Many contextual 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 guidelines for selecting appropriate ramp design speeds, a summary review of the state of the practice, and recommended revisions for the AASHTO Green Book. The guidelines should facilitate the selection of an appropriate ramp design speedbased on a combination of contextual considerations and quantitative information. The guidelines should address the definition of ramp design speed, specifically the portion(s) of the ramp to which design speed should be applicable. The guidelines should also address the known 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. Development of the guidelines may include the examination of field data and site observations supplemented with safety modeling considerations and traffic operational simulations. Speed transitions within ramps should be reviewed for performance and design policy implications.
Accomplishment of the project objectives will require at least the following tasks.
(1). Review relevant practice, performance data, research findings, and other information related to ramp design speed selection. This information shall be assembled from technical literature and from unpublished experiences of planners, engineers, designers, safety professionals, construction managers, operations managers, and others. Information on actual field performance and empirical knowledge—especially as it relates to the results of currently applied design strategies and state procedures—is of particular interest. (2). Analyze the literature and practices collected in Task 1 to develop (a) recommended definitions of ramp design speed terms (and their applicability) suitable for adoption by the relevant AASHTO committee(s), (b) a recommended data collection and analysis plan to be conducted in Phase II, and (c) a detailed outline for the guidelines to be developed in Phase II. (3). Prepare an interim report on the information developed in Tasks 1 and 2. The interim report shall also contain (a) a summary of agency practices and (b) a detailed Phase II work plan.
(4). Carry out the approved Phase II work plan to develop (a) the guidelines for selecting appropriate ramp design speeds and (b) recommended revisions for the AASHTO Green Book.
The ramp design speed guidelines should address at a minimum:
- System and service interchanges
- Entrance and exit ramps
- Different types of ramp interfaces at crossroads
- Single and multi-lane ramps
- Degree of physical site constraints, environmental and social impacts, and right-of-way costs
- Effects of ramp grade
- Use of transition curves into the controlling alignment element
- Presence of ramp meters
- Ramp terminal configuration (e.g., tapered and parallel, presence of auxiliary lane, etc.)
- Applicability of ramp design controls as opposed to mainline design controls for certain connections
(5). Submit a final report that documents the entire research effort and includes (a) the summary of agency practices, (b) the guidelines as a stand-alone document, (c) the recommended Green Book revisions, and (d) an updated PowerPoint presentation describing the research and results suitable (upon revision) for posting on the TRB website.
STATUS: Research in progress. An interim meeting was held in September 2016. A draft final report is anticipated in August 2017.