Hmm. If you look at the number of train accidents there have been, over the years, and bear in mind that trains run on well-defined tracks, and that there are only two ways that a train can travel on that track, and that the technology for railway safety is very mature now, etc. etc. I think that the optimism for safe automatic vehicles running along a road is severely misplaced.
I used to work for both the Signals and the Mechanical Engineering (trains) departments of the Underground. They are rightly obsessed with safety and cost does not come into the equation when called upon to rectify anything that jeopardises safety. With road usage, we are talking about commercial enterprises who have such things as shareholders and, so long as the deaths are not Boris-scale*, affecting the share price, then there's nothing to worry about.
The concept of "fail-safe" is ingrained into everyone involved in railway signalling: I'd argue that such is not the case with road technology.
*Bodies piled high.