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Summary NCHRP Report 687: Guidelines for Ramp and Interchange Spacing assists roadway planners and designers as they consider the feasibility of new or rebuilt interchanges and ramps. The Guidelines are not intended to set ramp and interchange spacing standards. Rather, they are informational and present a process for assessing spacing within the context of each design environment. These Guidelines were produced as part of NCHRP Project 03-88, which studied the relationship between ramp and interchange spacing and geometric design, traffic operations, safety, and signing. These Guidelines define interchange spacing as the distance between the centerlines of successive crossroads with interchanges on a freeway. Ramp spacing is defined as the distance between the painted tips of successive ramps. The Guidelines were developed primarily for ramps and interchanges on fully controlled access freeways but could also be applied on ramp and interchanges on partially controlled access highways. 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 AASHTOâs Policy on Geometric Design of Highways and Streets (Green Book) stem from publications that date from the 1970s. Research conducted as part of NCHRP Project 3-88 was primarily focused on two areas. Operations research investigated the impact of ramp spacing on freeway speed. Safety research investigated the impact of ramp spacing on crash frequency and severity. Due to the wide variety of interchange forms and ramp designs, these Guidelines emphasize the importance of ramp spacing versus interchange spacing. The Guidelines contain 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 on 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 03-88. 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 purpose, scope, and applicability of the Guidelines. Chapter 2 presents policy considerations, Chapter 3 presents geometric 1
design and signing considerations, and Chapter 4 presents traffic operations and safety considerations. Chapter 5 provides users with a framework for evaluating the adequacy of ramp and interchange configurations with regard to spacing and includes insights into factors that influence minimum dimensions. Appendix A provides five case studies to illustrate and apply the framework and considerations from Chapter 5. Appendix B provides additional traffic operations data from NCHRP Project 03-88 that addresses scenarios not directly addressed by the Highway Capacity Manual. The data quantifies the impact of ramp spacing on freeway speed and the benefit spaced entrance-exit ramp pair. (speed increase) associated with adding an auxiliary lane between a closely 2 Guidelines for Ramp and Interchange Spacing
