Adding an additional white light to traffic lights could enable autonomous vehicles (AVs) to control traffic flow and improve travel time, computer simulations run by engineers from North Carolina State University have found.

Under normal circumstances, the proposed four-light system operates in the same way as current systems three-light systems – red means stop and green means go etc. However, if a threshold number of self-driving cars are detected approaching an intersection the white light phase is triggered, overriding the three-light system.

This signals to human drivers that self-driving cars have taken control of the traffic flow and are coordinating their movements via wireless communication. Any cars being driven by a human are then simply required to follow the car in front – if the car in front stops, they stop; if the car in front goes through the intersection, so do they.

“This concept we’re proposing for traffic intersections, which we call a ‘white phase,’ taps into the computing power of AVs themselves,” said co-researcher Dr Ali Hajbabaie, an associate professor of civil, construction and environmental engineering at NC State.

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

“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 colour of the ‘white light’ doesn’t matter. What’s important is that there be a signal that is clearly identifiable by drivers.”

The simulations found that small improvements in traffic flow occurred when as little as 10 per cent of the vehicles at an intersection were autonomous. And as the percentage of AVs increased so too did the traffic flow.

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“Even if only 10 per cent of the vehicles at a white phase intersection are autonomous, you still see fewer delays,” said Hajbabaie.

“For example, when 10 per cent of vehicles are autonomous, you see delays reduced by 3 per cent. When 30 per cent of vehicles are autonomous, delays are reduced by 10.7 per cent.”

Although AVs are not ready to adopt the new approach in the immediate future, it would be relatively straightforward to implement with only minor modifications to intersections and AV software updates, the researchers say.

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Jason Goodyer
Jason GoodyerCommissioning editor, BBC Science Focus

Jason is the commissioning editor for BBC Science Focus. He holds an MSc in physics and was named Section Editor of the Year by the British Society of Magazine Editors in 2019. He has been reporting on science and technology for more than a decade. During this time, he's walked the tunnels of the Large Hadron Collider, watched Stephen Hawking deliver his Reith Lecture on Black Holes and reported on everything from simulation universes to dancing cockatoos. He looks after the magazine’s and website’s news sections and makes regular appearances on the Instant Genius Podcast.