So I just watched this video where a couple of guys showed at Defcon how they can hack the shit out of a prius.
http://news.cnet.com/8301-1009_3-57596847-83/car-hacking-code-released-at-defcon/
Being in a diriverless car when this happens would not be very pretty.
Also, think about this: What if the government demands backdoors to be able to assume direct control of your (autonomous) cars? (like they do with encryption keys on skype etc.)
From now on, I'm building my own cars, out of horses and wood.
Im going to find it funny when people "jailbreak" their cars and load driveing programs and such on them that will bend driveing laws past what they are programmed.
as for a backdoor in a car to make it stop I would doubt it because driverless cars are always going to have a controller to manualy drive them and that will override any kind driverless system (no one is going to buy a pure 1st gen driverless car) and on another note what probally going to happen is that while cars will drive them selfs they'll be autonomus in there own actions the first implementations of these systems are not going to know where every car is 24/7 they be limited to there own sensors.
what I do expect is that the goverment is going to want the gps signal from these cars and they'll track where you drive as they track who you call.