If they get in a situation they cannot handle, the car can stop and pull over to the side of the road before asking the human to take over. And it can refuse to navigate to areas where it wouldn't be able to do that. (And if you're in an area where an emergency stop is unsafe, it's unsafe for human drivers too. Getting rear-ended for an emergency stop is that fault of the following car for that reason.)
Or you'll need to call a tow truck when it gets disabled, or you'll need a stage 5 vehicle rather than stage 4, or you'll have to subscribe to an emergency low-speed teleoperation service that has been hypothesized.
