Either way, in many places (like San Francisco) the constraint isn't places that are too far away, it's places which have too much traffic between here and there. If you could replace a freeway full of human-directed cars with a freeway full of robot-directed cars, this would let you put more people not just further into rural areas, but around existing neighbor cities too. (One of the reasons the SF peninsula is interested in new "transit-oriented developments" near Caltrain instead of other developments: highway capacity on US-101 and 280.)