Oregon study shows traffic signal countdown timers lead to improved driver responses


Research at Oregon State University (OSU) suggests countdown timers that let motorists know when a traffic light will go from green to yellow lead to safer responses from drivers.

The OSU’s findings are important because of mistakes made in what traffic engineers refer to as the ‘dilemma zone’; the area in which a driver is not sure whether to stop or keep going when the signal turns yellow. A traffic signal countdown timer (TSCT) is a clock that digitally displays the time remaining for the current stoplight indication of red, yellow or green. Widely adopted by over 20 countries around the world, traffic signal countdown timers are not used in the USA. Crosswalk timers for pedestrians are allowed, but TSCTs are prohibited by the US Department of Transportation (USDOT). Several automakers have developed cloud-connected in-car countdown timers that are currently being trialed in several cities in vehicle-to-infrastructure (V2I) projects, and unlike the traffic-signal-mounted timers, the onboard systems are allowed in the USA.

There were more than 37,000 traffic fatalities in the USA in 2016, and around 20% of those occurred at intersections. It is not known exactly how many of the country’s intersections are signalized because no agency does a comprehensive count, but the National Transportation Operations Coalition estimates the number to be greater than 300,000. A significant percentage of those feature fixed-time signals, which are recommended in areas with low vehicle speed and heavy pedestrian traffic.

In the OSU study, which used a green signal countdown timer (GSCT) in the university’s driving simulator, the clock counted down the final 10 seconds of a green indication. A subject pool of 55 drivers ranging in age from 19 to 73, produced a data set of 1,100 intersection interactions, half of which involved a GSCT. The presence of the countdown timer increased the probability that a driver in the dilemma zone would stop by an average of just over 13% and decreased deceleration rates by an average of 1.5 feet per second.

“These results suggest that the information provided to drivers by GSCTs may contribute to improved intersection safety in the USA,” said David Hurwitz, transportation engineering researcher at OSU and co-author of the report. “When looking at driver response, deceleration rates were more gentle when presented with the countdown timers, and we did not find that drivers accelerated to try to beat the light; those are positives for safety. Drivers were significantly more likely to slow down and stop when caught in the dilemma zone. The results in the lab were really consistent and statistically convincing.”

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About Author


Tom has edited Traffic Technology International (TTi) magazine and its Traffic Technology Today website since May 2014. During his time at the title, he has interviewed some of the top transportation chiefs at public agencies around the world as well as CEOs of leading multinationals and ground-breaking start-ups. Tom's earlier career saw him working on some the UK's leading consumer magazine titles. He has a law degree from the London School of Economics (LSE).