OBJECTIVE

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

STATUS

Tasks 1 through 4 were completed and the contract was terminated.  The remainder of the work is being carried out in NCHRP Project 15-31A.

BACKGROUND

The design values for freeway ramps in the AASHTO Policy on Geometric Design of Highways and Streets (Green Book) rely heavily on research conducted in the late 1930s and early 1940s, predating the development of the Interstate system. The studies relied entirely on passenger cars for acceleration and deceleration rates, without consideration of trucks and busses; this was based on the assumption that the acceleration distances for heavy vehicles would be "entirely out of reason." Vehicle characteristics have changed since the original research. For example, the weight-to-horsepower ratio for trucks in 1965 was 400 lb/hp compared with the currently used ratio of 200 lb/hp.

According to the Green Book, the mainline terminal of a ramp is that portion adjacent to the through traveled way, including speed-change lanes and tapers. There are two basic designs for freeway ramp terminals: tapered and parallel. Transportation agencies tend to adopt one of these designs as a standard (often different for entrances and exits), but there is little objective information available for designers on their relative strengths.

TASKS

PHASE I

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.

PHASE II

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.