Red means stop, green means go, yellow means…?


I went through a yellow light today. I’d glanced away at the wrong moment, looked up to see the light had gone yellow, and realized I couldn’t stop without slamming on the brakes and probably skidding into the intersection.

Later, I was crossing a street downtown when a van went through the yellow in front of me. It looked to me like the driver had plenty of time to stop—but no doubt he had his own excuse.

It’s a rare driver who doesn’t run through a yellow light on occasion, and in most cases it’s barely even a conscious decision. You have a split second to decide to brake, keep going…or even speed up.

So how do we make that decision?

A transportation engineering graduate student at the University of Cincinnati recently decided to see what he could learn about the factors influencing the decision to run a yellow light.

In cooperation with the Ohio Department of Transportation and with the help of his advisor, Professor Heng Wei, Zhixia Li conducted research in Akron, Cleves and Fairfield, Ohio. The results were set forth in a paper called “Analysis of Drivers’ Stopping Behaviors Associated with the Yellow Phase Dilemma Zone—An Empirical Study in Fairfield, OH,” and were presented at the 2010 American Society of Highway Engineers National Conference held last week in Cincinnati.

Unfortunately the paper itself is not yet online, but a press release about his findings was put out a few days ago.

According to the release, Li found that lane position, type of vehicle, travel speed, speed limit and the timing of the light all figure in the running of yellow lights.

For example, he found that people in the right lane are 1.6 times more likely to speed through a yellow light than drivers in the left lane.

Drivers in heavy trucks are more likely to pass through a yellow light than drivers of automobiles, SUVs, vans or pickup trucks. I suspect that’s a matter of momentum: it takes a heavy vehicle longer to stop than a lighter one, and once it’s stopped, it’s harder to get going again.

I also suspect, though Li’s research has nothing to say on the matter, that in Saskatchewan in the winter time the incidence of people running yellow lights increases dramatically because suddenly all of us are dealing with the problem of momentum: brake too hard on an icy road and you’ll skid through the intersection, possibly out of control. Even if you do manage to stop, you may find it almost impossible to get going again. In effect, winter turns us all into heavy trucks.

(Er, turns our vehicles into heavy trucks. Although, after a month of Christmas goodies…)

Travel speed is a pretty obvious factor: the faster a vehicle is travelling at the onset of the yellow light, the more likely it is to pass through it. And that naturally means that the higher the posted speed limit, the more likely vehicles are to pass through the yellow light at an intersection.

Finally, there’s the timing of the light.

Yellow lights are typically set to last somewhere from three to five seconds. Drivers coming to an intersection with a longer yellow light are more likely to pass through it (presumably because they’re familiar with the intersection and know how long the light is going to be).

In fact, Li found that for every additional second a yellow light persists, drivers are three times more likely to pass through the intersection under yellow. In other words, drivers are three times more likely to pass through a four-second yellow than they are a three-second yellow, and three times more likely than that to pass through a five-second light, which if my math is right means that they’re a whopping nine times more likely to pass through a five-second yellow than a three-second one.

This kind of empirical data should be of great use to traffic engineers attempting to make better, smoother and safer the flow of traffic through cities.

It might even, the press release suggests, “help drivers consider their own actions when in the yellow-light dilemma zone.”

But that, I’ll believe when I see.

Permanent link to this article:

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Easy AdSense Pro by Unreal