It has to be able to handle every situation safely, but doesn't need to be able to continue operating in every situation. If unexpected construction blocks the road, it needs to be able to stop and pull over, it can't go haywire and just accelerate right into a bulldozer. But it doesn't need to be able to navigate every situation. For example, a self-driving car which couldn't self-drive at night would still be hugely useful to me, I'd pay double for that feature.