> How is there not a very basic “pull over and wait” final fallback
Barring everything else, the proper failsafe for any vehicle should be to stop moving and tell the humans inside to evacuate. This is true for autonomous vehicles as well as manned ones–if you can't figure out how to pull over during a disaster, ditching is absolutely a valid move.
They now apparently run these things on the interstate, the car needs to do more than just stop.
If the alternative is that the vehicle explodes, sure. And since GP did say "final fallback", I suppose you're right. But if the cars are actually reaching that point, they probably shouldn't be on the road in the first place.
The not-quite-final fallback should be to pull over.