On the other hand, humans also have the common sense not to (e.g.) follow the tracks of a car that has spun off the road.

I'd expect logic at that level might be more difficult to train for than the relatively low-level visual task of identifying and following tracks. But I guess existing self-driving systems already need to work at that level to predict the actions of other cars and pedestrians, so presumably the current data-driven training techniques would also handle it OK? (Assuming a lack of pranksters with shovels...)

What happen when you are the first on the road with no tracks to follow from previous cars and suddenly it turns into a blizzard/zero visibility? I am guessing the car can pull over and turn on emergency flasher...I don't know but to me I believe a Human will always have to take over at some point because you never know what mother nature will throw at you, keep in mind I am talking about a future where we aim for 100% autonomous car.