The fact that automated vehicles operate with much broader safety margins with regard to driving conditions definitely contributes to them being safer overall - humans choosing to drive in worse conditions objectively makes them more dangerous drivers. I also suspect that what others are getting at - that under the same driving conditions safety margins as robocars, humans have a lower accident rate - isn't true, considering the bulk of motor vehicle accidents occur during normal commutes during daylight hours in nominal weather at low speeds. I suspect that even if you control for the elderly, the drunk, and inclement weather, humans will still have worse statistics. I think that humans have a tolerance for risk and ambiguity that makes for statistically worse, albeit more predictable, driving behaviour. But much like you, I don't have any better data to back this up.