Self-driving cars in Phoenix have seen their share of harassment over the last few years. But that's nothing compared to the road rage they'll inspire if one transportation planner's prediction comes to pass.

Self-driving cars could exacerbate city gridlock, according to a paper in the journal "Transport Policy."

That's because cruising around at low speeds will likely prove cheaper than paying for parking.

Using game theory and traffic models, author Adam Millard-Ball of University of California, Santa Cruz, showed that as few as 2,000 self-driving cars could slow traffic to less than two miles per hour in downtown San Francisco.

Many cities use high parking rates to manage congestion.

Millard-Ball suggests instead imitating cities like London, where commuters pay a fee to enter urban centers.

He adds that cities should put such fees in place before the cars become commonplace.