Anytime you make something cheaper, easier to use, more useful and/or more efficient you get an increase in usage. Autonomous electric cars are all 4, and will lead to a dramatic increase in the number of miles driven. Some of those miles we can imagine (for instance sending your car on a dead-head journey somewhere for somebody else to use it), but most of the increase will probably be from new usage models that we…
I worry that self-driving cars are going to be used as an excuse not to expand public transit, especially in the Bay Area. The problem is scale -- once the population reaches a certain size, the roads are going to be gridlocked even if every car is self driving. I'm not sure if the Bay Area is at that point yet, but NYC certainly is, as are most of the major cities of the world.
By reducing people's sunk cost in cars, it may well free up the public mind to alternative forms of transportation. A lot of political inertia comes in the form of personal sunk costs. Also, self driving vehicles could themselves be used to expand public transit. IIRC, In Portland in the 90's, there was this thing called the "Custom Bus" where an late-night adapted bus route could be formed by people phoning in a request for a pickup within 2 miles of a normal bus route. Automated vehicles and networked computers could let us do an even better job of adaptive transportation.