[loeg]

I think you’re confusing “better” and merely “good,” as well as “average” and “mode.”

The average situation is maybe 0.99 accident-free (made up number for illustration). You improve on that by being 0.995 accident-free, which is better.

I agree that “safer than some human drivers in some conditions” is insufficient.

[ModernMech]

What I'm saying is that you can't design a car to handle the average driving situation and expect to really put a dent in the accident rate, because accidents are highly correlated to particular situations. For instance, if all of a town's accidents happen in a highly foggy area, but your driverless car cannot handle the fog because it's been designed to handle the average situation (not fog), then how will it reduce accidents? I would think that to reduce accidents it would have to be designed to work in the exceptional case (fog).

Maybe I'm confusing the idea of average and mode, so if I am an example of what you have in mind for an average situation would help.