Originally Posted by cyclocommuter

I have ridden my road bike with 700C x 23 tires and my cyclocross with non studded winter tires (Continental Top Contact Winter II) and studded winter tires (Schwalbe Marathon Studded) in various wintry conditions including in wet, snowy, slush, and icy conditions. In addition I have also ridden in trails in the winter.

Bottom line, the best tire for all adverse winter conditions (snowy, slush, icy) is the Schwalbe studded tire. It is noisy, heavy but I have peace of mind when I use it in winter. The non-studded Continental Winter tires are excellent in wet and slushy conditions but they will still slip and slide much more in icy conditions.

For winter riding, you adjust the pressure depending on the situation. If I am going to ride on snowy trails or roads covered with fresh snow, I lower the tire pressure to around 35 psi. The lower pressure enables, as others have noted, a bigger contact patch. On studded tires, lower pressure means more of the studs are also in contact with the ground. Note that with most studded tires, the studs are actually not in the center. If the tire pressure is high, less studs will be in contact with the ground and I actually increase the pressure when I know there is a slim chance of encountering ice on the roads.

Slick road bike tires are fine for salted roads and even slushy roads but I avoid using these on snow covered roads and especially when I am not sure if there is ice underneath the snow. For icy roads slick road tires can be dangerous. They are also more susceptible to slipping when over painted or leafy areas in the wet.

Some pics of the slick and studded tires on my bikes in winter:

Plowed, wet, salted, roads - slicks are fine, just avoid painted areas on the roads and obviously any ice patches you see: