Objectives:​ Few studies have focused on the effect of countdown timers at signalized intersections in China, where such timers are widely deployed for their perceived benefits of increased safety and capacity. This study examines the effect of countdown timers on driver behavior during the yellow interval.

Method:​ Signal phasing and traffic operations were videotaped at 4 comparable signalized intersections under normal conditions. Microscopic details were extracted manually at 25 Hz to yield 24 h of data on onset time of the yellow, onset time of the red, driver location and actions after the onset of the yellow, red light–running violations, etc. For comparable intersections with and without countdown timers, driver behavior measured by driver decision (stop or go) and vehicle entry time (when the vehicle crosses the stop line) were analyzed using binary logistical regression (BLR) and a nonparametric test, respectively.

Results:​ The results suggest that countdown timers can indeed influence driver behaviors, in terms of decisions to stop or cross the intersection as well as the distribution of vehicle entry times. There was a strong correlation between the presence of countdown timers and an increase in red light violations.

Conclusion:​ Countdown timers may lead to increased entrance into the intersection during the later portions of the yellow and even the red. This alarming finding calls for further research as well as for serious consideration before the field deployment of countdown timers.