Yeah some idiots would just let their car drive around all the time without them. I remember reading an article that they wanted to make a law specifically against this before it happens, but I can't find the link. Now if people can share the autonomous cars, that's a different story.
How? Being self-driving doesn't make a car take up less space.
A self-driving car will ideally drop you off in front of your commute target and go find a parking space at a convenient location that might even be outside town.
This would multiply traffic in cities, which would be a terrible alternative.
It would add some traffic to/from parking lots but reduce the “find a parking space near my target” traffic.
Multi-car parking areas (garages, e.g.) could be compressed somewhat if humans no longer need to be in the space. (When the car drops you off and then goes and parks itself.) There's space used for fault tolerance, and just opening doors, that would no longer be necessary given a "perfect" driver. I bet you could get another 20% usage spacewise. And there might even be other efficiencies to take advantage of. Even now there are parking garages where cars are stored on top of each other using some kind of mechanical lift.