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NCHRP 3-88 Final Report Guidelines for Ramp and Interchange Spacing iii Abstract This project considered the effects of geometry, traffic operations, safety, signing, and other factors to develop guidelines for understanding the considerations that influence minimum ramp and interchange spacing values. Phase I included conducting a literature search and other information gathering activities, developing a work plan to assess the impact of ramp spacing on traffic operations and safety, and developing a framework for the research guidelines. Phase II included creating microscopic simulation models of closely-spaced ramp combinations calibrated with field data, constructing a crash database and developing crash prediction models, developing a set of guidelines for ramp and interchange spacing, and recommending changes to major resource documents within the transportation profession. Traffic operations research investigated the impact of ramp spacing on mainline freeway speed and was primarily focused on two ramp combinations: an entry ramp followed by and exit ramp and an entry ramp followed by another entry ramp. Entry-exit combinations were studied with and without auxiliary lanes. These combinations were selected by the research team and project panel as having the greatest amount of vehicle interaction and thus would be most sensitive to the distance between the ramps. To supplement the findings of the simulation models, the research team reviewed datasets from several previous NCHRP projects. Safety research analyzed a dataset constructed specifically for this project and contained detailed information on ramp and interchange geometry. The team developed crash prediction models for entry-exit and entry-entry ramp combinations. Models considered the total number, type, and severity of crashes. The team attempted to include exit-entry and exit-exit ramp combinations in the analysis but this could not be done for sample size reasons. As a final product, the team wrote NCHRP Report 687: Guidelines for Ramp and Interchange Spacing (âGuidelinesâ) to assist users as they consider new or modified ramps and interchanges. The Guidelines stress the importance of ramp spacing, as opposed to interchange spacing, when considering new or modified ramp and interchange concepts and established clear definitions of ârampâ and âinterchangeâ spacing.. The Guidelines provide information on federal, state, and local policies related to ramp and interchange spacing, as well as the effects of geometry, traffic operations, signing, and safety. The Guidelines and intended to supplement existing resource documents such as the AASHTO Green Book, the Highway Capacity Manual, the Manual of Uniform Traffic Control Devices, and the Highway Safety Manual.
