A Matter of Timing

“It’s not my fault; the yellow light was too short!”

Sometimes it’s true: Flaws in traffic-signal timing and intersection design can force drivers to run red lights.

“There’s no question that a short yellow light is partly to blame in some intersections,” said Richard Retting, senior transportation engineer at the Insurance Institute for Highway Safety in Arlington, Va. He also chairs the Institute of Transportation Engineers’ safety council.

Most urban intersections require a three- to four-second yellow light to allow drivers to stop or continue through safely, Retting said. An intersection on a high-speed thoroughfare may need a five-second yellow light.

Cut it down to two seconds and even normally careful drivers will run the red light, according to Retting. In effect, “they get ticketed unfairly and make the environment unsafe,” he said.

Philadelphia took the most basic approach to stopping red-light violations: It eliminated the red lights. At 199 intersections with low traffic volumes, signals were replaced by stop signs; crashes were reduced by 24%.

Usually, though, the problem is much more subtle.

In San Francisco, a particularly gruesome red-light crash in 1994 launched the camera enforcement movement in California. A driver ran the light at 19th and Holloway avenues and slid through a crowd of 50 people at a bus stop. One of the city’s first enforcement cameras was installed at this intersection.

But traffic engineers discovered that traffic-signal timing was causing many of the red-light violations there because the signals were not coordinated with those at 19th Avenue and Crespi Drive only 250 feet away. Some drivers cruising normally past Crespi found themselves almost immediately confronted with a yellow or red light at Holloway.

After the signals were coordinated, very few red-light violations were recorded, said Tom Folks, senior traffic engineer.

“We think the prudent approach is to look at all the things you can do before getting to the point of installing the camera,” Folks said.