You are being too negative. In high density areas there is far less likely to be downtime, and the vehicles can wait just outside that area in a sensible parki

You are being too negative. In high density areas there is far less likely to be downtime, and the vehicles can wait just outside that area in a sensible parking place, such as one with electrical chargers. It is the suburban areas where they are more likely to need parking, and more likely to have it. I reckon self driving cars will also lead to significantly less car ownership. Taxis will become cheaper, more flexible and more plentiful (political shenanigans aside). The Zipcar model works too as the car can come to you. And of course the cars can be coordinated to alleviate congestion. For example the system may work out that a particular car going at 29mph for a stretch instead of the limit of 30mph would avoid a glut of traffic 5 minutes ahead, or take a different route, or tell it to wait 5 minutes when it is empty.