As far as I understand, the current behaviour of self-driving cars is to stop the cars safely in a safe place, to the best of their degraded ability. Once it's stopped, the human can either take over. If a human can't, you're stuck on the side of the road and you call a tow truck.
Which isn't really any different than any other sort of major mechanical failure.
That's all well and good if you make it to the side of the road, but what happens when the car decides it can't make it that far and stops in the middle lane of an Interstate?
I'd expect major mechanical problems with robots, like "the vision sensor died in the middle of an interstate," to be about as rare as major mechanical problems with humans, like "they had a heart attack in the middle of an interstate." If not rarer.
We're already okay with the possibility that a human might drop dead in the middle of an interstate and lose control and more humans might be injured or killed as a result. As long as it is not more likely that a self-driving car will drop dead, I don't see why it's any different.
(And this isn't even getting into things like the possibility of catastrophic failure being lower and the options for recovery being much higher. You can run two robots with redundant hardware; you can't run two humans quite as easily. A human who loses vision will lose their concentration; a robot that loses vision can use its last known positions of nearby cars to attempt to pull over immediately in an orderly fashion.)
What happens if your car breaks down such that it can't make it off the interstate?
It's the same principle. You try to do the safest thing you can, but yes, at some point a car will fail and be stuck in the middle lane of an interstate because the autopilot fails, just as can happen because of mechanical failure.
The same thing that happens when any other car breaks down in the middle of the road--a traffic jam. Cars (especially ICE ones) break down all the time, we know how to deal with it.
Which isn't really any different than any other sort of major mechanical failure.