Researchers are proposing that a fourth, white light for autonomous vehicles (AVs) should be added to the traditional red, amber and green at traffic lights.

The recommendation has come from transportation engineers at North Carolina State University, who say the measure could improve traffic flow.

In simulations, they found this approach allowed AVs to help “control” traffic flow, which resulted in quicker passage through intersections and reduced fuel consumption.

The rationale behind the idea, which the researchers have termed the “white phase,” is actually quite straightforward, and is based on the fact that as AVs approach an intersection, it is possible for them to communicate wirelessly with each other and also the computer that controls the traffic signals.

When a sufficient number of AVs are involved, their movements could be coordinated – in other words, those going in one direction would go through, while those going in the other direction would wait.

The suggestion is that when this is happening, the white light would be activated. This would inform the human drivers of non-automated vehicles of what is going on, and they would simply be required to follow the vehicle in front of them. If it stops, they stop, and if it crosses the intersection, they would do likewise.

When the majority of cars approaching the intersection are driven by humans – ie not automated – there would be no need for the white light, and the traffic lights would simply operate via their conventional green/amber/red pattern.

A paper has been published on the concept, and one of the authors, Ali Hajbabaie, explained: “Granting some of the traffic flow control to the AVs is a relatively new idea, called the mobile control paradigm.” 

Hajbabaie said researchers first investigated the concept of a “white phase” in 2020, and have developed the idea over the past couple of years, using microscopic traffic simulators to refine their findings. The simulators are complex computational models designed to replicate real-world traffic, down to the behavior of individual vehicles. 

“The simulations tell us several things,” he said. “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.

“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%.”

While an immediate full-scale overhaul of traffic lights across the world is clearly not feasible, the researchers believe their findings should be investigated further. 

“There are various elements of the white phase concept that could be adopted with only minor modifications to both intersections and existing AVs,” said Hajbabaie. He cited ports where there is a reasonable volume of automated commercial vehicle traffic as one specific location where trials could be carried out.
A paper on the research was recently published in the journal IEEE Transactions on Intelligent Transportation Systems.