Comment by Veserv

The commonly cited fatality statistics do not discuss fault, so, absent more precise information, a apples to apples comparison should not conditionalize on fault.

Why? Well let us try to imagine a world where Waymo has the statistically average fatal crash rate, yet is never at fault. In this world, every fatal crash occurs at the Waymos. You just have a bunch of crazy drivers roving the streets causing fatal crashes against (or involving) unsuspecting, not-at-fault victim Waymos.

If that is the world, then are these crazy drivers only crashing into Waymos? Probably not, these crazy drivers would almost certainly be crashing into a statistically average distribution of unsuspecting, not-at-fault victims.

So this imagined world is consistent with the overwhelming majority of human drivers engaging in 0 fatal accidents at fault with a small, crazy roving subset being at fault for all fatal accidents. So, in that world, a Waymo that is only ask good as the statically average driver would actually be infinitely worse than the overwhelming majority of human drivers. If you only replaced the 10th percentile driver (90% of drivers are better), you would actually be increasing the fatality rate. Only by displacing that crazy subset would you actually be decreasing the fatality rate.

Are we in that world? The Waymo numbers on fatal accident involvement with no fault actually kind of point in that direction (though the mileage numbers are still too low to make a strong conclusion). Seemingly massive crash and injury reduction due to massive reduction of at-fault crashes, but the fatality reduction does not seem proportional. Another possibility is that the Waymo driving distribution is biased toward being in situation that result in more no-fault fatal crashes.

But, that is all just conjecture and thought experiments. Unless we have better data, we unfortunately should be restricting ourselves to apples to apples comparisons which, in this case, do not conditionalize on fault.

Though, to nitpick on the original post, I think that the 2 fatal crashes in 170 million miles (~85 million miles per fatal crash which is similar to the USA human average of ~83 million miles per fatality), is probably not the correct analysis. The average fatal crash is probably 2 vehicles and 1 fatality. So, the 85 million miles per fatal crash versus 83 million miles per fatality is probably fine in the crash vs fatality direction. But I am pretty certain you would actually need to double the 170 million to account for the "statistically average mileage of the other cars".

Otherwise, if we have exactly two cars in the world each with 100 million miles and they engage in one fatal crash with one fatality, then we would conclude that car 1 has 1 fatality per 100 million miles and car 2 also has 1 fatality per 100 million miles. Naively averaging them, we could conclude that the overall fatality rate is 1 per 100 million miles. Therefore, over the 200 million miles the two of them drove, there were 2 fatalitys. Obviously incorrect, so we almost certainly need to weight them by their relative proportion when aggregating them.