This line of thinking is flawed because it assumes a smooth surface over the safety space, where if you make incremental improvements you will head towards some maxima of safety. e.g. : the wing fell off; investigate; find that you can't use brittle aluminum; tell aircraft manf. to use a more ductile alloy. Self driving technology isn't like that -- you can't just file a bug "don't mistake a human for a plastic bag", fix that bug and move on to the next one. No number of incremental fixes will make self driving that works as any reasonable human would expect it to work.