Development and analysis of a flexible signal phasing strategy for diamond interchange control

Abstract

There are many signal timing strategies for diamond interchanges. Due to the different geometric and traffic conditions, however, none of the available strategies is always optimal. The two most widely used strategies, three-phase, and four-phasewith-two-overlaps, each have limitations during certain traffic conditions. This research focused on the development of a prototype expert system to monitor traffic conditions at a diamond interchange and make adjustments in the phase length, phase sequence, and lane assignments. Utilizing the TexSIM traffic simulator, the expert system, also known as the PASSER III Optimization Manager, or P30M, was tested to determine how well it operated under changing traffic conditions. To evaluate the P30M, an interchange in Austin, Texas was modeled for two hours. There were three different versions of the P30M that were tested to determine if it was more beneficial to change phase lengths, phase order, lane assignment, or all three. The results indicated that the P30M can effectively control a diamond interchange by changing the phase order and the lane assignments. It was determined that the P30M was not reacting rapidly enough to the changing conditions, and that it occasionally over-reacted in an attempt to catch up. Most of the time, however, the P30M operated effectively by reducing delay and queue lengths. Some of the limitations of the p3OM may have been contributed to the communications with the controller and attempting to make a standard eight-phase NEMA controller switch from three to four-phase control.