The Effect of Vehicle Mix on Crash Frequency and Crash Severity (2024)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

2024 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 1103 The Effect of Vehicle Mix on Crash Frequency and Crash Severity Naveen Eluru Tanmoy Bhowmik Shahrior Pervaz Dewan Ashraful Parvez Lauren Hoover Mohamed Abdel-Aty University of Central Florida Orlando, FL Kai Wang John N. Ivan Shanshan Zhao Manmohan Joshi University of Connecticut Storrs Mansfield, CT Subscriber Categories Highways • Research • 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 1103 Project 22-49 ISSN 2572-3766 (Print) ISSN 2572-3774 (Online) ISBN 978-0-309-70946-0 Library of Congress Control Number 2024931664 © 2024 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, APTA, FAA, FHWA, FTA, GHSA, or NHTSA 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 spec- ifications. 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 program divisions of the National Academies of Sciences, Engineering, and Medicine. e mission of the Transportation Research Board is to mobilize expertise, experience, and knowledge to anticipate and solve complex transportation-related challenges. e Board’s varied activities annually engage about 8,500 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 1103 Waseem Dekelbab, Deputy Director, Cooperative Research Programs, and Manager, National Cooperative Highway Research Program Camille Crichton-Sumners, Senior Program Officer Mazen Alsharif, Senior Program Assistant Natalie Barnes, Director of Publications Heather DiAngelis, Associate Director of Publications NCHRP PROJECT 22-49 PANEL Field of Design—Area of Vehicle Barrier Systems Randy E. Bradley, Florida DOT, Tallahassee, FL (Chair) William Gordon Paille, BSC Group, Inc., Boston, MA Jeffrey A. Pulver, Maine DOT, Augusta, ME Karla C. Rodrigues Silva, University of Florida, Lake Mary, FL Ida van Schalkwyk, Washington State DOT, Olympia, WA Jonathan S. Wood, Iowa State University, Ames, IA Carol Tan, FHWA Liaison Kelly K. Hardy, AASHTO Liaison

NCHRP Research Report 1103: The Effect of Vehicle Mix on Crash Frequency and Crash Severity describes the methodology used to develop new crash prediction models that pro- vide crash frequency and severity predictions for user-provided data. The research report is complemented with new crash frequency and severity prediction spreadsheet tools and a guide to help practitioners incorporate the effect of vehicle mix in crash analysis using a preferred predictive model that is selected based on facility type. The research findings confirm that vehicle mix influences crash frequency and severity. The report and spreadsheet tools should be of interest to highway safety practitioners and others interested in or respon- sible for roadway safety analysis. The American Association of State Highway and Transportation Officials (AASHTO) Highway Safety Manual (HSM) includes methods for crash prediction for a variety of roadway facility types and site conditions. The current methods account for traffic volumes and other roadway characteristics but not for vehicle mix or the distribution of vehicle types. Recent studies suggest that heavy traffic adversely impacts roadway safety performance, and consid- eration of vehicle mix would improve predictive methods for estimating crash frequency and severity. Under NCHRP Project 22-49, the University of Central Florida was asked to develop and test a statistically valid predictive methodology to quantify the effect of vehicle mix on crash frequency and severity for several facility types. The research team developed spreadsheet tools for practitioners to quantify the effect of vehicle mix on safety performance and ana- lyzed two alternative statistical approaches. The research team calibrated the models using existing HSM methods to serve as a benchmark for the proposed alternative approaches. The team used predictive metrics to compare model performance for the estimation and validation of datasets and selected a single, preferred model system for each facility group. This project included analysis of two new facilities, rural arterial three-lane and rural arterial five-lane roadway segments. Though not included in the current HSM, the two new facilities exhibited a significant number of crashes across the states. To aid practitioners with crash frequency and severity analysis using the models, in addition to NCHRP Research Report 1103, several related products are available. This includes the report’s Appendices A through D, a PowerPoint presentation, a Tech Brief, and an Implementation Memo; all are available at https://doi.org/10.17226/27454. In addition, a suite of crash frequency and severity prediction Excel spreadsheet tools is available along with NCHRP Web-Only Document 393: User’s Guide for Quantifying the Effects of Vehicle Mix on Crash Frequency and Crash Severity, which provides detailed instructions on how to use the tools. These materials are available on the National Academies Press website (nap.nationalacademies.org) by searching for NCHRP Web-Only Document 393. F O R E W O R D By Camille Crichton-Sumners Staff Officer Transportation Research Board

1 Chapter 1 Introduction 1 1.1 Project Background 2 1.2 Report Organization 3 Chapter 2 Literature Review 3 2.1 Overview of Crash Prediction Models 6 2.2 Summary of Crash Frequency and Severity Approaches 11 2.3 Incorporating Vehicle Mix 13 2.4 Summary 14 Chapter 3 Data Description 14 3.1 Data Source 14 3.2 Facility Selection 19 3.3 Variable Assessment 21 3.4 Dependent and Independent Variable Creation 22 3.5 Approaches and Assumptions of Vehicle Mix Data 24 3.6 Summary 25 Chapter 4 Methodology 25 4.1 HSM Approach 28 4.2 Negative Binomial-Fractional Split Method 33 4.3 Multivariate Methods 37 4.4 Model Validation 38 4.5 Summary 39 Chapter 5 Model Selection and Facility-Specific Suggested Actions 39 5.1 Model Selection Process 43 5.2 Final Suggestions 44 5.3 Summary 45 Chapter 6 Suggested Model Parameters 45 6.1 Urban Limited-Access Facility Group 49 6.2 Rural Limited-Access Facility Group 49 6.3 Urban Arterial Facility Group 58 6.4 Rural Arterial Facility Group 66 6.5 Urban Intersection Facility Group 72 6.6 Rural Intersection Facility Group 76 6.7 Summary 77 Chapter 7 Conclusions C O N T E N T S

80 References 85 Acronyms and Abbreviations 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 nap.nationalacademies.org) retains the color versions.