Increasing Freeway Capacity by Efficiently Timing its Nearby Arterial Traffic Signals
- Author(s): Kan, Xingan David
- Advisor(s): Skabardonis, Alexander
- et al.
On-ramp metering at freeway bottlenecks is an effective method of reducing a freeway system’s delay because empirical studies have previously shown that metering can prevent capacity drop, or reduction of outflow as a result of queue formation. However, arterial traffic signals that facilitate access to freeway on-ramps operate independent of the traffic conditions on the freeway on-ramp. Consequently, the traffic signals employ long signal cycle lengths and thus long green durations to maximize capacity at the arterial signalized intersections, which result in long platoons of freeway-bound traffic advancing toward the on-ramps. This often causes queue spillback on the freeway on-ramps and the surface street network. Queue override, a function that terminates or significantly relaxes the on-ramp metering rate whenever a sensor placed at the entrance of the on-ramp detects a potential queue spillover of the on-ramp vehicles on the adjacent surface streets, has become a widely accepted method of resolving queue spillback on the freeway on-ramps and nearby surface streets. Unfortunately, queue override releases the queue into the freeway and negates the benefit of ramp metering during the peak hours with recurrent freeway congestion. Video data collected downstream of freeway/on-ramp merge in San Jose, California show that the bottleneck discharge flow diminishes when queue override is activated for a sustained period of time. Observations over a two-week period suggest that queue override reduces the bottleneck discharge flow by an average of 10%.
Recently, there has been significant interest in integrated corridor management (ICM) of facilities comprised of freeways and adjacent arterial streets. Significant benefits can be realized by preventing queue override and effectively storing the queued vehicles on the nearby arterial surface streets if the arterial traffic signals can account for traffic conditions on freeway on-ramps and avoid sending long platoons to the freeway on-ramp. A signal control strategy was developed and evaluated in this study. The algorithm takes available on-ramp storage and freeway ramp metering rate into account and dynamically reduces the cycle length and adjusts the green durations to prevent on-ramp queue spillback and mitigate unnecessary delay in the conflicting arterial directions. The proposed algorithm was tested through simulation and the results show that the proposed strategy reduces the freeway and system-wide delay even under fluctuations in traffic demand, at a modest penalty on the on-ramp bound traffic.