OPTIMIZATION OF LEFT-TURN PHASE SEQUENCE ON SIGNALIZED ARTERIALS

The traffic engineer has four variables available that can be adjusted to provide signal timing plans for signalized urban-suburban arterials. These are green phase time, offset, cycle length, and left-turn phase sequence. Up until recently, the last of these variables has received very little attention. In recent years, two signal optimization computer programs, MAXBAND and PASSER-II, have been developed. Through use of these programs, the impact of changing left-turn phase sequence so as to maximize the amount of green bandwidth on a two-way signalized arterial with left-turn phases at some or all of the intersections can be explicitly considered. However, there has also been a tendency to utilize signal optimization programs that use vehicular delay as a measure of performance rather than bandwidth. Unfortunately, the delay-based programs, TRANSYT-7F, SIGOP-III, and SSTOP, cannot optimally select left-turn phase sequence. Thus, the objective of this study was to examine whether using a bandwidth-based program for the selection of left-turn phase sequence subsequently followed by delay-based programs to determine the final offsets generates signal timing plans with lower delay than either class of programs individually. Data from seven arterials of widely varying characteristics were available for this study. It was found that optimizing phase sequence can substantially improve the performance of MAXBAND, both in terms of increased bandwidth and decreased delay and stops. The use of phase sequence patterns optimized by MAXBAND in TRANSYT-7F has the potential for further improving signal timing plans produced by the latter program. However, use of phase sequence patterns optimized by MAXBAND in SIGOP-III apparently has the potential for producing signal timing plans with reduced performance.