The city does clear the snow from bike lanes with little mini bobcat tractors, but there's only so much that can be done. In mid-winter, I think studded snow tires are a must. And even those aren't exactly magic. There were a couple of late season storms this year were the city basically didn't plow much at all because they knew it would melt; and while the slush was navigable easily with a car, trying to carve through that on a bike was pretty freaky. Studs are great on ice and hard pack but with mash potato snow they don't help much.

Even when things are cleared off, melting and refreezing can make the lanes treacherous. That said, I estimate the number of people riding mid-winter is probably less than 10% of what you see in Spring through Fall.