It’s likely that once all cars are self-driving, we’ll barely need any off-street parking spaces at all. During peak periods, virtually all cars will be on the roads driving people around. During off-peak periods, cars will still be on the roads, they’ll just pull over to the side of the road and stop.
As Brad Templeton points out in the middle of the night cars could double- or triple-park on 6- or 8-lane boulevards, park in front of driveways, and so forth. This won’t be a problem because they’ll be able to instantly get out of the way if they’re blocking the path of another car.
One question is whether a world of all self-driving cars will require less energy (transport will be so efficient) or more (we’ll use them more often).