Answer: our streets become much more congested. This thought-provoking post comes from Quartz:

Researchers at the University of California, Davis and UC-Berkeley gave a test group free private drivers to see what would happen when people get self-driving cars. Vehicle usage soared by 83%. That’s a potential nightmare in traffic-choked metropolitan areas, where increased travel could erase the efficiency gains of the last fifty years.

The outlines of this future are coming into focus in San Francisco. Between 2010 and 2016, congestion in the city rose by about 60%, city officials estimate. Half of this was attributed to Lyft and Uber as more people took ride-hailing services instead of transit and other non-car forms of transportation. Cars driving around waiting for passengers to hail them added to the problem, especially in congested areas of the city.

City planners say that to ease congestion, it’s key to keep private AVs on the periphery of dense urban centers, and focus on public transit and last-mile solutions at the center. If “personally-owned automated vehicles are superimposed on today’s patterns of usage… that will lead to the hell scenario,” says Daniel Sperling, founder of the Institute of Transportation Studies at UC-Davis. What, then, would heaven look like? “UberPool without the driver.”

This is interesting because AV-evangelists are always saying that self-driving cars will make our cities more pleasant.