Not just that, but human drivers have a lot of "hidden variables" affecting their accident risk -- level of experience, state of mind, aggressiveness, familiari

Not just that, but human drivers have a lot of "hidden variables" affecting their accident risk -- level of experience, state of mind, aggressiveness, familiarity with local roads, and so on. Insurance companies have to try to account for all of that when calculating premiums, based on a limited number of data points. With autonomous vehicles, two cars of the same model running the same software should be more or less interchangeable. So even leaving aside the number of accidents that occur, the level of uncertainty should be a lot lower.