OBJECTIVE

The objective of this project is to develop procedures to select ramp-management strategies for a freeway section under the threat of flow breakdown.

STATUS

The revised final report has been received and publication is expected in Summer 2015.

BACKGROUND

Research has shown that more vehicles can pass through a freeway bottleneck if the traffic is flowing freely rather than congested. These higher volumes are usually transitory, because it only takes a minor perturbation to cause the flow to breakdown. If these higher volumes could be sustained, the freeway could serve significantly more traffic.

Ramp management probably presents the greatest opportunity to delay or prevent flow breakdown. The Freeway Ramp Management and Control Handbook describes the following categories of ramp-management strategies: ramp metering, ramp closure, special use treatments, and ramp terminal treatments. Implementation of these ramp management strategies is often based on historical data although ramp metering usually uses real-time data from sensors at the ramp. Information on the status of traffic at the bottleneck should allow a better approach to ramp management that keeps the traffic flowing freely longer. Research is needed to develop a model for providing that information and to show how that model could be used to improve ramp management.

TASKS

Task 1. Review and summarize research findings to identify factors that affect freeway capacity, conditions that indicate imminent freeway-flow breakdown, maximum flows through freeway bottlenecks both before and after breakdown, and the impacts of ramp-management strategies on the onset of breakdown. Describe models that have been developed to predict breakdown or the effects of ramp-management strategies on the onset of breakdown.

Task 2. Develop a working definition of freeway-flow breakdown and describe typical flow-breakdown causes and processes at common types of bottlenecks.

Task 3. Describe ramp-management strategies that have the potential to delay or prevent freeway-flow breakdown. Describe information about the onset and nature of the breakdown that could be used to improve the effectiveness of these strategies in delaying or preventing breakdown.

Task 4. Identify the functional and operational requirements of a real-time model to predict freeway-flow breakdown and provide the information described in Task 3. The inputs of the model should consider, but not necessarily be limited to, the capabilities of existing freeway traffic-management system designs.

Task 5. Prepare a data collection and analysis plan to support the development of a real-time flow-breakdown prediction model. The plan should specify the data that will be collected, methods that will be used to collect the data, the number and types of field-data collection sites, and the methods that will be used to reduce and analyze the data. Candidate sites with their characteristics should be listed. Maximum use should be made of data from freeway traffic-management systems.

Task 6. Within 6 months of contract start, submit an interim report documenting the results of Tasks 1 through 5 and presenting a revised work plan for the remaining tasks.

Task 7. Carry out the approved data collection and analysis plan.

Task 8. Develop the real-time freeway-flow-breakdown prediction model. Assess the model's performance under real-world conditions at a few sites in cooperation with a transportation agency. Document the calibration procedures needed for the model.

Task 9. Develop procedures for using the outputs of the breakdown prediction model to select ramp-management strategies to prevent or delay breakdown.

Task 10. Quantify the effectiveness of the Task 9 procedures using microscopic-simulation modeling based on real-world sites. This evaluation must consider the stochastic nature of flow breakdown and compare the performance of the recommended strategies to that of traditional approaches to ramp management.

Task 11. Submit the final report describing the entire research effort, focusing on the detailed results of Tasks 8, 9, and 10. The report should describe other factors that must be considered in managing ramps (e.g., safety and equity). In an appendix, document the breakdown prediction model in a form that agencies can readily apply it in their systems.