Congestion-Responsive On-Ramp Metering: Recommendations toward a Statewide Policy
Congestion Responsive Ramp Metering (CRRM) is a strategy developed to address traffic congestion during non-peak hours and on the weekends using the available RM infrastructure implemented and operated on California highways. The objective of this study was to develop recommendations for the field operation of CRRM after quantitatively evaluating the benefits of the operation during extended hours. For this purpose, nine freeway corridors with known start dates for the operation of RM from Caltrans Districts 6, 7 and 8 were selected to collect 5-minute aggregated traffic count, occupancy and speed data so that a “before” and “after” comparison could be made to facilitate the quantitative evaluation. The study also investigated the effectiveness of setting the ramp meters on “Rest-in-Green” or “Black (OFF)” in general, when they were not in operation, considering energy consumption. The effectiveness of ramp metering during extended hours was then evaluated based on the following performance measures:
- Mainline freeway bottleneck and discharge flow, and corridor vehicle-miles-travelled (VMT)
- Freeway delays and vehicle-hours-travelled (VHT)
- Travel-time reliability: The 80th or 95th percentile of travel time distribution
- Maintain loop detection health close to 100 percent and data quality above 90 percent in order to operate CRRM. The operation of Local Responsive Ramp Metering (LRRM) (during peak hours) should be extended to non-peak hours progressively to achieve CRRM depending on readiness of the system, traffic detection and data quality, supporting staff availability, traffic situation of the freeway corridor, and public outreach influence. In addition to this, traffic engineers should tightly monitor the traffic health and check if the RM “ON” and “OFF” is executed correctly. Only corridors/areas that satisfy the above-mentioned criteria should be selected for this.
- Develop simple detector data fault checking and handling functionality in Universal Ramp Metering Systems (URMS). As data quality is critical for the operation of CRRM, being able to perform simple data health checks such as checking traffic flow for adjacent detector stations and inspecting the occupancy of detectors of all lanes would be beneficial.
- Adopt “Rest-In-Green” or “Black (OFF)” depending on the budget and staff-support availability. Energy consumption is a major factor in the operation of CRRM. Use of LED lights can help reduce energy consumption up to 80 percent with increased life cycle compared to incandescent lamps, although initial investment is high.
- Use integration and coordination of control strategies when extending the on-ramp storage is not feasible. This can be done by employing RM control for freeway corridors and signal controls for relevant arterial corridors. However, extending on-ramp storage at critical locations should be considered if budget is available and land-use if feasible as large on-ramp storage can significantly reduce the impact on mainline traffic during peak hours when the on-ramp demand is high.
- Work closely with different committees from local and regional government agencies to advocate for operation of RM during extended hours. It is important to communicate to local bodies that any negative impact on access to freeways due to RM can be compensated for by total travel time reduction of the overall traffic.