TRANSPORTATION RESEARCH BOARD 2024 EXECUTIVE COMMITTEE^*

OFFICERS

CHAIR: Carol A. Lewis, Professor, Transportation Studies, Texas Southern University, Houston

VICE CHAIR: Leslie S. Richards, General Manager, Southeastern Pennsylvania Transportation Authority (SEPTA), Philadelphia

EXECUTIVE DIRECTOR: Victoria Sheehan, Transportation Research Board, Washington, DC

MEMBERS

Michael F. Ableson, CEO, Arrival Automotive–North America, Detroit, MI

James F. Albaugh, President and CEO, The Boeing Company (retired), Scottsdale, AZ

Carlos M. Braceras, Executive Director, Utah Department of Transportation, Salt Lake City

Douglas C. Ceva, Vice President, Customer Lead Solutions, Prologis, Inc., Jupiter, FL

Nancy Daubenberger, Commissioner of Transportation, Minnesota Department of Transportation, St. Paul

Marie Therese Dominguez, Commissioner, New York State Department of Transportation, Albany

Garrett Eucalitto, Commissioner, Connecticut Department of Transportation, Newington

Chris T. Hendrickson, Hamerschlag University Professor of Engineering Emeritus, Carnegie Mellon University, Pittsburgh, PA

Randell Iwasaki, President and CEO, Iwasaki Consulting Services, Walnut Creek, CA

Ashby Johnson, Executive Director, Capital Area Metropolitan Planning Organization (CAMPO), Austin, TX

Joel M. Jundt, Secretary of Transportation, South Dakota Department of Transportation, Pierre

Hani S. Mahmassani, W.A. Patterson Distinguished Chair in Transportation; Director, Transportation Center, Northwestern University, Evanston, IL

Scott C. Marler, Director, Iowa Department of Transportation, Ames

Ricardo Martinez, Adjunct Professor of Emergency Medicine, Emory University School of Medicine, Decatur, GA

Michael R. McClellan, Vice President, Strategic Planning, Norfolk Southern Corporation, Atlanta, GA

Russell McMurry, Commissioner, Georgia Department of Transportation, Atlanta

Craig E. Philip, Research Professor and Director, VECTOR, Department of Civil and Environmental Engineering, Vanderbilt University, Nashville, TN

Steward T.A. Pickett, Distinguished Senior Scientist, Cary Institute of Ecosystem Studies, Millbrook, NY

Susan A. Shaheen, Professor and Co-director, Transportation Sustainability Research Center, University of California, Berkeley

Marc Williams, Executive Director, Texas Department of Transportation, Austin

EX OFFICIO MEMBERS

Michael R. Berube, Deputy Assistant Secretary for Sustainable Transportation, U.S. Department of Energy, Washington, DC

Shailen Bhatt, Administrator, Federal Highway Administration, U.S. Department of Transportation, Washington, DC

Amit Bose, Administrator, Federal Railroad Administration, Washington, DC

Tristan Brown, Deputy Administrator, Pipeline and Hazardous Materials Safety Administration, U.S. Department of Transportation, Washington, DC

Steven Cliff, Executive Officer, California Air Resources Board, Sacramento

Rand Ghayad, Senior Vice President, Association of American Railroads, Washington, DC

LeRoy Gishi, Chief, Division of Transportation, Bureau of Indian Affairs, U.S. Department of the Interior, Germantown, MD

William H. Graham, Jr. (Major General, U.S. Army), Deputy Commanding General for Civil and Emergency Operations, U.S. Army Corps of Engineers, Washington, DC

Robert C. Hampshire, Deputy Assistant Secretary for Research and Technology, U.S. Department of Transportation, Washington, DC

Zahra “Niloo” Parvinashtiani, Engineer, Mobility Consultant Solutions, Iteris Inc., Fairfax, VA, and Chair, TRB Young Members Coordinating Council

Sophie Shulman, Deputy Administrator, National Highway Traffic Safety Administration, Washington, DC

