Update of Highway Capacity Manual: Merge, Diverge, and Weaving Methodologies (2023)

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2023 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 1038 Update of Highway Capacity Manual MERGE, DIVERGE, AND WEAVING METHODOLOGIES Bastian J. Schroeder Burak Cesme Zifeng (Lilian) Wu Azhagan (Azy) Avr Paul Ryus Nagui M. Rouphail Kittelson & Associates, Inc. Wilmington, NC Yinhai Wang Shuyi Yin Wei Sun University of Washington Seattle, WA Subscriber Categories Highways • Design • Operations and Trafc Management 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 1038 Project 07-26 ISSN 2572-3766 (Print) ISSN 2572-3774 (Online) ISBN 978-0-309-69852-8 Library of Congress Control Number 2023906116 © 2023 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 programs of the National Academies of Sciences, Engineering, and Medicine. e 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. e 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. 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 1038 Christopher J. Hedges, Director, Cooperative Research Programs Waseem Dekelbab, Deputy Director, Cooperative Research Programs, and Manager, National Cooperative Highway Research Program Ahmad Abu-Hawash, Senior Program Officer Sheila A. Moore, Program Associate Natalie Barnes, Director of Publications Heather DiAngelis, Associate Director of Publications NCHRP PROJECT 07-26 PANEL Field of Traffic—Area of Traffic Planning Katherine J. Lloyd, Tennessee Department of Transportation, Nashville, TN (Chair) Carine Choubassi, Jacobs, New York, NY William E. Knowles, CDM Smith, Austin, TX Xiaoyue Cathy Liu, University of Utah, Salt Lake City, UT Sushmita Mitra, Maryland Transportation Authority, Baltimore, MD Benjamin Rouleau, Wisconsin Department of Transportation, Madison, WI Trey Young Tillander, Florida Department of Transportation, Tallahassee, FL Andrew J. Warren, Arkansas Department of Transportation, Conway, AR James D. Colyar, FHWA Liaison

NCHRP Research Report 1038: Update of Highway Capacity Manual: Merge, Diverge, and Weaving Methodologies evaluates and presents important updates to the freeway operations methods in the Highway Capacity Manual (HCM). The research team developed new meth- odologies for analyzing the capacity and speeds at freeway merge, diverge, and weaving segments based on a nationwide dataset. The research produced updated versions of HCM Chapters 13, 14, 27, and 28 utilizing the new methodologies for consideration by the TRB Committee on Highway Capacity and Quality of Service in the next update of the HCM. Implementation of the research findings by traffic and safety engineers at state departments of transportation could improve traffic flow and reduce congestion. Freeway congestion usually originates at freeway merge, diverge, and weaving segments that have the potential to develop into bottlenecks. To alleviate or mitigate the impacts of congestion at these segments, active management operational strategies have been imple- mented such as ramp metering, hard shoulder running, managed lanes, etc. The current freeway merge and diverge methodologies in Chapter 14 of the 6th edition of the HCM were developed more than 25 years ago using limited field-collected data. Although the weaving segment analysis method in HCM Chapter 13 was updated more recently, the relationship with the merge and diverge methodologies has not been clearly addressed. In addition to limited data, the methodology does not conform to the fundamental relation- ship of traffic flow, namely, that flow is the product of speed and density. The HCM does not offer any method ology for lane drops or additions, which often occur in the vicinity of freeway merge/diverge segments. In the past decade, the data available to traffic engineers have expanded exponentially with broad sensor coverage of urban freeways and probe vehicle coverage of entire roadway networks. These new datasets provide a wealth of information to support the development of updates or changes to the merge, diverge, and weaving segment methodologies and potentially complement traditional data sources. Under NCHRP Project 07-26, the Kittelson & Associates, Inc. research team was asked to develop new methodologies for analyzing freeway merge, diverge, weaving segments and pro- pose updates to the HCM. The proposed new methodologies address data requirements and collection methods; different geometric configurations; consistency across freeway bottle- neck types; and compatibility with basic segment methodologies parameters such as free- flow speed and capacity adjustment factors. Pilot studies in Pennsylvania and Wisconsin were conducted to test the new methodologies, and the methods were revised based on insights gained during the pilots. F O R E W O R D By Ahmad Abu-Hawash Staff Officer Transportation Research Board

