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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2021. Diverging Diamond Interchange Informational Guide, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/26027.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2021. Diverging Diamond Interchange Informational Guide, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/26027.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2021. Diverging Diamond Interchange Informational Guide, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/26027.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2021. Diverging Diamond Interchange Informational Guide, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/26027.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2021. Diverging Diamond Interchange Informational Guide, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/26027.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2021. Diverging Diamond Interchange Informational Guide, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/26027.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2021. Diverging Diamond Interchange Informational Guide, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/26027.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2021. Diverging Diamond Interchange Informational Guide, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/26027.
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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 959 Diverging Diamond Interchange Informational Guide SECOND EDITION Christopher Cunningham Thomas Chase Yulin Deng Chris Carnes Kihyun Pyo InstItute for transportatIon research and educatIon Raleigh, NC Pete Jenior Bastian Schroeder Brian Ray Thomas Urbanik II Julia Knudsen Lee Rodegerdts Shannon Warchol KIttelson & assocIates, Inc. Portland, OR Alison Tanaka City of Portland, Oregon Subscriber Categories Highways • Design • 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.nationalacademies.org and then searching for TRB Printed in the United States of America NCHRP RESEARCH REPORT 959 Project 03-113 ISSN 2572-3766 (Print) ISSN 2572-3774 (Online) ISBN 978-0-309-67369-3 Library of Congress Control Number 2020951593 © 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 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 NCHRP Project 03-113a and 03-113b through the Institute for Transpor- tation Research and Education at North Carolina State University (ITRE NCSU); Kittelson & Associates, Inc. (Kittelson); Kimley-Horn and Associates, Inc. (Kimley-Horn); Wayne State University; and Horrocks Engineers. ITRE NCSU was the contractor for this study. The work undertaken by other project team members occurred under subcontracts with ITRE. The authors of this guide are Chris Cunningham (Principal Inves- tigator) (ITRE NCSU); Thomas Chase, Yulin Deng, Chris Carnes, and Kihyun Pyo of ITRE; Pete Jenior, Bastian Schroeder, Brian Ray, Thomas Urbanik II, Julia Knudsen, Lee Rodegerdts, and Shannon Warchol of Kittelson; and Alison Tanaka of the City of Portland, Oregon. During the project, Bastian Schroeder and Shannon Warchol transitioned from ITRE NCSU to Kittelson; Alison Tanaka transi- tioned from Kittelson to the City of Portland, Oregon. The authors wish to thank other members of the project team who contributed to this document, including Tim Nye, Katy Salamati, and Chunho Yeom, all formerly of ITRE NCSU; Brandon Nevers, Jim Bonneson, Ralph Bentley, Sara Parks, Alek Pochowski, and John Ringert of Kittelson; Stacie Phillips of Kimley-Horn; Taylor Honeycutt of Exult Engineering (formerly of Kimley-Horn); Joe Hummer of North Carolina Department of Transportation (formerly Wayne State University); and Michael Murkley of the City of Orem, Utah (previously Horrocks Engineers and Pinetop Engineer- ing). These individuals oversaw experiments, performed supple- mental analysis, served as expert advisors, and provided graphics support. The authors also wish to thank the states of Georgia, Utah, and Missouri for hosting focus groups to identify safety and geometric research needs and the participants in the focus groups. The research team also acknowledges the NCHRP Project 07-25 project team. Chapter 3 of this diverging diamond interchange (DDI) guide was developed from content currently being prepared for NCHRP Project 07-25, “Guide for Pedestrian and Bicycle Safety at Alternative Intersections and Interchanges.” Finally, the project team thanks the authors, FHWA reviewers, and FHWA task manager of the first edition guide, which formed the basis of this guide and undoubtedly influenced a large percentage of the DDIs in existence in the United States today. CRP STAFF FOR NCHRP RESEARCH REPORT 959 Christopher J. Hedges, Director, Cooperative Research Programs Lori L. Sundstrom, Deputy Director, Cooperative Research Programs B. Ray Derr, Senior Program Officer Anthony P. Avery, Program Associate Eileen P. Delaney, Director of Publications Natalie Barnes, Associate Director of Publications NCHRP PROJECT 03-113 PANEL Field of Traffic—Area of Operations and Control James H. Dunlop, North Carolina Department of Transportation, Raleigh, NC (Chair) Gilbert M. Chlewicki, Advanced Transportation Solutions, LLC, Silver Spring, MD Michael LaCroix, Vermont Agency of Transportation, Montpelier, VT Lori Lange, City of Brentwood, TN Laurence Lucius Lambert II, Vectura Consulting Services, LLC, Baton Rouge, LA Zhixia Li, University of Louisville, Louisville, KY Melissa Ruth Longworth, Michigan Department of Transportation, Lansing, MI Eric Rasband, Utah Department of Transportation, Orem, UT James R. Sturdevant, Indiana Department of Transportation, Indianapolis, IN David Warrick, Oregon Department of Transportation, Salem, OR Wei Zhang, FHWA Liaison Richard A. Cunard, TRB Liaison

