People slow down from 35 to 30 or whatever. Humans don't slow down to the point at which an accident is physically impossible for all unexpected movements, because that would be zero considering there are frequently objects within feet or inches.

Self driving cars can emulate humans, but that won't bring them to human level performance without the ability to model other actors. If they try to mathematically rule out the possibility of accidents without such models, they won't be able to go anywhere.