If the roads are wet and slushy....just my usual commuter tires. That covers a lot of winter riding.

But, ice and snow can be a problem for me. When either are in the mix (under snow there can always be ice), I use studded tires. We put studded tires on a couple of old beater hybrids last winter. Best thing I ever did.

Cycling can bring out the kid in you, but never so much as riding fresh snow. There is something about riding when intellectually you think you shouldn't and having a great time doing it, that makes you feel like a 10 year old beating the system.

Also, our bike trail gets sort of plowed in the winter, but what it really means is that it ices up. The studded tires let me ride across the ice without any slippage.