The Yellow Light Conundrum

Even before attending a driver’s education class, most would-be drivers know how traffic signals work: green means go, red means stop and yellow means slow down.

Or does it?

Real-world experience suggests that for many drivers, yellow is a cue to speed up, an indication that if you want to make it through the intersection, you’d better gun it.

What dictates whether a driver hits the brakes at a yellow light or races through it? A new study by University of Cincinnati doctoral student Zhixia Li identifies factors influencing the split-second decision that follows what he refers to as the “yellow light dilemma.”

Li conducted his research in cooperation with the Ohio Department of Transportation at intersections in Akron, Cleves and Fairfield. It will be presented this week at the 2010 American Society of Highway Engineers National Conference in Cincinnati.

He found that lane position, type of vehicle, travel speed, speed limit and light timing all influence drivers’ reactions to yellow lights.

Some findings were obvious — the faster a car is going when the light switches to yellow, the more likely it is to pass through the light. And the higher the posted speed limit is, the more likely vehicles are to cruise through yellows.

Some are less so: Drivers in the right lane are 1.6 times more likely to speed through a yellow light than drivers in the left lane, and drivers in heavy trucks are more likely to go through a yellow than drivers of automobiles, SUVs, vans or pickup trucks (presumably because it would be harder for them to slow down).

The timing of an individual traffic light also influences drivers’ decisions. Most yellow lights are set to persist for 3 to 5 seconds, and drivers approaching an intersection with a longer yellow are more likely to continue through.

In fact, for every extra second the light stays yellow, the likelihood of a driver passing through it increases more than threefold. This means that a driver is three times more likely to drive through a 4-second light than a 3-second light, and the same is true for a 5-second light versus a 4-second one.

Anti-red-light-camera crusaders have long argued that lengthening yellow light time is one way to avoid red light violations, and although camera defenders contend that drivers become accustomed to the longer transition time, research in Virginia indicates that this isn’t the case.

What Li’s research means for the red-light debate remains to be seen, but it should give drivers something to think about the next time they approach that most ambiguous of all traffic signals: the yellow light.