Karl Simon, Director, Transportation and Climate Division, U.S. Environmental Protection Agency, Washington, DC

Paul P. Skoutelas, President and CEO, American Public Transportation Association, Washington, DC

Polly Trottenberg, Deputy Secretary of Transportation, U.S. Department of Transportation, Washington, DC

Jim Tymon, Executive Director, American Association of State Highway and Transportation Officials, Washington, DC

Veronica Vanterpool, Acting Administrator, Federal Transit Administration, Washington, DC

Michael Whitaker, Administrator, Federal Aviation Administration, U.S. Department of Transportation, Washington, DC

Vinn White, Deputy Administrator, Federal Motor Carrier Safety Administration, Washington, DC

___________________

^* Membership as of August 2024.

NATIONAL COOPERATIVE HIGHWAY RESEARCHP ROGRAM

NCHRP SYNTHESIS 634

Calibration and Development
of State-DOT-Specific Safety
Performance Functions

A Synthesis of Highway Practice

Vikash V. Gayah
Jakob C. Wiegand
Eric T. Donnell
DEPARTMENT OF CIVIL AND ENVIRONMENTAL ENGINEERING
THE PENNSYLVANIA STATE UNIVERSITY
University Park, PA

Subscriber Categories
Highways • Design • 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 transportation results in increasingly complex problems of wide interest to highway 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 initiated 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 Agreement 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 committee 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, universities, and industry; TRB’s relationship to the National Academies is an insurance of objectivity; and TRB maintains a full-time staff of specialists 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 identified 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 Directors 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.

NCHRP SYNTHESIS 634

Project 20-05, Topic 54-10

ISSN 0547-5570

ISBN 978-0-309-71011-4

Library of Congress Control Number 2024942425

© 2024 by the National Academy of Sciences. National Academies of Sciences, Engineering, and Medicine and the graphical logo are trademarks 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 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 specifications. 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.

Published reports of the

NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM

are available from

National Academies Press
500 Fifth Street, NW, Keck 360
Washington, DC 20001

(800) 624-6242

and can be ordered through the Internet by going to
https://nap.nationalacademies.org

Printed in the United States of America

The National Academy of Sciences was established in 1863 by an Act of Congress, signed by President Lincoln, as a private, nongovernmental 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 program divisions of the National Academies of Sciences, Engineering, and Medicine. The mission of the Transportation Research Board is to mobilize expertise, experience, and knowledge to anticipate and solve complex transportation-related challenges. The 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. 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.

COOPERATIVE RESEARCH PROGRAMS

CRP STAFF FOR NCHRP SYNTHESIS 634

Monique R. Evans, Director, Cooperative Research Programs

Waseem Dekelbab, Deputy Director, Cooperative Research Programs, and Manager, National Cooperative Highway Research Program

Edward T. Harrigan, Senior Program Officer

Daniel J. Magnolia, Senior Program Assistant

Natalie Barnes, Director of Publications

Heather DiAngelis, Associate Director of Publications

NCHRP PROJECT 20-05 PANEL

Joyce N. Taylor, Maine Department of Transportation, Augusta, ME (Chair)

