Guidelines for the Development and Application of Crash Modification Factors (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 991 Guidelines for the Development and Application of Crash Modification Factors Daniel Carter Raghavan Srinivasan The University of North Carolina at Chapel Hill Highway Safety Research Center Chapel Hill, NC Frank Gross Scott Himes Thanh Le VHB, Inc. Watertown, MA Bhagwant Persaud Craig Lyon Persaud and Lyon, Inc. Ottawa, ON James Bonneson Kittelson and Associates, Inc. Portland, OR Subscriber Categories Highways • Operations and Traffic Management • Safety and Human Factors Research sponsored by the American Association of State Highway and Transportation Officials 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. 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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 991 Project 17-63 ISSN 2572-3766 (Print) ISSN 2572-3774 (Online) ISBN 978-0-309-68686-0 Library of Congress Control Number 2022935073 © 2022 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; 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.

The 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. The 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. The 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. The 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. The 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. The Transportation Research Board is one of seven major programs of the National Academies of Sciences, Engineering, and Medicine. The 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. The 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. The 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 991 Christopher J. Hedges, Director, Cooperative Research Programs Lori L. Sundstrom, Deputy Director, Cooperative Research Programs Waseem Dekelbab, Associate Program Manager, Project Delivery, National Cooperative Highway Research Program Sid Mohan, Associate Program Manager, Implementation and Technology Transfer, National Cooperative Highway Research Program David Jared, Senior Program Officer Clara Schmetter, Senior Program Assistant Natalie Barnes, Director of Publications Heather DiAngelis, Associate Director of Publications NCHRP PROJECT 17-63 PANEL Field of Traffic—Area of Safety Kohinoor Kar, Arizona Department of Transportation, Phoenix, AZ (Chair) Alia Awwad, City of Fort Lauderdale, Fort Lauderdale, FL Michael Curtit, Jacobs, St. Louis, MO Dean C. Kanitz, Michigan Department of Transportation, Lansing, MI Cheryl Bornheimer Kelley, Burns & McDonnell, Kansas City, MO John S. Miller, Virginia Department of Transportation, Charlottesville, VA Grant G. Schultz, Brigham Young University, Provo, UT Ming-Bang Shyu, Washington State Department of Transportation, Seattle, WA Jason J. Siwula, Kentucky Transportation Cabinet, Frankfort, KY Karen Scurry, FHWA Liaison Kelly K. Hardy, AASHTO Liaison Bernardo B. Kleiner, TRB Liaison AUTHOR ACKNOWLEDGMENTS The research reported herein was performed under NCHRP Project 17-63 by the University of North Carolina Highway Safety Research Center (HSRC); Vanasse Hangen Brustlin, Inc. (VHB); Persaud and Lyon, Inc. (P&L); and Kittelson and Associates, Inc. (KAI). Daniel Carter, P.E., at HSRC was the principal investigator. The other authors of this report are Dr. Raghavan Srinivasan at HSRC; Dr. Frank Gross, Dr. Scott Himes, and Thanh Le at VHB; Bhagwant Persaud and Craig Lyon at P&L; and Dr. James Bonneson at KAI. Dr. Ezra Hauer contributed material for supporting appendixes to this report. In addition, the project team would like to thank and acknowledge the following people and entities for supplying data for this project: Dr. Mohamed Abdel-Aty, University of Central Florida Mr. Mark Sujka, Washington State DOT Ms. Anusha Patel Nujjetty, Highway Safety Information System

NCHRP Research Report 991 provides guidelines for developing and applying crash modification factors (CMFs) in road safety practice and interactive tools to support their use. These guidelines will be of interest to practitioners responsible for making informed decisions about CMFs. CMFs provide transportation professionals with quantitative information they need to make decisions about which safety improvements to design and implement. Although sig- nificant progress continues to be made in developing CMFs, broader guidelines for their development and application in practice were needed. In this project, guidelines were devel- oped for selecting and adjusting CMFs for sites at which key characteristics may be different, applying CMFs for multiple treatments in a single location, and developing future CMFs in functions that account for differences in key characteristics. Under NCHRP Project 17-63, “Guidelines for the Development and Application of Crash Modification Factors,” the University of North Carolina at Chapel Hill was asked to provide guidelines for developing and applying CMFs in various road safety scenarios. The guide- lines were developed in three parts. The first part provides a process to select and adjust a CMF for use at a site of interest which may differ in one or more characteristics from the sites where the CMF was developed. The second part provides a process by which an analyst can determine a combined effectiveness of two countermeasures implemented at the same location. Finally, the third part provides direction on developing CMFs that account for differences in key site characteristics, particularly in creating crash modification functions. The guidelines are accompanied by two interactive tools. One tool can be used to conduct a statistical analysis of CMFs for a common treatment or change in site characteristic, and the other tool can be used to test two or more CMFs for the same treatment to determine if they are similar enough to be combined into a single CMF value. These materials can be found by searching www.nap.edu for NCHRP Research Report 991. F O R E W O R D By David Jared Staff Officer Transportation Research Board

1 Summary 4 Chapter 1 Background 4 Introduction 5 Chapter 2 Research Approach 6 Chapter 3 Findings and Applications 6 Task 2 Guidelines for Calibration of Existing CMFs for Different Site Characteristics 25 Task 3 Quantifying the Effect of Multiple Treatments at a Single Location 90 Task 4 Recommended Procedures for Calibrating and Formulating Future CMFs That Identify Key Influential Site Characteristics 97 Chapter 4 Conclusions and Suggested Research 97 Combined Effect of Multiple Treatments 98 Enhancing Future CMF Research 100 References 103 Abbreviations A-1 Appendix A Procedure for Estimating the Effect of a Proposed Treatment at a Subject Site B-1 Appendix B Procedure for Estimating the Combined Safety Effect of Two Treatments C-1 Appendix C Guidelines for Developing Crash Modification Functions D-1 Appendix D User Guide for CMF Regression Software E-1 Appendix E User Guide for the CMF Combination Tool Spreadsheet F-1 Appendix F Enhancing Future CMF Research G-1 Appendix G Developing Consensus in Research About the Safety Effect of Manipulations C O N T E N T S 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.trb.org) retains the color versions.