It also gets better depending on the medical services nearby and there competence. So having more competent medical services in cities, lowers murder statistics and car-fatalities. Thus the actual accident rate would only be visible in the countryside.

If we want to get closer to the "real" number, we would also need to account for mechanic failures of the car, wildlife, and suicide. We don't usually attribute people jumping in front of trains as the fault of the person driving the train. Fatalities per mile driven may very well be biased in favor of waymo that has nothing to do with safety of drivers, which in turn create uncertainty when trying to estimate the statistical result of making all cars Autonomous. For example, car owners can have a higher variance in maintenance than waymo, they can use the car in areas with more wildlife than city traffic, and they can commit suicide through other means than personal being behind the wheel (suicide statistics is itself quite complicated).

Um, in those two examples where it was ridiculously not waymo's fault it was still absolutely the fault of a human driver. I'm struggling to imagine a significant amount of cases where it's ridiculously not the fault of any human driver.