INTEGRATED VARIABLE SPEED LIMIT AND RAMP METERING CONTROL FOR MANAGING RECURRENT FREEWAY CONGESTION
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Recurrent congestion due to highway bottlenecks is one major factor contributing to daily commuting delays in most traffic corridors. The resulting traffic queues often start from the bottleneck, and then spill back to further upstream segments to block their on-ramps and off-ramps. Consequently, the entire corridor -- freeways and their connected local arterials -- often suffers from severe gridlocks. To address such a critical issue, this research aims to develop an integrated control system, using both ramp metering and variable speed limits to improve the operational efficiency and to keep the traffic flows moving steadily near roadway capacity. The control system developed in this study includes a traffic state prediction model, a local bottleneck control module, and integrated corridor control strategies. The primary objective of the traffic state prediction model is to reliably predict the evolution of traffic conditions under the implemented control strategies, such as variable speed limit (VSL) and ramp metering. Based on the estimated compliance rate of drivers, the developed system with VSL is capable of capturing traffic flow dynamics – made up of VSL-complying and non-complying vehicles – and adjusting model parameters in real time, based on the on-line detected traffic data. The system’s predicted traffic dynamics will in turn serve as the basis for exercising a local and/or corridor integrated control. Grounded on the embedded mixed traffic flow model, the second component of this developed system is a local bottleneck control module, using both ramp metering and variable speed limits to tackle the recurrent congestion. This module is capable of selecting the activation time for each available control strategy – based on predicted traffic information – and determining the number of VSLs to be activated so as to ensure that the flow rate at the bottleneck segment will not exceed its capacity. The local control module can also activate the ramp metering in a timely manner to support the VSL operations within the target segment. The system under the local control state can also have the flexibility to select different control objectives based on the monitored traffic patterns. When traffic demand exhibits a sustained increase, the local bottleneck control alone may not be enough to prevent the freeway from breaking down. Hence, this study has further developed an integrated corridor control module, which can dynamically incorporate the upstream highway segments into control boundaries and exercise ramp metering controls to distribute excess volumes among different on-ramps. This will concurrently address both operational efficiency and equity concerns between freeway and ramp vehicles.