Someone is driving and stuck in a blizzard on the highway when Shabbat begins. It would most likely be life threatening to leave the car on the side of the highway and walk in a blinding blizzard. It's also life threatening to remain in the car in the cold with the engine off and leaving it on all Shabbat may cause CO build-up.
Given all these factors, can a person continue driving until he reaches the closest safe area? What if he is unfamiliar with the area he's in, but can manage to get to his home, which, in the snow, may take a few hours, but, at least, there, he knows he will be safe. Can he continue driving on Shabbat?