Do what humans do and drive without lines while trying to stay to one side of the road. This doesn’t seem like a major problem to self driving snow cars. More like, how do you deal with other drivers who are RWD with summer tires and are fish tailing all over the place.
If the sensors are up for it... Mountain roads covered in snow pack are a white blur, especially in certain light. Best bets for actually knowing where the road is might be previous tracks. I've dipped a wheel in a ditch more than once, not from carelessness, but snow drift obscuring the road... A road that I drive 500 times a year
