US researchers reckon we need a fourth traffic light colour for self-driving cars

US researchers reckon we need a new traffic light alongside the red, yellow and green ones so autonomous vehicles (AVs) know when they can move in convoy.

Speaking to TopGear.com, lead researcher Dr Ali Hajbabaie explained the team at North State Carolina University are looking at ways of keeping the roads less congested as the transition from manual to fully autonomous traffic happens.

Currently, self-driving cars will simply follow the existing manually-driven traffic because the numbers on the road are miniscule. As more self-driving cars make it onto our highways though, that’s arguably not the cleverest way of choreographing these smart vehicles, particularly at signal-controlled intersections.

Instead, the team proposes an extra signal in the red/yellow/green sequence. That signal communicates with a lead self-driving car, letting it know when it’s safe to begin a convoy and that would get a bunch of driverless cars through the lights more quickly in one direction. Then the same signal can be applied in another direction. It's like Edinburgh's Military Tattoo for future junctions.

The team’s research has shown that with 10 per cent of AVs on the road, the difference in road speed and congestion isn’t massively different. However, when there’s a 30-40 per cent mix of AVs and manual drivers, the team’s results show a significant positive impact and reduction in delays. Basically, the whole intersection is being controlled by the extra light, rather than having to wait for the established sequence we know today to cumbersomely cycle through.

The extra light, which Dr Hajbabaie says doesn’t have to be the white one they’ve used, just different to the current sequence, won’t stop manual drivers from getting through the lights either. Instead, they can fall in with the convoy, provided they adhere to specific rules, like maintaining a steady pace and not overtaking.

He said: “Any small change in signalisation in traffic lights needs to be done very carefully with a lot of research, and understanding that different drivers, even from the same region, from the same culture, respond differently.”

Dr Hajbabaie says he’s often asked how accurately manual drivers need to follow, but he clarifies that it’s just a case of naturally following as you would now.

If you’re thinking the manual driver might accidentally stall the car in the middle of the platoon, the model has already allowed for that. “If the gap becomes too big, then the signal goes back to green, yellow, and red [sequence] for a couple of cycles," said Hajbabaie.

Admitting he basically plays with giant Scalextric for his research, he added: “We have a test bed of 1:18 scale cars. They are electric, and some are autonomous but some, we control ourselves.” Testing with full-sized cars depends on funding but Hajbabaie hopes to translate the findings into real-world testing in about a year from now. What do you think?