Novel Freeway Traffic Control with Variable Speed Limit and Coordinated Ramp Metering

control freeway traffic in the United States, particularly in California. It is recognized that ramp metering can directly control the flow into the freeway (demand) and the average density immediately downstream, which indirectly affects the traffic upstream. After drivers enter the freeway, their collective behaviors are not controlled, which determines the traffic flow pattern. In addition, from the perspective of equity among the on-ramps along a corridor and the ramp queue length limits owing to road geometry, ramp metering has to be switched off if the demand from that on-ramp is too high to avoid traffic spilling back onto arterials. Therefore, from a systems and control viewpoint, using ramp metering alone cannot fully control the freeway traffic in practice. This is the motivation for investigating the combination of RM with VSL. A recent FHWA report (8) summarizes the benefits of using VSL, RM, and other strategies in active traffic management (ATM). The following acronyms are used in this paper: FD, fundamental diagram; MPC, model predictive control; TTD, total traveled distance (or vehicle miles traveled); TTS, total time spent; and TTT, total travel time (or vehicle hours traveled). This paper focuses on mobility improvements along a stretch of freeway using VSL and RM combined. Several possible ways exist to combine VSL and RM. At each time step: