Its ultimately legistation and the practicalities, not the technology that will inhibit / hold back autonomy. As above the scenario of 'kill two occupants or three at the bus stop' all need hammering out. Then when EVERY possible scenario has been worked out, deliberated by every single government in the world, who allow/disallow what the OEMs have decided, then the autonomous car will appear on the roads.... except they will get hacked. Hacking an RAF Typhoon aint so easy, not so much because the tech is impossible, but because no terrorist can get their hands on one for a weekend to dismantle and fiddle with...not so with your average car in the car park.
We still don't have 'theft proof' cars, 30+ years after immobilisers/deadlocks and so on, so hack proof, death-proof autonomous cars, well... its a lot further off than many people think. Again, its not the tech, but the implementation thats the killer.
PS thats before we hit something which may be totally unexpected... people may not actually take to the idea as much as people think...Amazon Kindle anyone? Death of the book, forever? Hmmm....