"I was about (sic) to bicycle commute at least 4 days a week all winter long"
I think you also have to factor in safety. You will inevitably take chances over time and one of those chances could lead to an injury that keeps you from working or gives you lifelong pain. Either from a mistake you make or from someone in a car or bus. (Of course this can happen in dry weather as well...)
Statistically you can also use the historical weather info on http://www.wunderground.com to look over several years data for the winter.
The way I factor in safety is by choosing my routes. Minneapolis is wonderful for this. Depending where you live and work you may be able to bypass roads completely.
In any case though, beyond reasonable precautions such as helmets and defensive riding, I'm not going to run statistics to figure out the safest way to live my life. Bicycle commuting in general isn't something I consider an extreme risk. I'd certainly rather take my chances at injury doing a real activity than become an invalid at age 60 because I never got any exercise—because if I have to go to a gym to get exercise "safely" it ain't gonna happen.
Yes, when a collision does happen, a car is undoubtedly a better place to be, for the same reasons a tank would be better still. However, when it comes to avoiding a collision, in a car one has a whole lot more kinetic energy, and a whole lot less visibility and maneuverability.