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How Driverless Cars Could Make Traffic Dramatically Worse

A new simulation shows that comfortable rides can come with big congestion costs.

Eric Brisson / Flickr

Safety is often celebrated as the biggest benefit of a world full of driverless cars, but two other presumed social improvements follow closely behind. One is that the technology could reduce traffic congestion, since shorter gaps between cars means more cars per lane. The other is that car travel will become more productive time for either business or pleasure—the way riding a train is today.

To wit: the way Mercedes envisions driverless interiors (top) isn't much different from the set-up already used in Amtrak's Acela (bottom):

Top: via Mercedes; bottom: The West End / Flickr

A new simulation-based study of driverless cars questions how well these two big secondary benefits—less traffic and more comfort—can coexist. Trains are conducive to productivity in large part because they aren't as jerky as cars. But if driverless cars mimic the acceleration and deceleration of trains, speeding up and slowing down more smoothly for the rider's sake, they might sacrifice much of their ability to relieve traffic in the process.

"Acceleration has big impacts on congestion at intersections because it describes how quickly a vehicle begins to move," Scott Le Vine of Imperial College London, who led the research, tells CityLab via email. "Think about being stuck behind an 18-wheeler when the light turns green. It accelerates very slowly, which means that you're delayed much more than if you were behind a car that accelerated quickly."

For their study, Le Vine and colleagues simulated traffic at a basic four-way urban intersection where 25 percent of the vehicles were driverless and the rest were standard. In some scenarios, the driverless cars accelerated and decelerated the way that light rail trains do—more comfortable than, say, riding in a taxi, but still a little jerky at times. In other scenarios, the cars started and stopped with the premium smoothness of high-speed rail.

Within these broad scenarios the researchers also tested alternatives that reduced speeds but improved smoothness, such as longer yellow lights or following distances. All told they modeled 16 scenarios against a baseline with all human-driven cars. The researchers then ran each simulation for an hour, repeated it 100 times, and calculated the average impact that scenario had in terms of traffic delay and road capacity.

In every single test scenario, driverless cars designed to create a comfortable, rail-style ride made congestion worse than it would have been in a baseline scenario with people behind every wheel.

The final traffic tolls ranged from annoying to frightening. In the baseline situation, without any driverless cars, each vehicle experienced a delay of 20 seconds at the intersection. When driverless cars accelerated and decelerated in the style of light rail, the congestion worsened from 4 percent (21 seconds) to 50 percent (30 seconds). The number of cars traveling through the intersection—at 1,793 in the baseline scenario—also fell between 4 percent (1,724 cars) and 21 percent (1,415 cars).

The HSR-smoothness scenario was even scarier. Against the same baseline, autonomous cars that started and stopped like high-speed rail increased delay anywhere from 36 percent (27 seconds) to nearly 2,000 percent (6 minutes and 44 seconds!). Meanwhile, intersection capacity fell between 18 percent (1,469 cars) and 53 percent (850 cars).

In other words, if we want riding in a driverless car to be as comfortable as riding in a train, we need to consider the possibility that more traffic will be the result. Le Vine and company conclude:

Our findings suggest a tension in the short run between these two anticipated benefits (more productive use of travel time and increased network capacity), at least in certain circumstances. It was found that the trade-off between capacity and passenger-comfort is greater if autonomous car occupants program their vehicles to keep within the constraints of HSR (in comparison to LRT).

The work is a reminder that the full benefits of a driverless-car world might take quite some time to materialize—and that we should prepare for the challenges, too. Le Vine acknowledges that congestion might very well clear up once every vehicle in the fleet is autonomous, or even once there are enough to create driverless platoons. Until then, however, the traffic outcomes are much less predictable and very possibly negative.

Consider, for instance, that these simulations didn't include pedestrians. Doing so no doubt would have led to even more starting and stopping, and thus more delay. And if seatbelts remain mandatory in driverless cars, that might require smoother acceleration and deceleration; much of the comfort of a train ride, after all, is the lack of seat restraints. Traffic behavior would also change if manufacturers offer people several driving profile options—say, from ultra-smooth to aggressive.

All the more reason to think driverless cars will complement, rather than immediately replace, public transportation in cities.

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