More likely scenario will be self driving cars cooperating with each other to drive from start to finish without any stops. Whether on freeways or local streets, cooperation amongst vehicles will raise the average speed and the volume of vehicles you can process through a given area. Pools work to a certain degree if everyone is starting and ending at the same location. When they don't, then it actually takes longer than driving by yourself.

Final point, you can certainly build out less parking spaces, but pre-existing spaces won't go away without redevelopment.