Another issue is that people will expect perfection from computers. If we say, look, we can eliminate 80% of accidents by switching from all human drivers to all computer drivers, that sounds great. And it may be absolutely possible to do that. But people are going to freak out about those remaining 20% (or whatever) of accidents that are caused by the computer's limitations, even though on average everyone is better off. We're much more willing psychologically to accept human error, than to accept the possibility that a computer will betray us and crash us into something.