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The National Academies

NCHRP 15-56 [Completed]

Guidelines for Selecting Ramp Design Speeds

  Project Data
Funds: $400,000
Research Agency: MRIGlobal
Principal Investigator: Dr. Darren Torbic
Effective Date: 11/10/2015
Completion Date: 6/9/2021
Comments: Research is complete. Publication decision pending.

Many factors should be considered in selecting an appropriate ramp design speed. Selecting a design speed for highway ramps can greatly influence project costs, right-of-way needs, and the potential project environmental and social impacts. General ramp design considerations, including ramp design speeds, are presented in Chapter 10 of the AASHTO Green Book, but the rationale of the existing guidance is in need of review. Additionally, the application of the design speed along all portions of a ramp and across the spectrum of interchange forms does not sufficiently take into account the inherent speed profiles of different ramp types. Research is needed to provide enhanced guidance on ramp design practices to facilitate the selection of more cost efficient, safer, and contextually appropriate interchange designs in all regions of the United States.
 
The objectives of this research are to develop a guide to assist in the selection of appropriate ramp design speeds, a summary review of the state of the practice, and recommended revisions for the AASHTO Green Book. The guide should facilitate the selection of an appropriate ramp design speed based on a combination of contextual considerations and quantitative information and address the definition of ramp design speed, specifically the portion(s) of the ramp to which design speed should be applicable. Content covers the safety and operational aspects of ramp design speed selection over a full range of interchange forms, ramp types, and area environments (rural vs. urban). The geometric design dimensions resulting from the suggested ramp design speeds (such as minimum radius of curvature) must also consider driver expectations and behaviors over a range of traffic conditions and the functional classification of the two interchanging roadways.  

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