NC State University engineers propose fourth traffic light for autonomous vehicles

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Transportation engineers at North Carolina State University in the USA have proposed a fourth, white light at traffic lights which would enable autonomous vehicles to help control traffic flow and which would also let human drivers know what is happening.

Having conducted computational simulations, the university’s engineers have found the approach greatly improves travel time when moving through intersections and say it can help to reduce fuel consumption, too.

“This concept we’re proposing for traffic intersections, which we call a ‘white phase,’ taps into the computing power of autonomous vehicles (AVs) themselves,” said Ali Hajbabaie, author of the paper and an associate professor of civil, construction and environmental engineering at NC State. “The white phase concept also incorporates a new traffic signal, so that human drivers know what they are supposed to do. Red lights will still mean stop. Green lights will still mean go. And white lights will tell human drivers to simply follow the car in front of them.”

The team’s white light concept relies on AVs being able to communicate wirelessly with one another, and also with the computer controlling the white traffic signal. If a certain number of AVs reach the intersection at the same time, the white light is activated. This signals to humans that AVs will now conduct their own movements through the intersection to make traffic movements more efficient. Vehicles being controlled by humans will be required to follow the vehicle in front of them, in terms of turning or stopping and starting. If there are too few AVs, then the traffic lights work conventionally.

“Granting some of the traffic flow control to the AVs is a relatively new idea, called the mobile control paradigm,” said Hajbabaie. “It can be used to coordinate traffic in any scenario involving AVs. But we think it is important to incorporate the white light concept at intersections because it tells human drivers what’s going on, so that they know what they are supposed to do as they approach the intersection. And, just to be clear, the color of the ‘white light’ doesn’t matter. What’s important is that there be a signal that is clearly identifiable by drivers.”

The “white phase” traffic intersection concept was first introduced by the team in 2020. This initial concept, however, relied on a centralized computing approach, with the computer controlling the traffic light being responsible for receiving input from all approaching AVs, making the necessary calculations, and then telling the AVs how they should proceed through the intersection.

“We’ve improved on that concept, and this paper outlines a white phase concept that relies on distributed computing – effectively using the computing resources of all the AVs to dictate traffic flow,” said Hajbabaie. “This is both more efficient, and less likely to fall prey to communication failures. For example, if there’s an interruption or time lag in communication with the traffic light, the distributed computing approach would still be able to handle traffic flow smoothly.”

To test the performance of the distributed computing white phase concept, the researchers made use of microscopic traffic simulators. These simulators enable highly complex computational models to be designed to replicate real-world traffic, down to the behavior of individual vehicles. Through the use of these simulators, the team was able to compare traffic behavior at intersections with and without the white phase, and how the number of AVs involved influenced that behavior.

“The simulations tell us several things,” said Hajbabaie. “First, AVs improve traffic flow, regardless of the presence of the white phase. Second, if there are AVs present, the white phase further improves traffic flow. This also reduces fuel consumption, because there is less stop-and-go traffic. Third, the higher the percentage of traffic at a white phase intersection that is made up of AVs, the faster the traffic moves through the intersection and the better the fuel consumption numbers.”

If only 10% to 30% of traffic at the lights was made up of AVs, however, the simulations found there were relatively small improvements in traffic flow. As the percentage of AVs at the white phase intersections was increased, so did the traffic flow.

“That said, even if only 10% of the vehicles at a white phase intersection are autonomous, you still see fewer delays,” commented Hajbabaie. “For example, when 10% of vehicles are autonomous, you see delays reduced by 3%. When 30% of vehicles are autonomous, delays are reduced by 10.7%.”

The engineering team acknowledges that AVs are not ready for the distributed computing approach yet and the researchers are aware that governments are not going to install new traffic lights at every intersection in the near future.

“However, there are various elements of the white phase concept that could be adopted with only minor modifications to both intersections and existing AVs,” explained Hajbabaie. “We also think there are opportunities to test drive this approach at specific locations.

“For example, ports see high volumes of commercial vehicle traffic, for which traffic flow is particularly important. Commercial vehicles seem to have higher rates of autonomous vehicle adoption, so there could be an opportunity to implement a pilot project in that setting that could benefit port traffic and commercial transportation.”

The paper, titled White Phase Intersection Control Through Distributed Coordination: A Mobile Controller Paradigm in a Mixed Traffic Stream, is published in IEEE Transactions on Intelligent Transportation Systems. First author of the paper is Ramin Niroumand, a PhD student at NC State; the paper is co-authored by Leila Hajibabai, an assistant professor in NC State’s Edward P Fitts Department of Industrial and Systems Engineering.

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After spending six years working as a mechanic for various motorsport and high-end performance car companies, Callum joined UKi Media & Events in February 2020 as an assistant editor. In this role he uses his vast practical knowledge and passion for automotive to produce informative news pieces for multiple vehicle-related sectors. Currently, he is responsible for content across UKi Media & Events' portfolio of websites while also writing for the company's print titles.

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