Google claims that the wrecks their cars were in were almost all the other human's fault. That's probably technically true, though the little Google pod cars had a top speed of 25 even on streets with much higher limits so it's highly likely that their slow speed and sudden moves would occasionally confuse or surprise somebody who was expecting something a little more normal. Of course, stopping at stop signs and red signals is sometimes not expected by human drivers.

During the brief period when Uber was testing driverless SUVs in SF, they were observed running red lights several times. Perhaps they learned the wrong lessons from their attendant humans.

Driverless trains, however, do exist. Several transit lines and most airport shuttle trains are driverless. Even BART, technically, is driven by the computer; in normal circumstances, the driver (originally called "train attendant") only holds the doors and watches for the need for an emergency stop. Manual control on a BART train is mainly for emergencies and yard work - top speed 25. Main issue regarding driverless on a regular railroad is the lack of separation from the rest of the world and its randomness; driverless transit is fully separated.