In winter sometimes the road markings aren't visible for weeks to months. In winter the lane positions are not absolute but emergent things that flocking humans create day to day by following each other. In winter the edges of ther oad are not straight, or absolute, but instead change from day to day and week to week. In winter, the road surface and road edges are made of the same material: snow. In winter it is slippery. And in winter sensors freeze and obscure, like you say.

All of these are very hard problems that aren't quite addressed by a bus driving on cleared bare roads. It's good to see someone tried to work on them instead of just pretending all of earth is like SF. Too bad it seems to have failed.