters to bring the findings of research directly to those in a position to use them. The program is developed on the basis of research needs iden- tified by chief administrators and other staff of the highway and transportation departments, by committees of AASHTO, and by the FHWA. Topics of the highest merit are selected by the AASHTO Special Committee on Research and Innovation (R&I), and each year R&I’s recommendations are proposed to the AASHTO Board of Direc- tors and the National Academies. Research projects to address these topics are defined by NCHRP, and qualified research agencies are selected from submitted proposals. Administration and surveillance of research contracts are the responsibilities of the National Academies and TRB. The needs for highway research are many, and NCHRP can make significant contributions to solving highway transportation problems of mutual concern to many responsible groups. The program, however, is intended to complement, rather than to substitute for or duplicate, other highway research programs. Published research reports of the NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM are available from Transportation Research Board Business Office 500 Fifth Street, NW Washington, DC 20001 and can be ordered through the Internet by going to https://www.mytrb.org/MyTRB/Store/default.aspx Printed in the United States of America NCHRP RESEARCH REPORT 972 Project 15-65 ISSN 2572-3766 (Print) ISSN 2572-3774 (Online) ISBN 978-0-309-68719-5 Library of Congress Control Number 2022942183 © 2022 by the National Academy of Sciences. National Academies of Sciences, Engineering, and Medicine and the graphical logo are trade- marks of the National Academy of Sciences. All rights reserved. COPYRIGHT INFORMATION Authors herein are responsible for the authenticity of their materials and for obtaining written permissions from publishers or persons who own the copyright to any previously published or copyrighted material used herein. Cooperative Research Programs (CRP) grants permission to reproduce material in this publication for classroom and not-for-profit purposes. Permission is given with the understanding that none of the material will be used to imply TRB, AASHTO, FAA, FHWA, FTA, GHSA, NHTSA, or TDC endorsement of a particular product, method, or practice. It is expected that those reproducing the material in this document for educational and not-for-profit uses will give appropriate acknowledgment of the source of any reprinted or reproduced material. For other uses of the material, request permission from CRP. NOTICE The research report was reviewed by the technical panel and accepted for publication according to procedures established and overseen by the Transportation Research Board and approved by the National Academies of Sciences, Engineering, and Medicine. The opinions and conclusions expressed or implied in this report are those of the researchers who performed the research and are not necessarily those of the Transportation Research Board; the National Academies of Sciences, Engineering, and Medicine; the FHWA; or the program sponsors. The Transportation Research Board does not develop, issue, or publish standards or speci- fications. The Transportation Research Board manages applied research projects which provide the scientific foundation that may be used by Transportation Research Board sponsors, industry associations, or other organizations as the basis for revised practices, procedures, or specifications. The Transportation Research Board; the National Academies of Sciences, Engineering, and Medicine; and the sponsors of the National Cooperative Highway Research Program do not endorse products or manufacturers. Trade or manufacturers’ names or logos appear herein solely because they are considered essential to the object of the report.

e National Academy of Sciences was established in 1863 by an Act of Congress, signed by President Lincoln, as a private, non- governmental institution to advise the nation on issues related to science and technology. Members are elected by their peers for outstanding contributions to research. Dr. Marcia McNutt is president. e National Academy of Engineering was established in 1964 under the charter of the National Academy of Sciences to bring the practices of engineering to advising the nation. Members are elected by their peers for extraordinary contributions to engineering. Dr. John L. Anderson is president. e National Academy of Medicine (formerly the Institute of Medicine) was established in 1970 under the charter of the National Academy of Sciences to advise the nation on medical and health issues. Members are elected by their peers for distinguished contributions to medicine and health. Dr. Victor J. Dzau is president. e three Academies work together as the National Academies of Sciences, Engineering, and Medicine to provide independent, objective analysis and advice to the nation and conduct other activities to solve complex problems and inform public policy decisions. e National Academies also encourage education and research, recognize outstanding contributions to knowledge, and increase public understanding in matters of science, engineering, and medicine. Learn more about the National Academies of Sciences, Engineering, and Medicine at www.nationalacademies.org. e Transportation Research Board is one of seven major programs of the National Academies of Sciences, Engineering, and Medicine. e mission of the Transportation Research Board is to provide leadership in transportation improvements and innovation through trusted, timely, impartial, and evidence-based information exchange, research, and advice regarding all modes of transportation. e Board’s varied activities annually engage about 8,000 engineers, scientists, and other transportation researchers and practitioners from the public and private sectors and academia, all of whom contribute their expertise in the public interest. e program is supported by state transportation departments, federal agencies including the component administrations of the U.S. Department of Transportation, and other organizations and individuals interested in the development of transportation. Learn more about the Transportation Research Board at www.TRB.org.

