The coldest I've ever been riding in is about -34C [~-45 with the windchill]. At that point I had:
- a thick wool toque with heavy fleece neck tube covering everything on my head except a 1/2" slit for my eyes [no helmut]
- t-shirt
- Polartec 300 expedition weight fleece jacket topped by nylon-based Gore-tex jacket
- nylon pants covering 200 weight Polartec pants, with expedition weight long underwear
- Gore-tex shell mits covering 300-weight polartec fleece mitts covering polypro liner gloves
- all-leather day hikers with heavy wool hiking socks and liner socks
I tried wearing glasses, but they fogged instantly. I had to squint and hope my eyes didn't freeze shut. My ride to school was about 10 km each way, but there wasn't much traffic. You really have to go by how cold you get. I get cold hands and feet really easily, so that's where I have to bundle up the most. Other people get cold heads and such. I guess it's better to be a little too warm than a little too cold, so dress for a little colder than what you think the weather might be.