Posted Speed Limit Setting Procedure and Tool: User Guide (2021)

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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 966 Posted Speed Limit Setting Procedure and Tool USER GUIDE Kay Fitzpatrick Subasish Das Michael P. Pratt Karen Dixon Texas A&M Transportation Institute College Station, TX Tim Gates Michigan State University East Lansing, MI 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. 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 966 Project 17-76 ISSN 2572-3766 (Print) ISSN 2572-3774 (Online) ISBN 978-0-309-67404-1 Library of Congress Control Number 2021938279 © 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 AUTHOR ACKNOWLEDGMENTS The research reported herein was performed under National Cooperative Highway Research Program Project 17-76 by the Texas A&M Transportation Institute (TTI), which is a member of The Texas A&M University System, and Michigan State University (MSU). Kay Fitzpatrick, TTI senior research engineer, was the principal investigator. The authors of this report are: • Kay Fitzpatrick (TTI). • Subasish Das (TTI). • Tim Gates (MSU). • Michael P. Pratt (TTI). • Karen Dixon (TTI). The work was performed under the general supervision of Dr. Fitzpatrick. The research reported here was supported by several staff members at TTI and MSU. The authors wish to acknowledge the many experts who contributed to this research by participating in discussions on posted speed limits and providing comments during presentations and early drafts. The authors appreciate the time and effort of these individuals. CRP STAFF FOR NCHRP RESEARCH REPORT 966 Christopher J. Hedges, Director, Cooperative Research Programs Lori L. Sundstrom, Deputy Director, Cooperative Research Programs David Jared, Senior Program Officer Clara Schmetter, Senior Program Assistant Eileen P. Delaney, Publications Senior Advisor Natalie Barnes, Director of Publications Scott E. Hitchcock, Senior Editor NCHRP PROJECT 17-76 PANEL Field of Traffic—Area of Safety Peter D. Buchen, Minnesota Department of Transportation, Roseville, MN (Chair) John E. Fisher, City of Culver City, South Pasadena, CA Kevin J. Haas, Oregon Department of Transportation, Salem, OR Thomas Hicks, Brudis Associates, Towson, MD Michelle Nickerson, Tennessee Department of Transportation, Nashville, TN Robert J. Pento, Pennsylvania Department of Transportation, Harrisburg, PA Steven Cole Strength, Louisiana Department of Transportation and Development, Baton Rouge, LA William C. Taylor, Michigan State University, East Lansing, MI Michael Matzke, FHWA Liaison Bernardo B. Kleiner, TRB Liaison

NCHRP Research Report 966 provides a procedure for setting speed limits and a practitioner- ready user manual explaining the speed limit setting procedure (SLS-Procedure). Addi- tionally, it provides an automated version of the SLS-Procedure via a spreadsheet-based Speed Limit Setting Tool (SLS-Tool). The guidebook will be of interest to engineers respon- sible for making informed decisions about the setting of speed limits. Several factors are considered within engineering studies when determining the posted speed limit for a speed zone. Currently, the predominant method for setting speed limits uses the 85th percentile speed. This method is viewed as being a fair way to set speed limits based on the driving behavior of most drivers (85 percent), representing reasonable and prudent drivers since the fastest 15 percent of drivers are excluded. The 85th percentile speed is also believed to represent a safe speed that would minimize crashes. The SLS-Procedure is based on decision rules that consider both driver speed choice and safety associated with the roadway. The SLS-Procedure was designed to be applicable for different roadway types and contexts by having a set of unique decision rules for four combinations of roadway types and contexts: Limited-Access, Undeveloped, Developed, and Full-Access facilities. The SLS-Procedure provides a fact-based, transparent set of decision rules to determine the suggested speed limit for a specific roadway segment. Under NCHRP Project 17-76, “Guidance for the Setting of Speed Limits,” Texas A&M Transportation Institute was asked to investigate factors that influence operating speed and safety through a review of the literature and an analysis of the relationships of speed, safety, and roadway characteristics on urban/suburban streets. That knowledge and a review of existing speed limit setting practices were used to develop the SLS-Procedure and accompanying SLS-Tool. Note that the SLS-Tool is provided in two formats, one with macros and one without. The without macros version is made available for users who are not able to use macro codes on their computers. The research team also conducted several workshops and presentations during the development of the SLS-Procedure, and these presentations provided opportunities to obtain feedback on its potential format. The SLS-Procedure and SLS-Tool are accompanied by NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool, which details the research activities and methods. The SLS-Tool and NCHRP Web-Only Document 291 are available on the TRB website (TRB.org) by searching for “NCHRP Research Report 966.” F O R E W O R D By David Jared Staff Officer Transportation Research Board

