Supposing that there would exist car sharing systems such that a single car may be used throughout the day by several people, then we could cut back on parking spots, but I'm sure there will be many that own their own self driving car and share it with no one.
During morning rush hour the only difference from now would be that commuters could do the crossword puzzle instead of focusing on traffic.
I think you'll see some gains in efficiency, but negligible compared to how efficiently we can move people via rapid transit. At the end of the day you're still going to be having most often a single person in a large vehicle that takes up a lot of room on the road.
With sufficiently good self-driving systems, it is obviously the case that you can safely drive cars more closely together than you can safely do with people. It is not obviously much smaller - you only get to shrink the portion of time it takes the human to react, not everything else involved in stopping or maneuvering the vehicle.
Supposing that there would exist car sharing systems such that a single car may be used throughout the day by several people, then we could cut back on parking spots, but I'm sure there will be many that own their own self driving car and share it with no one.
During morning rush hour the only difference from now would be that commuters could do the crossword puzzle instead of focusing on traffic.