NCHRP Research Report 959 presents a comprehensive guide to the design and operation of diverging diamond interchanges, and updates material found in the FHWA’s Diverging Diamond Interchange Informational Guide. It addresses the needs of planners, designers, and operators, and considers all modes of travel. The diverging diamond interchange (DDI, also known as a double crossover diamond interchange) is a relatively new design to the United States. This design can increase through- put and safety without widening bridge structures. Determination of the best geometric and traffic signal design depends on the appropriate use of analysis tools, particularly microscopic simulation models. Many traffic signal designers and operators and geometric designers lack experience with this novel design. Under NCHRP Project 03-113, “Guidance for Traffic Signals at Diverging Diamond Interchanges and Adjacent Intersections,” the research team led by the Institute for Trans- portation Research and Education of North Carolina State University developed guidance on the geometric and traffic signal design of DDIs and safety and operational analysis of design alternatives. The research included a literature review for this new design that continued throughout the project, interviews with experienced practitioners, identification of problematic situa- tions that arise at DDI installations (including for pedestrians and bicyclists), simulation of promising design and control strategies, and validation of the guidance with agencies. Ten training workshops were conducted with agencies that represented a broad range of experience with DDIs; the workshops were funded under NCHRP Project 20-44, “NCHRP Implementation Support Program.” F O R E W O R D By B. Ray Derr Staff Officer Transportation Research Board

1 Chapter 1 Introduction 1 1.1 Overview of Alternative Intersections and Interchanges 1 1.2 Intersection Control Evaluations and Considerations 2 1.3 Organization of the Guidelines 2 1.4 Scope of the Guide 2 1.5 DDI Overview 3 1.6 Application 7 1.7 Geometric Design Considerations 7 1.8 Resource Documents 8 1.9 References 9 Chapter 2 Policy and Planning 9 2.1 Planning Considerations for Alternative Intersections and Interchanges 9 2.1.1 Serving Pedestrians and Bicycles 10 2.1.2 Traffic Volume Relationships 11 2.2 Stakeholder Outreach 13 2.3 Policy Considerations 15 2.3.1 Access Management 15 2.4 Planning Considerations 16 2.5 Planning Challenges 17 2.6 Project Performance Considerations 17 2.6.1 Accessibility 17 2.6.2 Mobility 17 2.6.3 Quality of Service 17 2.6.4 Reliability 17 2.6.5 Safety 18 2.7 Project Development Process 18 2.7.1 Planning Studies 18 2.7.2 Alternatives Identification and Evaluation 18 2.7.3 Preliminary Design 19 2.7.4 Final Design 19 2.7.5 Construction 19 2.8 References 20 Chapter 3 Multimodal Considerations 20 3.1 Introduction 20 3.1.1 Benefits and Challenges 21 3.1.2 Anticipating Multimodal Needs, Behavior, and Patterns 22 3.2 Facility Selection 22 3.2.1 Pedestrians 28 3.2.2 Bicycles C O N T E N T S

32 3.3 Pedestrian & Bicycle Assessment 32 3.3.1 Assessment Methodology 33 3.3.2 Pedestrians—Key Safety Challenges 34 3.3.3 Bicycles—Key Safety Challenges 44 3.4 Intersection-Level Concepts 44 3.4.1 Shared-Use Path/Inner Walkway Concept 45 3.4.2 On-Street Bike Lane/Outer Walkway Concept 45 3.4.3 Separated Bike Lane/Inner Walkway Concept 48 3.5 Detailed Design Techniques 49 3.5.1 Pedestrian Phase Coordination 50 3.5.2 Pedestrian Channelization and Wayfinding 51 3.5.3 ADA and Accessibility 54 3.5.4 Turning Movements at Ramps 55 3.5.5 Vehicle Movements from Counterintuitive Directions 56 3.5.6 Indirect Paths 56 3.6 Transit 56 3.6.1 Buses on Cross Road 57 3.6.2 Transit on Freeway 57 3.6.3 Light-Rail on Cross Road 58 3.7 References 59 Chapter 4 Safety 59 4.1 Safety Principles 59 4.1.1 Conflict Points 59 4.1.2 Right Turn at Exit Ramp 61 4.1.3 Left Turn at Exit Ramp 62 4.1.4 Wrong-way Maneuvers 62 4.1.5 Incident Response Considerations 62 4.2 Observed Safety Performance 62 4.2.1 Literature Summary 63 4.2.2 Predicted Crash Modification Factors 66 4.3 References 67 Chapter 5 Conceptual Operations 67 5.1 Preliminary Operations Considerations 67 5.1.1 Directional Naming Conventions 67 5.1.2 Operating Zones 69 5.1.3 Critical Origin-Destination Movements 70 5.2 Signal Timing Conventions 70 5.2.1 Movement Numbering 72 5.2.2 Phase Numbering 73 5.2.3 Phases Versus Intervals 73 5.2.4 Ring-and-Barrier Diagram 75 5.2.5 Time-Space Diagram 76 5.2.6 Signal Timing Parameter Considerations 80 5.2.7 Phasing Scheme Layout 80 5.3 Phasing Schemes 81 5.3.1 Two Critical Movements 81 5.3.2 Three Critical Movements 85 5.3.3 Four Critical Movements 88 5.3.4 Specialized Signal Timing Applications 91 5.3.5 Understanding the Benefits and Challenges of DDI Phasing Schemes

