Yellow Lights: Red Flags for Driver Safety

When approaching a yellow light, every driver faces that moment of decision: Slow down or keep going?


Many factors affect your ability to safely navigate an intersection without causing a collision or being rear-ended. Now new research of these factors is challenging the status quo on how yellow lights are programmed.


Since 2005, researchers at the Center for Sustainable Mobility at the Virginia Tech Transportation Institute have been studying drivers’ yellow light behavior to determine safer signal times.


Most yellow lights are timed for 4.2 seconds on a 45 mph road, and longer for higher posted speed limits. The timing is based on 1 second of driver reaction time, and 3.2 seconds required for a vehicle to comfortably stop from 45 mph. The study revealed driver reaction time is actually slightly longer than 1 second, and, more importantly, driver deceleration levels are significantly higher than the signals were designed to handle—meaning drivers need more than 3.2 seconds to comfortably stop.


The study also revealed most drivers make the go/no-go decision just 3 seconds before the stop line, drivers who keep going typically maintain their speed rather than speeding up or slowing down, and 43 percent of drivers who crossed the stop line during yellow were not able to clear the intersection before the light turned red. Researchers also identified drivers over age 60 are more likely to try to stop than younger drivers, though they have to brake harder to do so due to their longer reaction times.


Poor road conditions and exceeding posted speed limits were cited as additional factors impacting driver safety and yellow light timing.


So what’s next? We may see more use of caution lights in advance of traffic lights, which alert drivers to light changes earlier. In-car technologies customized for each driver’s reaction times are also being studied, along with ways in which the lights themselves can communicate with your vehicle.


In the meantime, though the Virginia Driver’s Manual is clear about driver responsibility and yellow lights—“If you have not entered the intersection, stop. If you are already in the intersection, go through it. Do not speed up to beat the light.”—always use your best judgment for safe driving.

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