But roughly only a half of accidents could be blamed on each car driver, so even safest driver would get only 50 per cent reduction in accident rate.

Sure, if you include when and where people drive as part of what you blame on them. (Safety conscious people might move to particular places, or spend evenings in, or so on, and so even if they're just as good at avoiding accidents conditioned on condition the total distribution is weighted by conditions, which they have some control over.)