Right now, per mile, self-driving cars are much less safe than humans. As has been noted before though, we don't have that many miles of data on self-driving cars, and the tech is still quite young and there's (probably) a lot of room for improvement.

Not all self driving car programs were created equal. Tesla and Uber have been acting like cowboys from the start. Most of the advanced lane keeping type setups GM supercruise and Nissan Propilot are exposed to similar risk as Autopilot, but excepting the gong show that is Uber, all the other companies developing L4 have a pretty stellar track record.

And, as you mentioned, we need to accumulate billions of test miles before we'll have statistically comprehensive data.

You can't compare statistics unless the underlying miles are comparable, though. The only miles that humans drive which should be compared are those that a given AI is capable of driving.

The fatality rate for human drivers is 1 death per 141 million km driven (US only, many other industrialized countries are significantly better)[1]

There are currently at least 4 confirmed deaths from autonomous vehicles[2]. While beat is correct in stating that those are not enough deaths to draw reliable conclusions from (and Model S owners are not like the average driver), there would have to be more than 600 million km driven autonomously to be comparable to the death rate of normal cars. Probably significantly more, as the cohort of human drivers that is equivalent to Model S owners is way less often involved in accidents in the first place.

[1] = https://en.wikipedia.org/wiki/List_of_countries_by_traffic-r...

[2] = https://en.wikipedia.org/wiki/List_of_autonomous_car_fatalit...