It seems to me that diverless cars will [at first] only work on prepared roads.

These are roads where only driverless cars will be allowed (perhaps just a lane on an existing road), and all the traffic, and lane, markings are designed for the computer to read. The roads are very carefully mapped out.

In some ways kind of like a train track.

The computer knows when it's almost time to exit the road and go to a regular road, and can alert the driver in advance. (Or simply pull off into a parking lot.)