I happened to run across statistics (iihs I think) for fatalities to (human) drivers per registered vehicle-years, and the number is about 30 per million. You may or may not accept that this is the right type of metric, but it's interesting to think about what the number implies. I read that Uber had a fleet of 250 cars in testing. If an average human driver has a fatal accident once in ~33,000 years, then to demonstrate parity in self driving, Uber would have to operate their 250 vehicles for an average of 130 years between fatalities. Granted, this is driver fatalities, so hitting pedestrians would not count. But even so, it seems it is a high bar and you would need hundreds of years of testing at the current rate to be confident you are better than human drivers rather than having a statistical fluke.