Controlling traffic flow near the transition to the synchronous flow phase

For constant incoming flow far upstream of a freeway on-ramp, the flow downstream (throughput) and the rate of merging are studied with simulations using a generalized optimal velocity model. For large enough merge rates, a transition to synchronous flow occurs and the throughput is reduced by 0.5-0.7 vehicle on average for each vehicle that merges. For smaller merge rates there is free flow on the freeway and the throughput is the sum of the merge rate and the flow upstream of the on-ramp. Thus, there is an optimum merge rate that maximizes the throughput for a given incoming flow rate. These results hold for a wide range of initial vehicle position and velocity profiles and for single- as well as double-lane freeways. The results show that the transition to synchronous flow is due to the dynamics of the merge process, rather than to a limitation on the capacity of the downstream portion of the freeway. As a consequence, a new on-ramp metering algorithm, which controls the merge rate to prevent the transition to synchronous flow and concomitantly to maximize flow, has been developed.