There are many technologies that will assist driverless cars to navigate to where they need to go. One of those technologies is "beacon" technology, for lack of a better term, which tells the car where it is in the network. (If they are not common now, they most certainly will be necessary when multiple driverless cars are on the road at the same time -- they will tell the cars not only where they are, but how far apart they are from each other, for example, if they create special "driverless car lanes" in the highways in the manner that they have HOV lanes now).
Aircraft already use such a process to navigate. They switch frequencies as they navigate across the globe.
The driverless cars will end up doing the same thing.
These beacons for the driverless cars will basically be transmitters, in the same way that transmitters are used for the aircraft. The beacons should have certain standards that allow them to navigate ACROSS STATE LINES as well as within the states.
A good place to start would be communication between the individual states about this.
Information: Here's a good explanation of the distinction between cars that are "just" autonomous, versus those that are autonomous and also connected. It's my absolute strongest belief that the "connected" versions of the autonomous vehicles will be the ones that are most cost-efficient for governments to use, and the ones that will most empower those with disabilities.