The National Academies

NCHRP 15-31A [Final]

Design Guidance for Freeway Mainline Ramp Terminals

  Project Data
Funds: $380,000
Research Agency: Midwest Research Institute
Principal Investigator: Darren Torbic
Effective Date: 11/11/2008
Completion Date: 6/10/2011

The objective of this research was to develop improved design guidance for freeway mainline ramp terminals suitable for inclusion in the AASHTO Green Book. As appropriate, the guidance also addresses issues related to the design of the gore area and any transitional area to the ramp proper.


PHASE I [NOTE: Tasks 1-3 were carried out in NCHRP Project 15-31]
 Task 1.  Review relevant literature on freeway mainline ramp terminal design, operation, and safety and on vehicle-fleet mix and characteristics.
 Task 2.  Develop conceptual models for freeway entrance and exit maneuvers that represent the driver's thought processes and decision points. List critical factors and issues for ramp terminal design and describe interrelationships between them. Identify characteristics of the ramp proper that affect the design of the terminal.

Task 3.  Determine the design vehicles that may be appropriate for developing the design guidance and specify their characteristics.
 Task 4.  Prepare an interim report that documents the research performed in Phase I and includes an updated work plan for collecting data, analyzing data, and developing the products described in Phase II. Following review of the interim report by the NCHRP, the research team will be required to make a presentation to the project panel. Work on Phase II of the project will not begin until the interim report is approved and the Phase II work plan is authorized by the NCHRP.

 Task 5.  For entrance ramps, determine visibility and driver-decision requirements needed for merging cars and trucks to find a gap in the mainline traffic. These requirements may apply to the ramp proper and gore area in addition to the ramp terminal.
 Task 6.  Develop acceleration and deceleration curves plotting speed (0 to 75 mph) versus distance for the design vehicle(s) defined in Task 3 on various grades. These curves must be based on physical data rather than computer modeling (though models may be used for interpolation if appropriate). These curves will be useful in developing the design guidance but will also be useful to designers analyzing other geometric elements such as climbing lanes.
 Task 7.  Evaluate different designs for multilane entrances and exits (including interior merges) and determine the most effective.
Task 8.  Develop design guidance for parallel and tapered freeway mainline ramp terminals.
 Task 9.  Assess how low-speed ramps affect the safety and operation of ramp terminals and how the design of the terminal and gore area should be altered to compensate.
 Task 10.  Assess how the presence of a ramp meter and a high-occupancy-vehicle bypass lane affect the design of an entrance ramp gore area and terminal, particularly in providing the needed acceleration distance from the stop bar.
 Task 11.  Develop specific recommendations for the AASHTO Green Book, including a clear explanation of the merging process and a discussion of the advantages and disadvantages of parallel and tapered ramps. These recommendations should be presented in a red-line/strikeout format. Summarize the differences between the recommendations and the current text.
 Task 12.  Submit a final report documenting the entire research effort. The Green Book recommendations shall be provided in an appendix to the report.

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