Imagine if instead of paying for downtown parking, you could just have your car cruise the streets by itself while you attended to your business, and then return to pick you up when summoned? A new study suggests that's exactly what driverless cars could do – and it may cause a big problem.

According to the University of California, Santa Cruz's Assoc. Prof. Adam Millard-Ball, it would cost car-owners an estimated 50 cents an hour for most self-driving automobiles to cruise city-center streets at typical traffic speeds. That's much cheaper than paying to park at a metered area, or in a lot. Unfortunately, however, all those "circling" cars would also increase traffic congestion.

Additionally, the operating costs would be lower at slower speeds. This could encourage owners to set their vehicles to cruise at such speeds, making the congestion even worse.

Utilizing game theory and a traffic micro-simulation model, Millard-Ball predicts that even just 2,000 driverless cars circling in downtown San Francisco would slow the traffic flow to under 2 mph (3 km/h). He likens the problem to that which occurs at airports' arrivals areas, where drivers slowly circle the area in order to avoid paying for parking.

So, what's the solution? Adam suggests a congestion fee – this could consist of a flat amount that would have to be paid in order to enter the city center, or it could even be based on a combination of factors such as miles driven, cruising speed, time of day, and location.

"As a policy, congestion pricing is difficult to implement," he admits. "The public never wants to pay for something they've historically gotten for free. But no one owns an autonomous vehicle now, so there's no constituency organized to oppose charging for the use of public streets. This is the time to establish the principle and use it to avoid the nightmarish scenario of total gridlock."

A paper on the study was recently published in the journal Transport Policy.