There are plenty of documented accidents involving autonomous cars, the latest fatal Tesla crash being one of the most famous,
http://www.theverge.com/2016/6/30/12072408/tesla-autopilot-car-crash-death-autonomous-model-s
I would never trust a computer to drive a car for me. The computerised Dubai train system is a fantastic thing but it's on rails, it has nothing to crash into. The open road is a whole other story. There are just too many variables and unknowns. The Tesla didn't see a 40 foot truck? What else is it not going to see? What happens if the road becomes covered in sand and the car can't see it anymore? What if the white line is obscured? What if there is a break in the roadside barrier?
Check this out, an autonomous car almost swerving into another car which is only saved by the drivers quick reactions,
There are plenty of other videos showing near misses which were only avoided by the driver taking action.
One question I would like to raise is, how many of these people promoting autonomous vehicles would be prepared to give up their own vehicles and travel exclusively autonomously for the rest of their life? I don't think there would be very many.