Empirical Study of Ramp Metering and Capacity
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.