I adjust based more on my speed than weather or visibility. At low temperatures, I'm a lot slower, sometimes half of my summer speed, and on major roads the overtaking speed of cars is just too high for me to be comfortable.

I'm more likely to use MUPs and (yes) sidewalks, especially on longer uphills, since winter pedestrians are few and far between. The additional slow-down at intersections is much less of an issue since I am already going pretty slow.

In addition, the MUPs tend to have less slush and salt and be a better surface to ride on.

When there is actually snow falling or on the road, the speed of traffic is reduced almost to zero, so I'm more likely to be back on the main roads.