One question I think is what do we mean when we ask how safe self-driving cars are.
I think deaths per mile isn't a very meaningful measure. For example, suppose in the normal use case of day to day driving, a SDC will have an accident at a rate that is 50% less than a human, and let's define accident as any incident in which people or property are damaged. This certainly seems "safe".
However, suppose in the case of rare events, such as a pedestrian's trajectory intersects that of the cars, the SDC has an accident rate that approaches 90% - meaning, in 90% of such rare events, the car will have an accident. And let's assume for humans that this rate is low, comparable to the overall rate of accidents.
If these events are rare enough, then the total accidents per unit mile of an SDC may well be less than that of human drivers. However, if SDCs have known flaws where they will likely fail, such as consistently plowing into pedestrians, then in those particular instances they should be considered extremely unsafe.
Does a SDC need to outperform humans in all possible cases to be safe? If, on occasion, a SDC will plow down pedestrians (including children), but in the most common scenarios perform better than humans, resulting in overall fewer deaths per mile, are they safe or not?