1 Summary 3 Section 1 Introduction 3 Background 3 Objective 4 Organization of User Guide 5 Section 2 Speed Limit Relationships and Practices 5 Speed and Crashes 5 Ongoing Debate on How to Set a Posted Speed Limit 6 The Consequences of Speed 6 Challenges with the Relationship Between Posted Speed and Operating Speed 8 Achieving Target Speeds Through Roadway Configuration and Traffic Control 10 Section 3 Procedure to Calculate the Suggested Speed Limit 10 Overview 10 Speed Limit Setting Tool 11 Section 4 Decision-Making Steps Within the Suggested Speed Limit Procedure 11 Roadway Segment Context and Type 11 Roadway Context 12 Roadway Type 13 Matrix 14 Speed Limit Setting Groups 15 Speed Distribution 15 Consideration of Geometric Variables, Human Factors, and Safety 17 Decision Rules for Each Speed Limit Setting Group 17 Speed Limit Setting Group: Limited-Access 17 Speed Limit Setting Group: Undeveloped 18 Speed Limit Setting Group: Developed 18 Speed Limit Setting Group: Full-Access 21 Section 5 Variables for Decision-Making Procedure 21 Roadway Context 21 Roadway Type 22 Roadway Segment Input Variables for Speed Limit Setting Groups 24 Speed Data Input Variables for Speed Limit Setting Groups 24 Speed Data Variable: 50th Percentile Speed (All SLSGs) 24 Speed Data Variable: 85th Percentile Speed (All SLSGs) 24 Speed Data Variable: Maximum Speed Limit (All SLSGs) C O N T E N T S

24 Roadway Segment Data Input Variables for Speed Limit Setting Groups 24 Roadway Segment Variable: AADT (Limited-Access, Undeveloped SLSGs) 24 Roadway Segment Variable: Adverse Alignment Presence (All SLSGs) 24 Roadway Segment Variable: Angle Parking Present (Developed and Full-Access SLSGs) 24 Roadway Segment Variable: Bicyclist Activity (Developed and Full-Access SLSGs) 24 Roadway Segment Variable: Design Speed (Limited-Access SLSG) 25 Roadway Segment Variable: Directional Design-Hour Truck Volume (Limited-Access SLSG) 25 Roadway Segment Variable: Grade (Limited-Access SLSG) 25 Roadway Segment Variable: ISW (Limited-Access SLSG) 25 Roadway Segment Variable: Lane Width (Undeveloped SLSG) 25 Roadway Segment Variable: Median Type (Undeveloped, Developed, and Full-Access SLSGs) 26 Roadway Segment Variable: Number of Access Points (Undeveloped, Developed, and Full-Access SLSGs) 26 Roadway Segment Variable: Number of Interchanges (Limited-Access SLSG) 26 Roadway Segment Variable: Number of Lanes (All SLSGs) 26 Roadway Segment Variable: Number of Traffic Signals (Developed and Full-Access SLSGs) 27 Roadway Segment Variable: On-Street Parking Activity (Developed and Full-Access SLSGs) 27 Roadway Segment Variable: Outside (Right) SW (Limited-Access SLSG) 27 Roadway Segment Variable: Parallel Parking Permitted (Developed SLSGs) 27 Roadway Segment Variable: Pedestrian Activity (Developed and Full-Access SLSGs) 27 Roadway Segment Variable: Segment Length (All SLSGs) 27 Roadway Segment Variable: SW (Undeveloped SLSG) 28 Roadway Segment Variable: Sidewalk Buffer (Developed and Full-Access SLSGs) 28 Roadway Segment Variable: Sidewalk Presence/Width (Developed and Full-Access SLSGs) 28 Combination of Roadway Segment Variables 28 Roadway Segment Combination of Variables: Grade and Design Speed (Limited-Access SLSG) 29 Roadway Segment Combination of Variables: ISW, Number of Lanes, and Hourly Truck Volume (Limited-Access SLSG) 29 Roadway Segment Combination of Variables: Number of Lanes, Median Type, AADT Combination (Undeveloped SLSG) 29 Roadway Segment Variable: Sidewalk Presences/Width, Sidewalk Buffer, and Pedestrian Activity (Developed and Full-Access SLSGs) 29 Crash Data Input Variables for Speed Limit Setting Groups 30 Crash Variables 30 Average Crash Rate 32 Critical Crash Rate 32 Crash Rate Scenarios

33 Section 6 Speed Limit Setting Tool 33 Overview of SLS-Tool Requirements 34 Data Entry 35 Advisory, Calculated, or Warning Messages 36 How to Handle Situations When Data Are Not Available for One of the Variables 38 Section 7 SLS-Tool Case Study Examples 38 Example 1: Limited-Access 38 Example 2: Undeveloped 39 Example 3: Developed 41 Example 4: Full-Access 44 Section 8 Other Considerations When Setting Posted Speed Limits 44 Why 85th or 50th Percentile Speed? 45 Identifying the Segment Limits 46 Gathering Operating Speed 46 Gathering Crash Data 47 Design Speed 47 Relationships Among Safety, Speed, and Roadway Characteristics, Including Posted Speed Limit 47 Traffic Variables 49 TCD Variables 49 Roadway Geometry Variables 50 Variables Associated with Roadway Surroundings 51 Section 9 Related Reference Materials 51 Speed Management Safety Website 51 Speed Management ePrimer for Rural Transition Zones and Town Centers 51 Traffic Calming ePrimer 52 Highway Safety Manual 52 Speed Management Program Plan 52 Methods and Practices for Setting Speed Limits: An Informational Report 52 Speed Concepts: Informational Guide 53 NCHRP Report 500: Guidance for Implementation of the AASHTO Strategic Highway Safety Plan, Volume 23: A Guide for Reducing Speeding-Related Crashes 53 MUTCD for Streets and Highways 53 USLIMITS2 53 Speed Enforcement Program Guidelines 55 Acronyms and Abbreviations 56 References 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.