GPS is helpful, and key, to autonomous cars. But let's not forget the various ways GPS can be hacked. We need multiple methods of navigation (visual/radar/lidar to stay within lines and avoid obstacles, inertial/GPS like aircraft use to sanity check your inputs, ground-based authenticated systems to identify current location (imagine being able to poll an intersection for its identity, we already have computers at every intersection with traffic lights)).

If we rely too strongly on GPS, we eliminate many existing roadways with tunnels, and many convenient constructs like parking decks, or new tunnels where they make sense.