Stop Or Go: What Do You Do At Yellow Lights?

September 25, 2012

No matter how many years you’ve been driving, you’ve likely encountered situations where you need to make a decision about what to do at a yellow light. Depending on where and when you drive most regularly, you may experience this on a frequent basis. The question becomes: do you stop or go?

The answer depends, for the most part, on how far away you are from the traffic signal when it first turns yellow. But it’s not as simple as that. Some drivers have slower reaction time than others, particularly older drivers. The weather and road conditions also have a bearing – or should – on your actions.

To get a clearer picture of driver behavior when dealing with yellow lights, we spoke with Hesham Rakha, director for Sustainable Mobility at Virginia Tech Transportation Institute (VTTI). Not coincidentally, Rakha is deeply involved in research to come up with new ways of designing yellow light times to account for the differences in drivers, as reported in a recent story in Claims Journal.

Virginia Smart Road test site at VTTI

Virginia Smart Road test site at VTTI

“Every driver, depending on their characteristics, actually needs a different yellow time than another driver,” said Rakha. “What we found in the field was the drivers were typically more aggressive in their deceleration than what is used in the design. However, depending on the age of the driver, the gender, also the weather conditions, those strategies will change.”

Rakha said that the current procedures for determining yellow light time use a fixed perception-reaction time of one second and a deceleration level of 3 meters per second squared. But research by Rakha and his colleagues found is that drivers are different. “If you spend a longer time perceiving and reacting, in order to overcome that you typically press on your brakes harder in order to compensate for the time you lost in reacting to the change,” Rakha explained.

Dilemma zone test site at VTTI

Dilemma zone test site at VTTI

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The dilemma zone

Based on field results, Rakha said that no matter how well yellow times are designed, there is the possibility that someone could be caught in the dilemma zone, also called the decision zone.

“When you’re very far away from the traffic light the decision is easy. It is also very easy when you’re close to the traffic light and it changes. When you’re close you know you should run, continue through the light at your current speed, because there is no way you could stop. And when you’re very far away, you know you could stop. There’s no decision involved.

“The dilemma zone comes when you do not have a correct decision to make,” Rakha said. “If you tried to stop and with the parameters the lights were designed with – a one second perception-reaction and 3 meters per second squared deceleration rate–you will not be able to stop before the light changes and, if you’re trying to run, you will not be able to run before it changes to red. You have no correct decision to make, unless you can pull yourself out of that dilemma zone, either by reacting quicker or being more aggressive on the brake pedal.”

An unfortunate consequence off suddenly slamming on the brakes is a possible rear end collision. If you’re traveling faster than the speed limit, or if you spend more time distracted and take longer to react, you can actually be caught in the dilemma zone.

Designing longer yellow times

Studies of driver reaction times and vehicle deceleration rates used in determining appropriate yellow and all-red change intervals were conducted more than 25 years ago (although some recent studies have occurred during the past couple of years).  Additional studies are required to validate whether these reaction times and deceleration rates are still appropriate.

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