Limitations of Simultaneous Gap-Out Logic

The current practice of specifying simultaneous gap-out logic places constraints on signal controller logic. These constraints cannot be achieved under high traffic flow conditions, and degraded signal efficiency and dilemma zone protection often result. This study documents the phenomenon described above with set-back detectors at an instrumented intersection in Noblesville, Indiana, and characterizes the problem of dilemma zone protection as being traffic-volume dependent, a factor that should be carefully considered before the simultaneous gap-out logic is applied. Implementation of simultaneous gap-out logic led to max out ranging from 3.5% to 40% of cycles per hour during peak traffic flow periods and about 200 dilemma zone incursions per day. Results also indicate that simultaneous gap-out logic performs inefficiently and unsafely under high-volume conditions, whereas its performance is satisfactory under low-volume conditions. Analysis suggests an upper bound on potential savings of about 400 s of green time per day and a 25% reduction in dilemma zone incursions.