A car with no driver, nor even a steering wheel, pulls up at your door. You jump in, bound for the airport. The car pulls away, turns off your street onto a main road and… hits traffic? It turns out that even when autonomous cars become reality, they won’t help much to ease the gridlock in our cities: for that, we need to overhaul traffic lights too.

With ordinary traffic lights, autonomous vehicles don’t improve traffic flow at all in most cases, according to new computer simulations by Carlos Gershenson and Jorge Zapotecatl, at the National Autonomous University of Mexico. Only at the highest traffic densities near gridlock do autonomous cars make a small increase in flow efficiency, of 7 per cent.

But giving intelligence to traffic lights as well created a different picture. In this scenario, Gershenson and Zapotecatl saw an improvement of 200 per cent in traffic flow compared to the situation with human drivers and ordinary traffic lights. Their smart lights were very simple: each set had sensors that could detect how many cars were approaching on each street and give priority to those with more traffic.


No communication

This set-up doesn’t require any special communication between the self-driving cars and the traffic lights. All the cars have to do is detect the colour of the set of lights ahead of them. “The advantage of autonomous vehicles is basically that they can react much faster to changes of speed,” says Gershenson.

The simulation also showed that combining autonomous cars and smart traffic lights meant the grid could handle much higher traffic densities before getting jammed.

Steven Shladover at the University of California, Berkeley, is sceptical of an assumption in the model that the amount of time different traffic lights are green for in a given direction evens out over time. A more realistic scenario is that engineers would program lights to display green for longer on higher-traffic roads, he says. Gershenson says his approach was inspired by Mexico City, where there is roughly equal traffic from all directions, so it doesn’t make sense to favour one.

Shladover says of the simulations with ordinary traffic lights that “I’m certainly not surprised that they didn’t see much effect whether the vehicles were automated or not”. The limitation on an urban traffic grid, he says, is the time it takes a vehicle to start up when a traffic light changes to green – something autonomous cars don’t improve.

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