The NYT has a little piece looking at how the impact of driverless cars will change cities. There's some interesting thoughts in there but I will happily bet goodly sums that the major impact is going to be to expand the cities, massively, in geographic area. Here's their set up:
While driverless cars might still seem like science fiction outside the Valley, the people working and thinking about these technologies are starting to ask what these autos could mean for the city of the future. The short answer is "a lot."
As they point out parking will be a lot easier for you just skip out where you want to be and the car goes park elsewhere. We might well find that we currently have a surplus of parking in the centre of cities as a result. It's also likely that driverless cars will reduce congestion: for two slightly different reasons. The first is that we rather expect them to be better at driving than we mere mortals. Better drivers can be packed more closely on the extant roads. The second is that speed will become less important: as we're going to be able to do other things than simply drive while commuting then we'll not get so uptight about a few minutes here or there in those commutes. This again will make traffic flow more easily.