White traffic lights, adding a fourth colour.

I hadn't heard of this concept before.
Since AV can communicate wtih themselves there needs to be a way for them to communicate with human drivers.
The white light will mean follow the car infront.

https://www.traffictechnologytoday.com/news/intersections/researchers-propose-fourth-light-on-traffic-signals-white-for-avs.html

Three-colour traffic signals have been in use for over 100 years with little change to their functionality, but now researchers at North Carolina State University believe the time may have come for a change – they propose a fourth ‘white light’ to signal to human drivers when it is time to let autonomous vehicles dictate traffic flow.

The proposed new system would enable autonomous vehicles to help control traffic flow – and let human drivers know what’s going on. In computational simulations, the new approach significantly improves travel time through intersections and reduces fuel consumption.

“This concept we’re proposing for traffic intersections, which we call a ‘white phase,’ taps into the computing power of autonomous vehicles (AVs) themselves,” says Ali Hajbabaie, corresponding 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.”