Minnesota's New Ramp Control Strategy: Design Overview and Preliminary Assessment

Abstract

Freeway ramp control has been successfully implemented in the Minneapolis-St. Paul area since the early 1970s. However, the recent ramp metering controversy highlighted the need for a less restrictive ramp control strategy that maximizes freeway capacity utilization while limiting ramp wait times. As a result, a new multilayer ramp control strategy called stratified ramp control was recently developed and deployed systemwide in the Twin Cities metropolitan area. This strategy determines the metering rates from freeway conditions as well as from real-time ramp demand and ramp queue size, indicating a shift of emphasis away from freeway flow toward the balance between both freeway efficiency and reduced ramp delays. Minnesota's new ramp control strategy is detailed along with a preliminary assessment of its effectiveness. The evaluation is accomplished by comparing the new strategy to its predecessor, ZONE metering, through rigorous micro-simulation. The preliminary results suggest that the stratified ramp control strategy is effective in reducing ramp delays and limiting ramp wait times below the prescribed value. However, peak-hour freeway congestion is extended in both time and space as opposed to that with the ZONE metering strategy and reveals a compromised freeway performance in favor of virtually reducing ramp delays.