In addition to the revised HCM chapters, three other products were developed during this project, including the following: • NCHRP Web-Only Document 343: Traffic Modeling Document containing supplemental details on the collected data and modeling results. • Spreadsheet-based computational engines implementing the proposed methods. • Presentation providing a comprehensive summary of the completed research and the proposed enhancements to methods in the HCM that is intended for a training session that could be delivered virtually or in person. NCHRP Web-Only Document 343, the draft revisions to the HCM chapters, and the products associated with this report can be found on the National Academies Press website (nap.nationalacademies.org) by searching for NCHRP Research Report 1038: Update of Highway Capacity Manual: Merge, Diverge, and Weaving Methodologies. 16497-00b_FM-3rdPgs.indd 6 5/4/23 3:00 PM

1 Summary 6 Chapter 1 Background 6 Overview 7 Research Problem Statement 7 Background 7 Objectives 7 Research Approach 8 Literature Review 8 Introduction 9 General Considerations 11 Merges 18 Diverges 19 Freeway Weaving 26 Potentially Relevant Recent Freeway Basic Segment Research 27 Literature Synthesis and Knowledge Gaps 28 Merge Segments 30 Diverge Segments 31 Weaving Segments 33 Relevant Basic Segment Research 34 Results of Literature Review 34 HCM Methodologies 34 Overview of the Current HCM Freeway Analysis Methodology 43 Modeling Freeway Management Strategies 45 Summary 47 Chapter 2 Research Approach 47 Overview 47 Research Needs Prioritization 50 Data Collection 51 Data Sources 51 Site Selection Process 54 Data Quality Control Procedure 56 Operations Data Collection 60 Metadata Collection 60 Field-Capacity Estimation Approach 60 Speed and Flow Rate Aggregation 60 Lane-to-Approach Aggregation 61 Capacity Estimation 63 Weibull Parameter Estimation 63 Data Cleaning 64 Using the Speed–Flow Curve Instead of the Weibull Distribution to Estimate Capacity 64 Removal of Sites from Model Development C O N T E N T S

66 Final Sites 66 Modeling Framework 66 Weaving Analysis in the HCM 6th Edition 68 Weaving Analysis in NCHRP 07-26 70 Merge and Diverge Analysis in the HCM 6th Edition 72 Merge and Diverge Analysis in NCHRP 07-26 73 Summary 75 Chapter 3 Findings and Applications 75 Overview 75 Model Results for Speed Estimation 75 Weaving Sites 81 Simple Merge Sites 84 Simple Diverge Sites 86 Speed Estimation Sensitivity Tests 88 Variable Ranges for Speed Estimation 88 Model Results for Density-at-Capacity Estimation 88 Empirical Evidence for Revisiting Density-at-Capacity Assumptions 89 Simple Weave Sites 90 Simple Merge Sites 91 Simple Diverge Sites 91 Model Results for Capacity Estimation 91 Simple Weave Sites 92 Simple Merge Sites 94 Simple Diverge Sites 94 Model Validation 95 Speed Model 98 Capacity Model 98 Ramp Metering 100 Pilot Implementation Testing 100 Workshop Preparations 100 Workshop Outline 103 Workshop Summary 106 Recommended Changes to HCM Chapters 107 Chapter 4 Conclusions and Suggested Research 107 Overview 107 Recommended Speed Model 107 Weaving Segments 108 Merge Segments 108 Diverge Segments 108 Discussion 109 Recommended Density at Capacity 109 Recommended Capacity Models 109 Weaving Segments 110 Merge Segments 111 Diverge Segments 111 Comparison of the Recommended Models to the HCM 6th Edition Models 113 Suggested Future Research 115 Chapter 5 References