Red means stop. Green means go. Both are definitive and unambiguous. But yellow means "caution." That means that a driver, instead of responding to a command, must make a judgment. Isn't that an accident waiting to happen?
"There are circumstances, as you approach a yellow light, where the decision is easy," said Hesham Rakha, professor of civil and environmental engineering at Virginia Tech. "If you are close to the intersection, you keep going. If you are far away, you stop. If you are almost at the intersection, you have to keep going because if you try to stop, you could cause a rear-end crash with the vehicle behind you and would be in the middle of the intersection anyway."
But where, exactly is that point at which you stop for a yellow light or go on through the intersection. Rakha, director of the Center for Sustainable Mobility, has been trying to answer that question since 2005.
His research group has been studying drivers' behaviors as they approach yellow lights. Their goal is to determine signal times for intersections that are safer and still efficient.
The goal is to reduce rear-end crashes and collisions with side-street traffic. Although observation-based research shows that only 1.4 percent of drivers cross the stop line after the light turns red, more than 20 percent of traffic fatalities in the United States occur at intersections.
In the dilemma zone
"If the yellow time is not set correctly, a dilemma zone is imminent," Rakha said.
"The dilemma zone occurs when the driver has no feasible choice. In other words the driver can neither stop nor proceed through the intersection before the light turns red. This can also occur if the approaching vehicle is traveling faster than the posted speed limit and/or if the driver's perception and reaction time is longer than the design one-second value."
On a road with a 45 miles per hour speed limit, the average yellow time on a traffic signal is set for 4.2 seconds. The time is extended on roads with a higher speed limit.
"These timings are based on two assumptions," Rakha said. "Namely, the driver requires one second to perceive and react to the change in signal indication and that the driver requires 3.2 seconds to stop from 45 mph at a comfortable deceleration level, assumed to be 3 meters per second squared or 10 feet per second squared."
Rakha and his team study actual drivers at a test road at the Virginia Tech Transportation Institute. So far they've found drivers of different ages react differently.
People over 60 years of age have a longer perception-reaction time, so they have to brake harder to stop. But they are more likely to try to stop, compared to younger drivers. However, if they keep going, they are unlikely to clear the intersection, the researchers report.
One strategy to make intersections safer might be more use of caution lights that tell drivers a green light is about to change, so the driver has a longer time to react. Such systems now are used on high speed roads, where the stopping distance is longer, and when the lighted intersection is not visible until the last seconds.
Rakha says a future strategy that researchers are investigating is in-car display systems that can be customized to each driver's reaction time. One driver might get a four-second warning but another might get the signal sooner.