University of California Transportation Center

Measurements taken downstream of freeway/on-ramp merges have verified that discharge flow diminishes when a merge becomes an active bottleneck. We show that metering the on-ramp can recover the higher discharge flow and thereby increase merge capacity. Detailed observations collected using video revealed that the outflow drop following activation was triggered by a queue that formed near the merge in the freeway shoulder lane and then spread laterally, as drivers changed lanes to maneuver around slow traffic. Once restrictive metering mitigated this shoulder lane queue, high outflows often returned to the median lane. Merge outflow could be increased to levels measured prior to the bottleneck activation by then relaxing the metering rate so that inflows from the on-ramp increased. Although outflows recovered in this fashion were unstable and never persisted for periods greater than 13 mins, the findings are the first real evidence that ramp metering can favorably affect the capacity of an isolated merge. The findings point to control strategies that might stabilize outflow and increase merge capacity even more.