...and the peds and fair weather cyclists vanish.

Layering is key, as temperature differences between morning and evening commutes can be dramatic. As a rough guide, I'll add a layer for each 10 degrees as the temps dip below 60. So in the 20s, 4 layers, all breathable. Below that, I usually drive. For extremities, various combinations of shoes, socks, gloves and head coverings. You'll work it out in a hurry, believe me.