I'm still pretty happy at 10 deg F. I've ridden as low as minus 5 or so. But somewhere in the single digits and below it's getting (and oohh, how I hate to say it) to be not so much fun anymore.

Icing glasses. Bummer. Breath (and snot) freezing on your face. Yuck. It's not too hard to keep your core warm. But the hands, feet, and face get uncomfortable.

Still, I would face up to it. But there are two things, I guess, that stop me. One is if I'm pretty sure to be dealing with a strong and/or steady headwind. The other is the snow pack that others have mentioned. It seems as though the roads are never clear when it's super cold. I guess that's because the ice just doesn't go away when it's that cold.

But I don't do too badly. The last two (very mild) winters I've ridden straight on through without giving much, or any, thought to transportation alternatives. Such as the car.

In more normal winters I've ridden all the time right up to Christmas or New Year. Then in January I've only managed two or three rides. February perhaps I ride a little more than I drive. By the end of Feb, it's riding season again.

That pattern, except in the real northern climates, is not just do-able. It's really not uncomfortable at all.