C O O P E R A T I V E R E S E A R C H P R O G R A M S CRP STAFF FOR NCHRP RESEARCH REPORT 972 Christopher J. Hedges, Director, Cooperative Research Programs Lori L. Sundstrom, Deputy Director, Cooperative Research Programs Waseem Dekelbab, Associate Program Manager, National Cooperative Highway Research Program William C. Rogers, Senior Program Officer (retired) Richard A. Retting, Senior Program Officer Dajaih Bias-Johnson, Senior Program Assistant Natalie Barnes, Director of Publications Heather DiAngelis, Associate Director of Publications NCHRP PROJECT 15-65 PANEL Field of Design—Area of General Design Erik O. Emerson, Wisconsin Department of Transportation, Madison, WI (Chair) Mark Ayton, Safe Roads Engineering, Stouffville, ON Amelia Beth DeGrace, New York City Department of Environmental Protection, Kew Gardens, NY John Patrick Donahue, Washington State Department of Transportation, Olympia, WA David D. Galloway, North Carolina Department of Transportation, Fletcher, NC Sally Marie Gunn, New Hampshire Department of Transportation, Concord, NH In-Kyu Lim, Federal Highway Administration (FHWA), McLean, VA Dean L. Sicking, University of Alabama, Birmingham, Birmingham, AL Richard B. Albin, FHWA Liaison Kelly K. Hardy, AASHTO Liaison Stephen F. Maher, TRB Liaison

NCHRP Research Report 972: Development of Safety Performance-Based Guidelines for the Roadside Design Guide identifies and prioritizes gaps and needs related to guidance in the current AASHTO Roadside Design Guide. The report also provides safety performance- based guidance to address high-priority needs that support quantitative design decisions and that promote consistency in interpretation and implementation. The report’s conclu- sions and proposals support American Association of State Highway and Transportation Officials’ (AASHTO’s) ongoing efforts to reduce fatal and incapacitating injury roadway departure crashes. Using a performance-based risk assessment procedure as described in this report will provide a valuable tool for roadside engineers to use in minimizing the risk of fatal and serious injury crashes and maximizing the safety of highway agency roadways. Roadway departure is a major source of traffic deaths and serious injuries. About 20 percent of U.S. motor vehicle crash deaths result from a vehicle leaving the roadway and striking a fixed object. Additional fatalities occur when errant vehicles leave the road and roll over without striking a fixed object. Roadway departure crashes result from a variety of contributing factors, including driver inattention, fatigue, speeding, impairment, icy/ snowy conditions, and evasive maneuvers to avoid animals in the road. In rare cases, vehicle malfunctions such as brake or steering failure can be a factor. While roadway owners cannot eliminate such factors that contribute to roadway depar- ture, state and local transportation agencies are encouraged to adopt roadside design practices that reduce the chance of harmful outcomes when drivers do leave the road. To support identification and implementation of best roadside design practices, the AASHTO publishes the Roadside Design Guide (RDG). This guide presents a synthesis of current information and operating practices related to roadside safety and focuses on safety treat- ments that can minimize the likelihood of serious injuries when a motorist leaves the road- way. Developed for use by design engineers and professionals involved in roadside safety, it is considered a significant tool that combines current research with practical experience. The AASHTO Technical Committee on Roadside Safety (TCRS), which maintains the RDG, develops, implements, and maintains guidance for reducing fatal and serious-injury roadway departure crashes. With increasing reliance in the highway design community on performance-based methods, the TCRS identified a need to review guidance in the current RDG; identify gaps and areas where updated guidance is needed; and explicitly express this guidance in terms of safety performance. Accordingly, NCHRP Research Report 972 identi- fies and prioritizes gaps and needs related to guidance in the AASHTO RDG and presents safety performance-based guidance to address high-priority needs that support quantita- tive design decisions and that promote consistency in interpretation and implementation. F O R E W O R D By Richard A. Retting Staff Officer Transportation Research Board

