The first objective of this research was to develop improved design guidance for interchange loop ramps and their connections (Tasks 1-10). The guidance considers the context of the interchange and the safety, operational, and constructability impacts of the design.
The second objective of this research was to assess the current AASHTO design policy for pavement/shoulder cross-slope breaks on the outside of horizontal curves to determine whether any updates in design criteria are needed (Tasks 11-16).
Interchanges are among the most complex and expensive components of the roadway network. An appropriate interchange design must safely and effectively accommodate the traffic volumes for the various movements, while considering the context of the location (e.g., rural/urban, right-of-way and environmental constraints, near-by traffic generators, upstream and downstream interchanges). Design decisions can be even more difficult when improving an existing interchange. Several interchange designs have been developed over the years, using a variety of ramp types and connections from the ramps to the intersecting roadways. Loop ramps are prominent in the traditional cloverleaf design and are also used in partial cloverleafs, trumpets, and other interchange designs. Despite their long use, information on the safety and operational performance of loop ramps is limited. This is particularly true for two-lane loop ramps because of the interactions between passenger vehicles and trucks traveling side-by-side on a fairly tight curve. In addition to conditions on the ramp proper, the safety and operation of the connections from the ramp to the intersecting roads must be considered. For freeway connections to and from a multi-lane ramp, the development or termination of the added lanes must be carefully designed to adequately accommodate diverging/merging movements and the necessary deceleration/acceleration. When a loop ramp terminates at a traffic signal or a ramp meter, the design must consider the expected queue.
Task 1. Review the literature, including state DOT design manuals, on the design and redesign of freeway interchange loop ramps and connections and their operational and safety characteristics. Summarize the state of the art and the state of the practice in loop ramp design.
Task 2. Develop a framework of the various types of loop ramp connections and typical geometric and operational configurations for each type. Describe design, safety, and operational challenges associated with each configuration, including acceleration and deceleration requirements.
Task 3. Describe promising models for determining the impact of critical design elements (e.g., curve radius, lane width) on speeds and capacity for one- and two-lane loop ramps. List the field data that will need to be collected to assess and develop these models and recommend a data collection plan. This task does not include the ramp connections because of the limited project funds and previous research efforts.
Task 4. Prepare a plan for developing a safety model for one- and two-lane loop ramps, including their connections. Describe the data sources that will be used to develop the model and recommend a data collection plan. The safety assessment should be based upon at least 3 years of existing data and should include crash type and severity.
Task 5. Prepare an interim report summarizing the work done in the previous tasks and presenting the data collection and analysis plans developed in Tasks 3 and 4. The plans should describe data collection procedures and characterize sites for data collection. A prospective list of data collection sites should also be provided. The interim report should also include an updated work plan and budget for the remaining tasks.
Task 6. Carry out the approved data collection plan, at the sites approved at the interim report panel meeting.
Task 7. Based on the work in Task 3 and the data collected in Task 6, develop a method to determine the impact of critical design elements (e.g., curve radius, lane width) on speeds and capacity for one- and two-lane loop ramps.
Task 8. Based on the work in Task 4 and the data collected in Task 6, determine the effect of design elements on safety for one- and two-lane loop ramps, including their connections.
Task 9. Develop draft material for potential inclusion in AASHTO’s A Policy on Geometric Design of Highways and Streets. The draft material should consider the following issues:
- Selection of design speed and vehicle,
- Upstream and downstream conditions and proximity to traffic generators,
- Maximum superelevation and its relationship to the minimum radius,
- Lane and shoulder widths (and the effect of off-tracking),
- Breakover tolerance for cross slopes,
- Drainage on the ramp proper,
- Appropriate compound and spiral curve configurations,
- Design elements of the connections entering and leaving the ramp,
- Speed differential between freeway through and ramp traffic,
- Longitudinal speed profile of traffic using the ramp,
- Presentation of the results of Tasks 7 and 8 in appropriate forms,
- Implications for the principle of lane balance,
- Comparison of the performance of one- and two-lane loop ramps,
- Incorporation of ramp metering and managed lanes, and
- Application of the material in retrofit and phased construction projects.
Task 10. Prepare a final report documenting all aspects of the research. The report should include the results of Task 9 as a stand-alone appendix. As appropriate, the report should also discuss suggestions for revisions to the Highway Safety Manual, Highway Capacity Manual, and the Manual on Uniform Traffic Control Devices.
Task 11. Review the literature and current practice related to pavement/shoulder cross-slope breaks.
Task 12. Develop a work plan for evaluating different pavement/shoulder cross-slope designs.
Task 13. Submit an interim report on the pavement/shoulder cross-slope break work.
Task 14. Execute the Task 12 work plan as approved by the panel.
Task 15. Develop design guidance on pavement/shoulder cross-slope breaks.
Task 16. Prepare a final report on the pavement/shoulder cross-slope break work.