Development of the Next Generation Stratified Ramp Metering Algorithm Based on Freeway Density

Project summary:

This project continues previous work aimed at improving the stratified freeway ramp metering strategy used by the Minnesota Department of Transportation (Mn/DOT). In developing a next generation strategy, Mn/DOT's Regional Traffic Management Center wishes to explore alternatives that address inherent limitations of the current Stratified Zone Metering (SZM) strategy. As earlier research has shown, occupancy values near capacity are quite stable, but bottleneck capacity is subject to stochastic variations; therefore, a control strategy based on flow thresholds is likely to under-load the freeway or produce traffic congestion.

For this project, a new coordinated, traffic-responsive ramp metering algorithm has been designed for Minnesota's freeways based on density measurements, rather than flows. Researchers developed a methodology to estimate densities with space and time based on data from loop detectors. This methodology is based on solving a flow conservation differential equation (using LWR theory) with intermediate (internal) freeway mainline boundaries, which is faster and more accurate from previous research using only external boundaries. To capture the capacity drop phenomenon into the first order model, researchers utilized a fundamental diagram with two values of capacity and provided a memory-based methodology to choose the appropriate value in the numerical solution of the problem. With respect to ramp metering, the main goals of the algorithm are to delay the onset of the breakdown and to accelerate system recovery when ramp metering is unable due to the violation of maximum allowable ramp waiting time.

The effectiveness of the new control strategy is being assessed by comparison with the currently deployed version of the Stratified Zone Algorithm (SZM) through microscopic simulation of a real 12-mile, 17 ramp freeway section. Simulations show a decrease in the delays of mainline and ramp traffic, an improvement of 8% in the overall delays, and avoidance of the maximum ramp delay violations.