Guidelines for Ramp and Interchange Spacing (2011)

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TRANSPORTAT ION RESEARCH BOARD WASHINGTON, D.C. 2011 www.TRB.org 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 REPORT 687 Subscriber Categories Highways • Design Guidelines for Ramp and Interchange Spacing Brian L. Ray James Schoen Pete Jenior Julia Knudsen KITTELSON & ASSOCIATES, INC. Portland, OR I N A S S O C I A T I O N W I T H Richard J. Porter UNIVERSITY OF UTAH Salt Lake City, UT Joel P. Leisch Bend, OR John Mason Auburn, AL Roger Roess Brooklyn, NY Traffic Research & Analysis, Inc. Phoenix, AZ Research sponsored by the American Association of State Highway and Transportation Officials in cooperation with the Federal Highway Administration

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CRP STAFF FOR NCHRP REPORT 687 Christopher W. Jenks, Director, Cooperative Research Programs Crawford F. Jencks, Deputy Director, Cooperative Research Programs Christopher Hedges, Senior Program Officer Danna Powell, Senior Program Assistant Eileen P. Delaney, Director of Publications Maria Sabin Crawford, Assistant Editor NCHRP PROJECT 03-88 PANEL Field of Traffic—Area of Operations and Control Larry F. Sutherland, Parsons Brinckerhoff, Columbus, OH (Chair) Kenneth T. Briggs, KCI Technologies Inc., Sparks, MD J. Michael Y. Ereti, City of Houston, Houston, TX Jeff C. Jones, Tennessee DOT, Nashville, TN Steve King, Kansas DOT, Topeka, KS Eil Kwon, University of Minnesota - Duluth, Duluth, MN Thomas A. Parlante, Arizona DOT (retired), Phoenix, AZ Zhongren Wang, California DOT, Sacramento, CA Mark Doctor, FHWA Liaison Stephen F. Maher, TRB Liaison 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

This report provides guidelines for ramp and interchange spacing based on design, oper- ations, safety, and signing considerations. The guidelines will be valuable to transportation agencies who need to balance system efficiency and safety with the need to provide access for local users. The guidelines are intended to aid the decision-making process when an agency is considering new ramps or interchanges on existing facilities, modifying ramps and interchanges of existing facilities, or when planning and designing new highway and inter- change facilities. The guidelines also offer standardized definitions measuring ramp and interchange spacing, which have varied in previous design manuals guides. Interchanges are essential components of freeways for providing reasonable access and mobility. However, interchanges can greatly diminish the traffic operations, safety, and capac- ity of the through lanes of the freeway. Transportation agencies are tasked with constructing new freeways with interchanges, reconstructing existing freeways and interchanges, and adding interchanges to existing freeways. The AASHTO Policy on Geometric Design of Highways and Streets contains guidelines on the distance between successive ramp terminals. On urban freeways and other facilities that carry large traffic volumes, two or more ramp terminals are often located in close succes- sion. To provide adequate space for signing, adequate gaps for entering motorists, and suf- ficient weaving lengths, the AASHTO policy provides minimum ramp terminal spacing dimensions for various ramp pair combinations. Spacing between successive ramp termi- nals depends on the classification of the interchanges involved, the function of the ramp pair (entrance vs. exit), and the potential for weaving. The guidelines provided in the AASHTO policy are acknowledged to be based on operational experience and recommend basing actual spacing on operations and safety procedures derived from applied research. Although the location and spacing of interchanges and ramps on freeways has a major effect on the ability of a freeway to carry traffic effectively, this is a topic for which little research or literature has been published. Recent research indicates that a majority of freeway acci- dents occur at interchanges and in weaving sections between closely spaced entrance and exit ramps. The spacing of interchanges on an urban road network can also result in tradeoffs between providing adequate service and access with both safety and operations. As a result, making sound decisions requires a clear understanding of the impacts of ramp and inter- change spacing on safety and operations. Under NCHRP Project 03-88, “Guidelines for Ramp and Interchange Spacing,” a research team led by Kittelson & Associates, Inc., evaluated and summarized design, operations, safety, and signing considerations that influence ramp and interchange spacing decisions. The team conducted simulation modeling, calibrated with field data, of closely-spaced pairs F O R E W O R D By Christopher Hedges Staff Officer Transportation Research Board

of ramps and developed safety performance models. The team then developed guidelines to assist practitioners in selecting ramp and interchange spacing values for their particular design context. The selection criteria include geometric design needs, operational perfor- mance, signing needs, and safety performance. The results will also provide information that can also be incorporated in future editions or updates of relevant AASHTO manuals, includ- ing the Policy on Geometric Design, the Highway Capacity Manual, the Manual on Uniform Traffic Control Devices, and the Highway Safety Manual. A final report documenting the full research effort will be posted on the TRB website as NCHRP Web-Only Document 169 and can be found at http://www.trb.org/Main/Blurbs/164815.aspx.

