Abstract
One of the major criticisms of ramp metering has been delay caused on the ramps because of the queues created by ramp control strategies. Recently, several researchers proposed strategies that explicitly consider ramp queues when metering rates are determined. An isolated "feedback"-based ramp-metering strategy (mixed control) is presented. In addition to regulation of ramp input from the freeway, the strategy calls for regulation of ramp queues by explicitly incorporating them into the model. Mixed control is tested using PARAMICS, a microscopic traffic simulation package, on a calibrated test network located in Hayward, California. In addition to mixed control, ALINEA and new control are implemented. Because the focus here is on evaluation of isolated ramp control strategies, change in demand caused by ramp metering is not considered. The system is defined as the upstream, downstream, and ramp links around the metered ramp. The performance of two freeway demand patterns (congested and overcongested) is compared. From the simulation results, all the strategies tested were found to be quite effective in optimizing freeway traffic conditions [reduction in mean congestion duration on the freeway downstream link, mean downstream occupancy, and travel time (upstream and downstream links)]. However, mixed control produced the best system results (upstream, downstream, and ramp links) by achieving optimal flow on the highway while keeping the queue length on the ramp small enough to prevent spillover onto the arterial network.
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