John Metcalfe was CityLab’s Bay Area bureau chief, covering climate change and the science of cities.
Take a trip to the "dilemma zone," the strip of road where auto accidents are born.
You're coming up on an intersection and the light turns yellow. What do you do: Apply the brakes, or slam on the gas like you're trying to bust though a Nigerian military checkpoint?
This is actually a question that traffic engineers have spent countless hours investigating. Over the years, they've found that exactly how drivers react when a light switches to yellow has a lot to do with whether they're within the "dilemma zone," kind of like the "Danger Zone" but less cool (it doesn't involved high-altitude dogfights). This zone is a stretch of road before an intersection, varying in length according to a car's speed, in which a driver who sees a light go yellow might feel great indecision over what to do.
Before the zone begins, a driver can be pretty sure that he doesn't have enough time to make a fresh yellow light, and thus begins to deaccelerate. And after it ends a driver knows that it's too late to mash the brakes, and that proceeding forward is probably the best option. But within the 'D' zone, a driver might either decide to slow to a screeching halt or gun it in an attempt to clear the traffic light. Those two scenarios are the recipe for rear-end wrecks and T-bone crashes, respectively.
Here's a good visual explanation of the concept:
That graphic was developed by Traficon, a roadway technology company based in Wevelgem, Belgium. This summer, at the yearly conference of the International Municipal Signal Association, Traficon debuted a gadget designed to hobble the dilemma zone's lethal nature. Called TrafiRadar, the device uses video and radar sensors to tell when a car has entered the zone and then, if warranted, extends the length of a green light to allow the vehicle to pass safely through the intersection.
This type of technology has been tried in other forms across the country, with mixed results. A decade ago, the Texas Department of Transportation launched a three-year trial of"Advanced Dilemma-Zone Detection" systems at rural intersections. The department found that they reduced red-light violations by 58 percent and "severe" crashes by 39 percent. However, according to a 2003 paper by University of Nebraska highway researchers, similar tests elsewhere in the U.S. determined there was a big drawback:
[The studies] noted the long allowable gaps permitted by [dilemma-zone detection systems] increase the frequency of reaching the maximum green interval at which point the green is terminated and the yellow is displayed immediately without regard to the presence of vehicles in the dilemma zone. The termination of green in this manner is referred to as “max-out.” Thus, when max-out occurs, the dilemma zone protection is lost, which becomes a greater problem as traffic volumes on the approach increase.
In the sluggish gridlock of big cities, you can imagine these max-outs happening all the time. The result would be a traffic light that's just as dumb as any other.
Top photo of an auto accident courtesy of Rian Castillo on Flickr.