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 it 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.

Traffic data near the junction of a single-lane on-ramp (with a ramp meter) and a three-lane freeway were measured for six weekdays during the rush and studied. On each of these days, the merge became a bottleneck with queue discharge rates that were substantially lower than the flows that had passed the merge prior to the bottleneck's activation. On some days, these earlier high flows persisted for many minutes. The bottleneck always occurred when inflows from the on-ramp surged in the presence of high flows arriving from the freeway. Often, the on-ramp surges persisted for no longer than a minute or two and a wide range of these surges was observed. The data show strong correlation between the magnitudes of the on-ramp surges and the merge area's outflows that were measured during the final minutes before the bottleneck activations. These short-run outflows were markedly higher on days when surges from the on-ramp were low. This implies on-ramp metering can be an effective means of postponing this bottleneck's activation, thereby prolonging higher outflows from the merge. Further study of the data indicates that vehicles often maneuvered from the shoulder lane to the center lane at locations just downstream of the merge. It appears that bursts in this lane-changing activity were what triggered the bottleneck's daily activations and that these bursts coincided with surges in inflow from the on-ramp. (These lane changes were evidently negotiated by drivers who had just originated from the on-ramp and/or by other drivers over-taking them). Thus, the benefits of metering inflows at this on-ramp seem to stem from the effects this has on limiting disruptive lane changing downstream.

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.

Traffic data measured near the junction of a single-lane on-ramp (with metered inflows) and a three-lane freeway were carefully studied for four days during the rush. The data showed the area around this merge junction became a bottleneck each day when the on-ramp's meter allowed its inflows to rise in the presence of high flows arriving from the freeway. Detailed study during these times further showed that queueing actually arose some distance downstream of the merge and that these queues were caused by drivers who, having just entered the freeway's shoulder lane from the on-ramp, slowed-down to negotiate discretionary lane-change maneuvers. The slowing was observed to spread quickly to all three freeway lanes and this had marked effects on the discharge flows from the bottleneck; i.e., average queue discharge rates were as much as 10 percent lower than the flows that had departed the merge prior to the bottleneck's activation. The latter of these flows, moreover, could be observed for many minutes (and therefore cannot be attributed to statistical variation). Even after the bottleneck had activated, its discharge flows continued to be influenced by vehicle slowing at, or just downstream of, the merge; i.e., queue discharge rates temporarily diminished with each recurring slow-down. Reductions of 800 vph or more could accompany such a slow-down and these lower rates could persist for some minutes. These undesirable effects were again triggered by vehicles from the on-ramp changing lanes soon after having merged. And notably, the discharge reductions became more (less) pronounced as the ramp's inflows surged (diminished). The findings show that service rates at this freeway site can be made higher 1) by eliminating, or at least postponing, the bottleneck's activation; and 2) by mitigating downstream slow-downs if or when the activation occurs. They further show, at times in dramatic fashion, that these objectives can be realized by carefully metering the on-ramp to limit the rates its vehicles join the freeway traffic stream. Certain issues concerning ramp metering in light of the present findings are discussed at the conclusion of this manuscript.