AASHTO Journal - Researchers Are Seeking a Fourth Traffic Light for AVs

Transportation engineers at North Carolina State University are proposing to add a fourth light to the traditional “red, yellow, green” traffic signal configuration to enable autonomous vehicles or AVs to help control traffic flow, while letting human drivers know what’s going on.

[Above image by NCDOT]

Ali Hajbabaie, an associate professor of civil, construction and environmental engineering at NC State, noted in a blog post that adding this fourth light – which would be a white light – significantly improves travel time through intersections and reduces fuel consumption in computer simulations.

“This concept we’re proposing for traffic intersections, which we call a ‘white phase,’ taps into the computing power of AVs themselves,” Hajbabaie explained.

“The ‘white phase’ concept also incorporates a new traffic signal, so that human drivers know what they are supposed to do,” he said. “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.”

In a paper, the professor noted that the “white phase concept” rests on the fact that it is possible for AVs to communicate wirelessly with both each other and the computer controlling the traffic signal.

When enough AVs are approaching the intersection, this would activate the white light. The white light is a signal that AVs are coordinating their movement to facilitate traffic through the intersection more efficiently.

Any non-automated vehicles – those being driven by a person – would simply be required to follow the vehicle in front of them: if the car in front of them stops, they stop; if the car in front of them goes through the intersection, they go through the intersection.

When too many vehicles approaching the intersection are being controlled by drivers, rather than AVs, the traffic light would revert to the conventional green-yellow-red signal pattern, he said.

“Granting some of the traffic flow control to the AVs is a relatively new idea, called the mobile control paradigm,” Hajbabaie noted. “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.”

He stressed, however, that the color of the fourth traffic light does not matter. “What’s important is that there be a signal that is clearly identifiable by human drivers,” Hajbabaie said.