When to Control the Ramps on Freeway Corridors? A Novel Stability-and-MFD-Based Approach

Coordinated ramp metering (CRM) is an efficient measure to mitigate traffic congestion on freeway corridors. One of the key issues of implementing CRM is to decide when to start the control on the ramps. Generally, the control timing for CRM is determined on the basis of the traffic state on a freeway corridor to prevent traffic jams on corridors. In principle, high throughputs and less congestions on corridors are aimed when determining the control timing of CRM. However, it is found that the traffic state at high densities is fairly unstable with high probabilities of traffic breakdown. This paper proposes to determine the control timing, by considering not only high throughputs but also stable flows on freeway corridors. A novel approach is developed by introducing an indicator of production stability (PS) to determine when to start the control of CRM. The indicator is established on the basis of a macroscopic fundamental diagram (MFD) and the instability of traffic flows. Empirical traffic data from urban freeway networks in the city of Shanghai, China, are utilized to verify the feasibility of using the PS indicator. It is found that the flows at high accumulations are very unstable with high probabilities to breakdown. The developed indicator PS has a clear peak, from which a critical accumulation level could be identified. The critical accumulation level derived according to PS is lower than that simply determined from the MFD, which implies that the control should start earlier to guarantee stable flows.