92 5.4 System Needs 93 5.4.1 DDI Corridor Signal Timing 98 5.4.2 Corridor Operational Considerations and Common Challenges 101 5.4.3 Assessing Signal Phasing Schemes in a Corridor Context 105 5.4.4 Strategies to Improve Corridor Operations 109 5.5 Operational Analysis 109 5.5.1 Selecting the Appropriate Level of Analysis 110 5.5.2 Planning-Level Analysis 116 5.5.3 HCM6 DDI Method 117 5.5.4 Microsimulation 120 5.6 References 121 Chapter 6 Geometric Design 121 6.1 Overview 121 6.1.1 Integrating with ICE Activities 121 6.1.2 Optimizing for Project Context and User Type 121 6.1.3 Considering Each Project Development Stage 122 6.2 Principles and Objectives 122 6.2.1 DDI Planning and Design User Considerations 124 6.2.2 Project Type 125 6.2.3 Project Context 125 6.2.4 DDI Performance Considerations 126 6.2.5 Crossover and Ramp Terminal Intersection Definitions 128 6.3 Project Constraints 128 6.3.1 Overall Footprint 129 6.3.2 Indirect Impacts 130 6.3.3 Adapting to Site Constraints 130 6.3.4 Constraints at the Crossover and Ramp Terminal Intersections 130 6.3.5 Matching to an Existing Highway Crossing 135 6.3.6 Existing Ramp Locations 135 6.4 Horizontal Alignment 136 6.4.1 Alignment Fundamentals 136 6.4.2 Developing a DDI Layout 137 6.4.3 Effect of Skew on Crossovers and Ramp Terminal Intersections 140 6.4.4 Lane Numbers and Arrangements 141 6.4.5 Design User and Type 145 6.4.6 Crossroad Alignment Design 146 6.4.7 Crossover Design 153 6.4.8 Ramp Terminal Intersection 155 6.4.9 Adjacent Driveways and Intersections 156 6.4.10 Developing a Design from the Inside Out 161 6.4.11 Performance Checks and Considerations 161 6.5 Cross Section 162 6.5.1 Overpass/Underpass Cross Section 163 6.5.2 Lane Width 166 6.5.3 Shoulder Width 166 6.5.4 Bicycle Lanes 167 6.5.5 Sidewalk 167 6.5.6 Crosswalks 167 6.6 Vertical Alignment

168 6.7 Performance Checks 170 6.7.1 General Performance Categories 176 6.7.2 DDI-Specific Performance Categories 186 6.8 References 188 Chapter 7 Traffic Control Devices and Illumination Applications 188 7.1 Traffic Control Device Design Principles and Approach 189 7.2 Regulatory and Warning Signs 189 7.2.1 Movement Prohibition and Lane Control Signs 190 7.2.2 Signs for Directing Traffic Around Islands 192 7.2.3 Advance Warning Signs 193 7.3 Guide Signs 195 7.4 Pavement Markings 195 7.4.1 Centerlines and Edge Lines 195 7.4.2 Lane Lines 197 7.4.3 Lane Use Arrows 197 7.4.4 Stop and Yield Lines 198 7.4.5 Pedestrian Crosswalk Markings 198 7.4.6 Bicycle Lanes 199 7.5 Signal Equipment 199 7.5.1 Number of Controllers 201 7.5.2 Signal Cabinet Details 202 7.5.3 Signal Poles and Displays 209 7.5.4 Detection 210 7.5.5 Communications 210 7.5.6 Preliminary Cost Estimate 210 7.6 Signal Timing Parameters 211 7.7 Illumination 211 7.7.1 Complete Versus Partial Interchange Lighting Systems 213 7.7.2 Pedestrian Lighting 214 7.8 Document Local Practices 214 7.9 References 215 Chapter 8 Construction and Implementation 215 8.1 Construction Staging 217 8.2 Cost Estimates 218 8.3 Maintenance 219 8.4 Snow Removal 219 8.5 Law Enforcement Needs 219 8.6 References A-1 Appendix Safety Details

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The diverging diamond interchange (also known as a double crossover diamond interchange) is a relatively new design to the United States. This design can increase throughput and safety without widening bridge structures.

The TRB National Cooperative Highway Research Program's NCHRP Research Report 959: Diverging Diamond Interchange Informational Guide, Second Edition presents a comprehensive guide to the design and operation of diverging diamond interchanges and updates material found in the FHWA’s Diverging Diamond Interchange Informational Guide.

A workshop summary is provided that includes an overview of key traffic signal timing concepts at diverging diamond interchanges—from terminology to timing considerations and from operational analysis to traffic signal equipment. Videos viewed during the workshop are also provided.

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