The roadside safety performance-based risk assessment procedure presented in NCHRP Research Report 972 is a quantitative performance-based method that can be used to assess roadside designs. The method quantifies the risk of a fatal or serious injury run-off-road (ROR) crash for each considered roadside design alternative and allows the engineer to select the design with the lowest risk. The roadside risk assessment procedure provides two alternative decision criteria: a relative risk criterion where the alternative design must result in fewer fatal and serious injury crashes than the existing roadside condition and an absolute measure of risk that can be compared to highway agency network-wide crash data in order to prioritize projects and needs. The values for those performance goals can be set by highway agencies to reflect local conditions and practices. Most components of the roadside risk assessment procedure are based on statistical modelling of observed data. This data can be updated, expanded, and refined without changing the underlying methodology such that over time the methodology will become increasingly specific and remain current. NCHRP Research Report 972 also identifies areas in the performance-based risk assess- ment method with limited observed data or where existing data is old or of suspect quality as well as recommendations for future research to fill these knowledge/data gaps. As knowl- edge gaps are filled, they can be integrated into the roadside risk assessment procedure such that results found using the procedures are improved over time while retaining con- sistent and measurable goals. While the implicit design objective for roadside design has always been to reduce the risk of the motoring public, this report provides a quantifiable design objective: to minimize the frequency of fatal and serious injury roadway departure crashes. Risk can be reduced by keeping vehicles from leaving the road in the first place, by reducing the chance of striking off-road fixed objects, and by reducing the crash severity if a crash does occur. The following appendices to this report, not printed herein, are available from the National Academies Press website (www.nap.edu) by searching for NCHRP Research Report 972: Development of Safety Performance-Based Guidelines for the Roadside Design Guide. The appendices include the following: • Appendix A: Roadside Risk Workbook, • Appendix B: Derivations, • Appendix C: Serious and Fatal Injury Crash Tables, and • Appendix D: Research Needs and Knowledge Gaps.

Note: Photographs, figures, and tables in this report may have been converted from color to grayscale for printing. The electronic version of the report (posted on the web at www.nap.edu) retains the color versions. 1 Chapter 1 Introduction 3 Chapter 2 Literature Review 3 2.1 Introduction 5 2.2 History 7 2.3 Roadside Features 7 2.3.1 Terrain 14 2.3.2 Fixed Objects 22 2.3.3 Roadside Safety Hardware 44 2.3.4 Other Road Users 46 2.4 Decision-Making in Roadside Safety 46 2.4.1 Decision-Making Methods 58 2.4.2 Basis for Decision-Making 75 2.5 Conclusion 77 Chapter 3 Roadside Risk Design Methodology 77 3.1 Background 78 3.2 Safety Performance Goal 80 3.3 Safety Performance Design 80 3.3.1 Method 83 3.3.2 Calculate Feature Risk (OUTCOMEj) 136 3.3.3 Segment Risk and Making Design Decisions 143 3.4 Updating and Maintaining the Data 144 Chapter 4 Results 144 4.1 Median Barrier Selection 150 4.2 Relocating Narrow Fixed Objects 151 4.3 Shielding Object-Free Sloped Terrain 152 4.4 Shielding Fixed Objects with Longitudinal Barriers 158 4.5 Bridge Rail Selection 164 Chapter 5 Conclusions and Proposals 164 5.1 Conclusions 165 5.2 Proposals 166 References 176 Appendices A–D C O N T E N T S