Atlantic Cities

The Quest for Safer Intersections

The Quest for Safer Intersections

It's happened to every driver. You're driving down the road, approaching an intersection and the green light turns yellow. Now you've got a quick decision to make: slow down and stop or keep going and beat the red. From certain distances, this is an easy decision to make. If the light turns yellow and you're still a block away, you're just not going to make the light so you slow down. If you're already driving through the crosswalk, there's no sense in stopping. But what if you're somewhere in between?

This is what the people who study traffic patterns refer to as the "dilemma zone" – do I have enough time to make it through, or should I slam on the brakes? This dilemma zone is of course also a danger zone, as the decision of whether to stop or keep going can have disastrous or even deadly results.

Researchers at Oregon State University are now trying to identify exactly what the size of the dilemma zone actually is, how it differs from street to street and how roads and stoplights can be improved to reduce the dangerous impacts it can have.

Much of the uncertainty or vagueness of this yellow light conundrum has to do with change interval timing, or how long it takes to go from green to yellow, yellow to red and red back to green. National standards, perhaps unsurprisingly, do not exist.

Most of the roadways in the U.S. are guided by one document: the Manual on Uniform Traffic Control Devices. It's a guidebook that standardizes road signs, markings and traffic signals. It doesn't, however, offer guidance on change interval timing, because the right timing can be dramatically different from block to block. The manual is updated periodically, though, which means that as new information comes to light, it can be integrated into how the nation's roads and streets are controlled.

There are a few standardized approaches to handling traffic light timing, according to a recent paper from the Oregon State researchers, including a relatively common calculation provided by the Institute of Transportation Engineers. The authors argue that a more standardized system is needed.

But in addition to light timing, many factors determine why drivers decide to stop or go, which makes understanding how to reduce accidents challenging. As explained in this paper [PDF] by David Hurwitz and others, one approach is to use the concept of "fuzzy logic" to help model intersection activity and understand exactly where these dilemma zones exist under varying conditions.

Driver decision making at a signalized intersection requires the estimation of vehicle position relative to the stop line, the speed and acceleration/deceleration capabilities of the vehicle, and the duration of the current indication. These quantities are continuously approximated by the approaching driver, and are therefore ideal fuzzy sets to be modeled with [fuzzy logic].

For parameters, the researchers went to some test intersections and observed horizontal and vertical curvature of the road, grades, clear zones, adjacent land uses, and the presence of guard rails. They also collected about 500 hours of video of cars driving through those intersections.

Through this data, they were able to establish with a fair degree of certainty how big the dilemma zones are at different intersections. By understanding where the dilemma zones are, engineers should be able to adjust the timing of lights and speed limits to reduce the potential for accidents. They also hope to ensure that drivers either going too late or stopping too soon will become less of a danger to themselves and others.

As the researchers note, laws concerning yellow lights vary from state to state and don't make it much easier to solve the dilemma zone on a large scale. Some only allow driving through yellow lights if it is "safe" to do so – the definition of "safe" being highly subjective. Other states allow cars to pass through intersections on a red light so long as the front axle of the vehicle is over the stop line before the light switches from yellow to red – again, an inexact science.

It's hoped that policymakers will be able to take the results of this study into consideration when crafting and refining laws and regulations around intersections. By creating a deeper understanding of how those last-millisecond yellow light decisions are made, cities may be able to help drivers to more consistently make the safest decision, or at least time their lights so that even when bad decisions are made, fewer accidents occur.

Top image credit: Ralph Orlowski/Reuters

Nate Berg is a freelance reporter and a former staff writer for The Atlantic Cities. He lives in Los Angeles. All posts »

Join the Discussion