If you think traffic in cities is bad now, just wait until autonomous vehicles arrive, cruising around to avoid paying pricey parking fees.

Perverse incentives: Driverless cars will snarl up city roads because cruising will cost less than parking, Adam Millard-Ball, associate professor of environmental studies at the University of California, Santa Cruz, writes in Transport Policy. Even worse, because cruising is cheaper at lower speeds, they’ll slow traffic to a crawl as they kill time, he says: "They will have every incentive to create havoc."

2 mph: Using a combination of game theory and traffic simulation models, Millard-Ball predicts that under the best-case scenario, even just 2,000 self-driving cars in San Francisco would slow overall traffic to less than two miles per hour.

The solution? He suggests cities impose congestion fees like those in London, Singapore, and Stockholm, where motorists pay a flat fee to enter the city center. And we should implement them now, before autonomous vehicles arrive.

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