http://jacquesmattheij.com/Autonomous+driving+cars+one+big+s...
The google car seems very much reliant on its gps data, in the article there is this passage: "He did [interrupt auto mode] so twice, once when a bicyclist ran a red light and again when a car in front stopped and began to back into a parking space. But the car seemed likely to have prevented an accident itself."
Seemed likely is not really good enough though, you have to be sure enough not to interrupt the auto-driving at all for this to be usable.
The biggest problem with auto-driving cars that have a human supervisor is that there is nothing more dangerous than having to be alert for a long time when nothing happens. Invariably you will be distracted when something does over the course of tens of thousands of miles.
Regular traffic is one thing on mapped streets is one thing, but that 100's of thousands of emergency situations that a normal career on the road throws at you over a lifetime contains quite a few where I really wonder how a 'robot' driver would deal with them.
I'm all for 'automated drivers', but I'd like the researchers to be sitting in the passenger seat and to have their kids playing around the car (or come out on their tri-cycle from between two parked cars) while they drive it to prove that it is safe. Overrode but the car would have probably done it is not good enough yet, even though the achievement is very impressive this is not as close as it seems.
Computer programs are great at dealing with everything that you can think of beforehand, it's the exceptions and the response to those exceptions that matters in an application like this.
I also wonder how they'd deal with liability issues if their driver was not actually paying attention to traffic and the software made a mistake and caused an accident.