Increasing freeway merge capacity through on-ramp metering

This research describes field studies of how on-ramp metering can increase the capacity of freeway merges. Some effects of on-ramp metering have been known for a long time. We have known that on-ramp metering can (1) increase freeway flow and speed upstream of a merge; and (2) reduce system-wide delay by alleviating gridlock-causing queues that have blocked off-ramps. However, past studies have not conclusively shown that on-ramp metering can increase the maximum outflow (capacity) of freeway merges.; The experiments conducted in the present study verify that on-ramp metering can increase freeway merge capacities. Detailed traffic data collected from videos for more than 30 rush periods at two merge bottlenecks unveil six major research findings: (1) merge capacity diminishes after merges became active bottlenecks; (2) the mechanism of "capacity drop" has been identified and was found to be reproducible across all days and at both sites. By metering the on-ramp in certain strategic ways, the capacity drop mechanism can be (3) reversed; and (4) even averted; (5) such metering strategies can be fully automated using loop detector measurements; and (6) control strategies other than ramp metering also hold promise for increasing merge capacities.; These findings provide much-needed information concerning how to control freeway traffic. They also offer basis for more realistic theories of merging traffic flow.