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Traffic study sees shades of gray in yellow lights

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Tuesday, June 29, 2010

Study sees shades of gray in yellow lights

Drivers know that green means go, red means stop and yellow means . . . "Can I make it?" Although the law is clear that yellow means slow down and prepare to stop, many drivers do not. New research sheds light on what factors come into play when a driver decides to run those yellows, and it turns out it's not just a matter of speed.

Researchers from the University of Cincinnati, funded by the Ohio Department of Transportation, monitored four "high-speed" traffic intersections in suburban areas of Ohio, using video cameras to track more than 1,500 drivers. They measured vehicle type, speed, driver's distance from the light when it turned yellow, and decision to stop or not in what they called the "dilemma zone."

They found that cars traveling in right-hand lanes tended to go through yellow lights, while those on the left did not. Truckers also tended to speed through yellows, as did drivers on streets with higher posted speed limits. Drivers of vehicles on streets marked by 55 mph speed limits were more likely to run yellows than those in 50 mph zones.

The researchers found that drivers of SUVs, pickups, sedans and vans tended to slow down at yellows more than drivers of heavy trucks. They speculated that vehicle weight may be the explanation, since heavy trucks have more difficulty decelerating rapidly than other vehicles.

Not surprisingly, how long the light remains yellow also matters. (Yellow-light times vary, but typically last about three to five seconds. Traffic engineers base the time on the average speed of the vehicles passing through the intersection.)

The longer the yellow persists, the more likely it is that drivers will not stop, said Zhixia Li, an engineering PhD student who worked on the study with his professor Heng Wei. In fact, he said, with a long yellow, "stopping is more dangerous," because other drivers are likely to keep going through the yellow, and someone who opts to stop runs a greater risk getting hit from behind.

Li knows this from experience. While visiting his parents in Shanghai last summer, Li borrowed his dad's black Chevy sedan. His father warned him that red-light cameras police many intersections. "He told me," Li said, " 'You better not run yellow.' "

When one light turned yellow, Li at the last minute quickly hit the brakes -- and a van behind him slammed into him. "I was in the dilemma zone," he said. Li wasn't hurt, but his car was shoved 20 feet into traffic.

-- Leslie Tamura


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