Evaluating the Effects of Signal Countdown Timers on Queue Discharge Characteristics at Signalized Intersections in China

Abstract

A study evaluated the effects of signal countdown timers on queue discharge characteristics of protected left-turn and through movements at signalized intersections in China. Using data collected from 13 approaches at seven signalized intersections, the research team evaluated the effects of countdown timers on saturation headways and start-up lost time for both through and protected left-turn movements. Countdown timers had a positive but generally limited impact on the capacity of signalized intersections. Signal countdown timers significantly affected driver starting response time and start-up lost time for both protected left-turn and through movements. On average, countdown timers reduced the start-up lost time by 0.6 s per cycle for protected left-turn movements and 2.25 s per cycle for through movements at selected sites. Signal countdown timers had little effect on saturation headways. On average, signal countdown timers resulted in a saturation headway 0.11 s higher for protected left-turn movements and a saturation headway 0.15 s higher for through movements at selected sites. The difference in saturation headways was statistically significant for through movements, but not for left-turn movements. The study also looked extensively at the impact of countdown timers on the compression of discharge headways at the end of each queue. The presence of countdown timers resulted in headway compressions at the end of each queue. For protected left-turn movements, headway compression usually started when 5 s remain in the left-turn green arrow time. For through movements, headway compression may start as early as when 14 s remain in the green time.