Evaluation of traffic signal controller transition methods

Abstract

A coordinated signal system achieves the best traffic progression when the signal plans are optimized at the correct offsets and intervals. When traffic conditions change and a transition to a new timing plan is warranted, it is important to reach the new desired offset as efficiently as possible. The timing plans are adjusted by transition methods in signal controllers to reach the new offset settings. According to Traffic Engineering, 1990, transition methods must satisfy constraints of safety, pedestrian walk time, driver expectancy, and traffic efficiency. However, the methodology for deciding on the type of transition method over another appears very little in literature. In this research, the performance of three different types of transition methods in an Eagle EPAC300 series Controller unit were evaluated. The three transition methods tested were a Shortway Transition method, Shortway Add Only Transition method, and Infinite Dwell Transition method. The goal was to develop a methodology to determine the "best" transition method when the signal timing plans changed by an offset correction of 10, 30, 50, 70, and 90 percent of the cycle length. It was based on hardware-in-the-loop simulation that was used to simulate real-time traffic flow at a coordinated signalized intersection. This real traffic signal control hardware interacts with the simulation model, CORSIM (CORridor SIMulation). The CORSIM simulation calculated the total delay of the intersection caused from the transition of timing plans for the simulation period. The test was implemented at the Wellborn Road and George Bush Drive intersection in College Station, Texas. The simulation runs were all based on a 35-minute study period. The traffic volume remained constant at approximately 85% capacity at the coordinated phase for the 30 minutes and at no volume for the remaining 5 minutes. The signal timing plans were fixed at a 120-second cycle length. The transition methods for each of the offset changes were tested to find the method that resulted in the least amount of delay at the intersection. The Shortway transition method was the "best" method under these constraints for all offset corrections. The Shortway method ads time to the coordinated phase(s) or subtract time equally spread across the phase(s) to reach the new offset. The maximum amount of time that can be added or subtracted during a single cycle while the system is in transition is 18.75% of the cycle length. This research provides a better understanding about the transitioning methods of the Eagle EPAC300 signal controller, responding to the change in offset. The default mode, Shortway method, performs better than the other methods when the time is subtracted from the phase(s) to reach the new offset. Ultimately, this will result in a more efficient utilization of the transition methods at signal control intersections. The signal controllers will provide less stops and delay to improve traffic efficiency. This hardware-in-the-loop methodology of evaluating the performance of transition methods is recommended for future use. The methodology is capable of testing actual signal controller transition methods in a laboratory with accurate results.