Cars are also unused for over %90 of the time, taking up space in some parking spot on premium land close to people's workspaces and homes. On almost every road in america, 2 lanes of traffic are taken up by car parking. In a self driving car future, we can free up those lanes and put excess cars in parking / maintenance towers. The software system managing can predictively dispatch them based on previous demand curves to prevent traffic jams.

The amount of cars we would need as a society will equal the peak demand curve, much like electricity demand today. It will also be very likely that those cars will have multiple people in them since a large part of the cost savings will come from car sharing. In the future, we will probably have 1 motor vehicle serving 10 households, vs the almost 1:1 to 1:0.5 ratio we have here in the US today.