National Academies Press: OpenBook
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2021. Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results. Washington, DC: The National Academies Press. doi: 10.17226/26439.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2021. Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results. Washington, DC: The National Academies Press. doi: 10.17226/26439.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2021. Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results. Washington, DC: The National Academies Press. doi: 10.17226/26439.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2021. Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results. Washington, DC: The National Academies Press. doi: 10.17226/26439.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2021. Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results. Washington, DC: The National Academies Press. doi: 10.17226/26439.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2021. Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results. Washington, DC: The National Academies Press. doi: 10.17226/26439.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2021. Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results. Washington, DC: The National Academies Press. doi: 10.17226/26439.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2021. Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results. Washington, DC: The National Academies Press. doi: 10.17226/26439.
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2021 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 983 Reliability of Crash Prediction Models A GUIDE FOR QUANTIFYING AND IMPROVING THE RELIABILITY OF MODEL RESULTS Raghavan Srinivasan Bo Lan Caroline Mozingo The University of North Carolina at Chapel Hill Highway Safety Research Center (HSRC) Chapel Hill, NC James Bonneson Kittelson and Associates, Inc. Portland, OR Craig Lyon Bhagwant Persaud Persaud and Lyon, Inc. Ottawa, ON Geni Bahar NAVIGATS Inc. Toronto, ON Subscriber Categories Safety and Human Factors • Operations and Traffic Management 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. 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 983 Project 17-78 ISSN 2572-3766 (Print) ISSN 2572-3774 (Online) ISBN 978-0-309-09424-5 Library of Congress Control Number 2021950186 © 2021 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 983 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 David M. Jared, Senior Program Officer Clara Schmetter, Senior Program Assistant Natalie Barnes, Director of Publications Heather DiAngelis, Associate Director of Publications Kami Cabral, Editor NCHRP PROJECT 17-78 PANEL Field of Traffic—Area of Safety Tim Harmon, Holly Springs, NC (Chair) Kelly K. Campbell, Idaho Transportation Department, Boise, ID Andrew H. Ceifetz, WSP, Walled Lake, MI Brian Hovanec, Mississippi DOT, Jackson, MS Rahul Jain, District Department of Transportation, Washington, D.C. Dean C. Kanitz, Michigan Department of Transportation, Lansing, MI Andrew G. Kaplan, Port Authority of New York and New Jersey, Hoboken, NJ Howard Lubliner, Burns and McDonnell Engineering Co., Lawrence, KS John McFadden, FHWA Liaison Kelly K. Hardy, AASHTO Liaison Bernardo B. Kleiner, TRB Liaison

NCHRP Research Report 983: Reliability of Crash Prediction Models: A Guide for Quan- tifying and Improving the Reliability of Model Results (hereinafter the Guide) describes methods and procedures to (a) quantify the impact of selecting or neglecting certain data parameters in safety estimate predictions, and (b) estimate, interpret, and improve the reli- ability of predictions and use of crash modification factors (CMFs). It also describes several scenarios encountered by practitioners in using crash prediction models (CPMs) and CMFs for safety estimate predictions. The Guide will be of interest to practitioners responsible for making informed decisions about CPMs and CMFs. The AASHTO Highway Safety Manual (HSM) provides fact-based, analytical tools and techniques to quantify the potential safety impacts of planning, design, operations, and maintenance decisions. Part C of the HSM contains predictive methods for rural two-lane roads, rural multilane highways, and urban and suburban arterials; however, it does not include methods for consistently ensuring model reliability. Since the publication of the HSM, the safety analysis state of practice has progressed and more has been learned about the impact on the accuracy of assumptions made during the development of CPMs using HSM procedures. Practitioners are also striving to fully understand and appropriately com- municate the benefits of the HSM methods and the results derived from these methods. Under NCHRP Project 17-78, “Understanding and Communicating Reliability of Crash Prediction Models,” the University of North Carolina at Chapel Hill developed the Guide that contains guidelines for (a) quantification of the reliability of CPMs for practitioner use; (b) interpretation of model reliability; and (c) application of CPMs accounting for, but not limited to, assumptions, data ranges, and intended and unintended uses. The Guide is accompanied by (a) a conduct of research report (published as NCHRP Web-Only Document 303: Understanding and Communicating Reliability of Crash Prediction Models) that details the research activities and methods, (b) a communications plan, (c) a flyer to promote the Guide, and (d) a PowerPoint presentation that summarizes the research effort. These materials are available on the TRB website (www.trb.org) and can be found by searching for “NCHRP Research Report 983.” F O R E W O R D By David M. Jared Staff Officer Transportation Research Board

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. 1 Chapter 1 Introduction and Background 9 Chapter 2 Quantifying the Reliability of CPM Estimates for Mismatches Between Crash Modification Factors and SPF Base Conditions 37 Chapter 3 Quantifying the Reliability of CPM Estimates for Error in Estimated Input Values 47 Chapter 4 Quantifying the Reliability of CPM Estimates for How the Number of Variables in Crash Prediction Models Affects Reliability 57 Chapter 5 Reliability Associated with Using a Crash Prediction Model to Estimate Frequency of Rare Crash Types and Severities 70 Chapter 6 Reliability Associated with Predicting Outside the Range of Independent Variables 81 Chapter 7 Reliability Associated with Crash Prediction Models Estimated for Other Facility Types 93 References C O N T E N T S

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The AASHTO Highway Safety Manual (HSM) provides fact-based, analytical tools and techniques to quantify the potential safety impacts of planning, design, operations, and maintenance decisions.

The TRB National Cooperative Highway Research Program's NCHRP Research Report 983: Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results complements the HSM by providing methods for consistently ensuring model reliability.

Supplemental to the report are NCHRP Web-Only Document 303: Understanding and Communicating Reliability of Crash Prediction Models, a communications plan, a flyer, and a PowerPoint presentation.

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