C O N T E N T S ix Preface 1 Summary 3 Chapter 1 Introduction 3 1.1 Ramp and Interchange Spacing Definitions 5 1.2 Intended Users 5 1.3 Scope of Guide 6 1.4 Relationship to Recommended Resource Documents 7 Chapter 2 Ramp and Interchange Spacing Overview 7 2.1 Introduction 9 2.2 Policy Considerations 13 2.3 Major Published Resource Documentation 14 Chapter 3 Design and Signing Considerations 14 3.1 Overarching Design and Operational Relationships 17 3.2 Interchange Categories 19 3.3 Ramps 29 3.4 Auxiliary Lanes 30 3.5 Terrain and Grades 30 3.6 Vehicle Fleet 31 3.7 Relationship to the AASHTO Policy 33 3.8 Relationship to State-level Guidance 34 3.9 Human Factors Considerations 41 Chapter 4 Operational and Safety Considerations 41 4.1 Traffic Operations Overview 42 4.2 Highway Capacity Manual Procedures 48 4.3 Other Planning-level Operational Guidelines 52 4.4 Microsimulation 53 4.5 Safety 62 Chapter 5 Spacing Guidance 62 5.1 Guidelines Framework 71 5.2 “Interchange” Versus “Ramp” Spacing 75 5.3 Ramp Spacing Assessments 91 5.4 Spacing Guidance Summary 92 Appendix A Scenario Based Case Studies 168 Appendix B Traffic Operations Tools 181 References

Preface Freeway facilities are intended to provide a high level of mobility and allow drivers to travel safety and efficiently. Freeways are characterized by limited access, with all connections to adjacent roads and land provided with ramps freeway performance, especially if placed too close to one another. Planners, designers, and operators of freeways must strike a balance between preserving the mobility of facility and providing access for local users. These Guidelines provide users with aids and tools to consider and evaluate potential impacts of ramp spacing, as well as information on many factors that influence ramp and interchange spacing needs. Under NCHRP Project 03-88, the research team conducted operational and safety assessments of two types of ramp pairs—an entry ramp followed by an exit ramp (EN-EX) and an entry ramp followed by another entry ramp (EN- EN). Future research results may allow similar assessments of other ramp combinations. Furthermore, NCHRP 03-88 research was focused on relatively simple, single lane, service ramps and interchanges. Additional research may be valuable to address larger, more complex designs such as multi-lane ramps and system interchanges. Until future research is completed the principles and fundamental approach suggested in these Guidelines should be applicable in considering those ramp and interchange spacing needs. These Guidelines present substantial discussions on geometric design, traffic operations, safety, and signing, and the role each of these play in determining ramp and interchange spacing needs. The Guidelines define “ramp spacing” and “interchange spacing” and recommend ramp spacing values be the primary consideration in freeway and interchange planning and design. Geometric design principles, as well as site-specific features, dictate minimum lengths needed for ramps and other interchange components. Traffic volumes can necessitate increased spacing beyond the dimensions needed purely for geometrics. Safety tradeoffs, which have rarely been quantified until recently, can now be considered in project decision making. Finally, signing and other human factors considerations should be taken into account at the earliest in the evaluation process when making choices about ramp and interchange spacing. The transportation profession is beginning to move away from rigid design criteria towards performance-based metrics that allow flexibility in design while still meeting the needs of system users. The prominence of safety effects of design choices is increasing within the profession as evidenced by the publication of the first addition of the Highway Safety Manual. Both of these trends are reflected in the Guidelines. The Guidelines present ranges of minimum recommended spacing dimensions for various conditions rather than single “one-size-fits-all” dimensions. Additionally, the Guidelines advocate for quantifiable safety analysis on par with operational analysis and evaluations. and interchanges. These ramps and interchanges are known to impact