Increasing Freeway Capacity by Efficiently Timing Its Nearby Arterial Traffic Signals

Abstract

Currently, arterial traffic signals adjacent to freeway on-ramps operate independent of freeway on-ramp metering. During peak periods, the traffic signals employ long signal cycles to maximize intersection capacity. This approach often results in long platoons of freeway-bound traffic advancing toward the on-ramp within a short period, which quickly fills up the on-ramp and causes spillback on the surface streets. Queue override is typically employed to prevent the queue spillback by suspending ramp metering or relaxing the metering rates. Unfortunately, this counteracts the objective of on-ramp metering, which is to prevent capacity drop at freeway merge bottlenecks. Significant benefits can be realized by integrating arterial signal timing with the adjacent ramp-metering system. A signal control strategy was developed and evaluated in this study. The algorithm takes available on-ramp storage and the freeway ramp-metering rate into account and dynamically adjusts the cycle length to prevent on-ramp queue spillback and activation of queue override. The proposed algorithm was tested through simulation and the results show that the proposed strategy reduces the freeway, arterial, and system-wide delay at a modest penalty for on-ramp bound traffic.