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

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.

“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.

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.

Rakha and his colleagues at VTTI came up with yellow times to account for various driver populations in extensive testing on the Virginia Department of Transportation’s Virginia Smart Road. A certain amount in yellow time that will result in 95 percent of the drivers not being caught in the dilemma zone and only potentially 5 percent being caught in the dilemma zone. For a result of 99 percent not being caught in the dilemma zone, a longer yellow time is required.

“If you’re designing the yellow times in Florida, where the majority of the drivers are older, you have to come up with a different yellow time than if you’re designing yellow times for here in Blacksburg, Virginia where the population is younger, and the majority of the population is students,” said Rakha.

Possible solutions
Although none of the possible solutions Rakha mentioned are likely to occur anytime soon, there are several distinct ones that could provide drivers with advance warning, more time to react, and better decision-making options.

In-car warning systems – “Potentially you could have a warning in your vehicle that basically recognizes your age, gender, and can warn you before the yellow light comes on, to give you that extra time you need to take your action,” Rakha said.

In a paper sent to the Department of Transportation, Rakha proposed that automakers could possibly create such a system. But it would have to be more than just a forward-collision warning system. It would have to be in communication with the traffic signal controller, vehicle-to-infrastructure (V2I) and infrastructure-to-vehicle (I2V).




Department of Transportation vehicle-to-vehicle (V2V) Program



V2I and I2V “The infrastructure would inform the vehicle that the light will be turning yellow in, say 10 seconds,” said Rakha. “When you’re approaching the traffic light, you’re going to get some kind of communication, and then the vehicle can react. It could be some kind of in-car driver display, providing a countdown to yellow to alert the driver to start slowing down, or giving the driver the actual decision by automatically applying the brakes. Of course, sensors would be required to measure wet and dry conditions.”
According to Rakha, his group’s proposed approach could be integrated within the Connected Vehicle Research program, which can gather information on the driver, the subject vehicle, and surrounding traffic conditions to execute safe and customizable change interval in-vehicle warnings.

Flashing yellow - Drivers approaching a high-speed intersection need a much longer yellow. A flashing yellow informs the driver earlier that it’s going to be yellow so that he can make the decision. “If I’m upstream of this, I should stop. If I’ve passed that flashing yellow, then I should run,” said Rakha.
This is something Rakha’s group suggested in their report to the DOT. Germany, for example, has a flashing yellow on the traffic lights a couple of seconds before the yellow comes on. 

Yellow line on roadway
“We also proposed to the DOT to draw a yellow line on the roadway,” Rakha said. “If the traffic signal turns yellow when you’re upstream of that line, you should stop. If you’re downstream of that line, you should continue at present speed through the intersection. You should be able to proceed without running a red light.”

But there are other potential benefits, such as minimizing emissions and vehicle fuel consumption. “Why should we incur an extra stop which consumes more fuel and emits more emissions?” Rakha asked.  “If you stop when you weren’t supposed to stop, you’ll stop very aggressively, which could cause an accident, a rear-end crash for someone behind you. The fact that you’re stopping does not mean that you’re a better driver. It could mean that you’re more dangerous.”

When will any of this take place? Rakha said that the DOT requires a rigorous process before changing their procedures for designing yellow interval durations. This is typically mandated federally from the Institute for Transportation Engineers (ITE) or AASHTO.

Rakha added that he plans to approach the DOT to see if his group can gather additional data to design specific yellow timings and test them in the field for real drivers.

The Car Connection Take:
With development occurring now on V-to-I and I-to-V communication systems, perhaps the horizon for longer yellow times isn’t all that far off. On the other hand, when and if self-driving cars become a reality, the need may be moot – at least on certain roadways.

In the meantime, we can perhaps draw our own conclusions from Rakha’s research about driver reaction times, approach speed and distance from a signalized intersection, in making a personal decision whether to stop or go. While Rakha emphasized he cannot recommend stop or go, what he can do is recommend to traffic engineers ways that they can come up with a better plan for yellow times.

Source:  The Car Connection