Anita K. Bush, Nevada Department of Transportation, Carson City, NV

Joseph D. Crabtree, Kentucky Transportation Center, Lexington, KY

Mostafa Jamshidi, Nebraska Department of Transportation, Lincoln, NE

Jessie X. Jones, Arkansas Department of Transportation, Little Rock, AR

Raymond J. Khoury, Virginia Department of Transportation, Richmond, VA

Brenda Moore, North Carolina Department of Transportation (retired), Cary, NC

Jesus Alberto Sandoval-Gil, Arizona Department of Transportation, Phoenix, AZ

Cynthia J. Smith, Mississippi Department of Transportation, Jackson, MS

Jean M. Wallace, Minnesota Department of Transportation, St. Paul, MN

Mary Huie, FHWA Liaison

Jim McDonnell, AASHTO Liaison

Stephen F. Maher, TRB Liaison

TOPIC 54-10 PANEL

Brenton Bogard, Ohio Department of Transportation, Columbus, OH

Daniel L. Carter, North Carolina Department of Transportation, Durham, NC

Kohinoor Kar, Arizona Department of Transportation, Phoenix, AZ

Jonathan K. Markt, HDR, Omaha, NE

Bonnie S. Polin, Massachusetts Department of Transportation, Boston, MA

Kishor Shrestha, Washington State University, Pullman, WA

Matt Hinshaw, FHWA Liaison

Kelly K. Hardy, AASHTO Liaison

Bernardo B. Kleiner, TRB Liaison

ABOUT THE NCHRP SYNTHESIS PROGRAM

Highway administrators, engineers, and researchers often face problems for which information already exists, either in documented form or as undocumented experience and practice. This information may be fragmented, scattered, and unevaluated. As a consequence, full knowledge of what has been learned about a problem may not be brought to bear on its solution. Costly research findings may go unused, valuable experience may be overlooked, and due consideration may not be given to recommended practices for solving or alleviating the problem.

There is information on nearly every subject of concern to highway administrators and engineers. Much of it derives from research or from the work of practitioners faced with problems in their day-to-day work. To provide a systematic means for assembling and evaluating such useful information and to make it available to the entire highway community, the American Association of State Highway and Transportation Officials—through the mechanism of the National Cooperative Highway Research Program—authorized the Transportation Research Board to undertake a continuing study. This study, NCHRP Project 20-05, “Synthesis of Information Related to Highway Practices,” searches out and synthesizes useful knowledge from all available sources and prepares concise, documented reports on specific topics. Reports from this endeavor constitute an NCHRP report series, Synthesis of Highway Practice.

This synthesis series reports on current knowledge and practice, in a compact format, without the detailed directions usually found in handbooks or design manuals. Each report in the series provides a compendium of the best knowledge available on those measures found to be the most successful in resolving specific problems.

FOREWORD

By Edward T. Harrigan

Staff Officer

Transportation Research Board

The AASHTO Highway Safety Manual (HSM) provides tools for predicting the safety performance of a roadway facility. These tools include safety performance functions (SPFs), which incorporate geometric and other conditions to predict the crashes expected on a facility. SPFs are developed using crash numbers, geometric conditions, traffic conditions, and other data from one or more states, resulting in less accurate safety performance measures when the SPFs are used for analyses in other states. SPFs can be customized for a specific condition or region using a calibration factor. The calibration factor is then multiplied by the HSM model results to yield crash predictions that better represent the observed crash number in that state. Jurisdictions can develop jurisdiction-specific SPFs using their own data, allowing analyses that more closely represent their individual experiences. Although the development of customized SPFs is generally considered more accurate for crash predictions, it requires more data and expertise, and it has a higher cost.

As the state of the practice in data-driven safety analysis advances, states are challenged to calibrate or develop models that meet their needs. Specific challenges include the availability of sufficient data or funding to collect data. In addition, states increasingly question whether and how to apply particular factors or models to facility types that are not exactly the same as the ones used to develop the models. Practitioners have questions about whether calibration factors or SPFs are transferable and could be used by other jurisdictions. An initial step to addressing these questions is to synthesize the work states have already done to calibrate the Highway Safety Manual SPFs or develop their own SPFs.

The objective of this synthesis is to document state DOT practice on calibration factors and development of jurisdiction-specific SPFs. Information for this study was gathered through a literature review, a survey of state DOTs, and follow-up interviews with selected DOTs. Case examples of five state DOTs provide additional information on calibration factors and development of jurisdiction-specific SPFs.

Vikash V. Gayah and his colleagues at the Pennsylvania State University, University Park, Pennsylvania, collected and synthesized the information and wrote the report. The members of the topic panel are acknowledged on page iv. This synthesis is an immediately useful document that records state DOT practices on calibration factors and development of jurisdiction-specific SPFs that were acceptable within the limitations of the knowledge available at the time of its preparation. As progress in research and practice continues, new knowledge will be added to that now at hand.

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