Determining Guidelines for Ramp and Interchange Spacing (2011)

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Chapter 4 Guidelines

NCHRP 3-88 Final Report Guidelines for Ramp and Interchange Spacing Chapter 4: Guidelines 4-1 Chapter 4 GUIDELINES Two final products were produced the NCHRP 3-88 project team: this final research report, and NCHRP Report 687: Guidelines for Ramp and Interchange Spacing (Guidelines). The Guidelines assist users as they consider the feasibility of new or rebuilt interchanges and ramps. The Guidelines were developed primarily for ramps and interchanges on fully-controlled access freeways, but could also be applied to ramps and interchanges on partially-controlled access highways. The Guidelines are not a standard, but rather a “how to” document that is intended to be informative and present a processed for assessing spacing values. As part of this process, The Guidelines provide insights into the factors that influence minimum ramp and interchange spacing dimensions for various interchange forms and ramp combinations. Chapter 5 of this report discusses proposed changes to the AASHTO Green Book to create consistency between it and the Guidelines. Prior to NCHRP Project 3-88, little research focused on ramp and interchange spacing had been conducted in recent decades. Rules of thumb such as one mile minimum interchange spacing in urban areas and two mile minimum interchange spacing in rural areas date from the early days of the Interstate Highway System. The minimum recommended ramp spacing values in the AASHTO’s Green Book stem from publications that date from the 1970’s. The Guidelines present substantial background information related to freeway and interchange geometric design, traffic operations, safety, and signing. The information is drawn from major resource documents such as the AASHTO Green Book, Highway Capacity Manual, Manual of Uniform Traffic Control Devices, Highway Safety Manual, and ITE Freeway and Interchange Geometric Design Handbook; other past studies; and research conducted as part of NCHRP Project 3-88. The Guidelines present a framework for evaluating ramp and interchange spacing, and provide insights into the factors that influence minimum ramp and interchange spacing dimensions for various interchange forms and ramp combinations. Chapter 1 introduces the Guidelines and presents their purpose, scope, and applicability. This chapter also defines and differentiates ramp spacing and interchange spacing. The Guidelines emphasize that ramp spacing, rather than interchange spacing, should be the primary considering when determining the adequacy of spacing. Chapter 2 presents an overview of project development, policies relevant to ramp and interchange spacing, and major resource documents used by transportation professionals. As the project development process advances, there is both an increasing amount of information that can be used to make

Final Report NCHRP 3-88 Chapter 4: Guidelines Guidelines for Ramp and Interchange Spacing 4-2 ramp and interchange spacing assessments and a decreasing flexibility in changing spacing values. Federal, state, and in some cases local agencies generally must approve new or substantially modified interchanges, and have policies and processes for doing so. When assessing ramp and interchange spacing and complying with relevant policies, users are likely to rely on the AASHTO Green Book, the HCM, the MUTCD, and the HSM. Chapter 3 presents geometric design and signing considerations. A multitude of geometric features influence the design of ramps and interchanges and choices about the spacing between them. The number and type of lanes, the types and forms of interchanges, the design of ramps, and the surrounding terrain all effect choices about ramp and interchange spacing. Additionally, the AASHTO Green Book and many state-level documents provide minimum recommended ramp spacing values for both ramps and interchanges. Signing plays a role in exit ramp spacing decisions. The MUTCD recommends that a certain sequence of signs be used in advance of an interchange, but also recommends limits on the number of sign panels and message units at any one location so that drivers are not overwhelmed with information. Chapter 4 presents traffic operations and safety considerations. Traffic operations analysis and safety analysis can be performed in the early stages of a project with planning level tools from the HCM, the ITE Freeway Handbook, and the research conducted for this project. Chapter 4 explains how to conduct such planning-level assessments, as well as complete HCM or safety analyses at later stages of a project when a complete traffic forecast is available and the level of design provides greater certainty with respect to spacing values. Traditionally, the transportation profession has addressed safety with a nominal approach – a given design is either “safe” or “unsafe”. “Safe” designs are presumed to result from compliance with standards and guidelines. The transportation profession is shifting away this approach, and instead developing an approach where expected crash frequencies for various design alternatives will be known and used in decision-making. The Guidelines take this latter approach to safety. Chapter 5 offers spacing guidance to practitioners. The chapter begins with a framework for considering ramp and interchange configurations. This framework integrates the considerations discussed in Chapters 2, 3, and 4. Interchange spacing guidance is provided with a table containing three pairs of interchanges (with different forms). For each paring, ranges of spacing that are likely not geometrically feasible, potentially geometrically feasible, and likely geometrically feasible are identified.

NCHRP 3-88 Final Report Guidelines for Ramp and Interchange Spacing Chapter 4: Guidelines 4-3 Ramp spacing guidance is offered based upon geometry, traffic operations, safety, and signing. • Based upon geometry, the Guidelines identified ranges of spacing that are likely not geometrically feasible, potentially geometrically feasible, and likely geometrically feasible for each of the four possible ramp combinations. • The Guidelines offer traffic operations considerations for closely spaced ramp combinations based upon the HCM. The Guidelines offer additional traffic operations considerations for closely- spaced entry-exit and entry-entry ramps based upon the findings of this project. • The Guidelines presents the relative crash risk associated with changing ramp spacing values for entry-exit and entry-entry ramp combinations. • Based upon signing, the Guidelines recommends minimum values for the spacing between consecutive exit ramps and the number of exit ramps on a one-mile segment of freeway. Chapter 6 contains five scenario-based case studies that illustrate and apply the principles of the Guidelines. Appendix A contains operational analysis tools developed from the HCM and from the simulation modeling conducted as part of this research project.