I grew up in Minneapolis, you should check out the bicycle commuting scene where you live. It varies by location, but in Minneapolis I was able to bicycle commute at least 4 days a week all winter long thanks to good plowing of bike paths, studded tires (only occasionally) and substituting a bus ride in right after significant snowfalls.
Winter bike commuting is not for everyone, but with the proper gear it's not as bad as it looks (less uncomfortable than getting in a cold car and waiting for it to warm up IMHO).
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.
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.
In a car one has restraints, a lot of armor to absorb impact, airbags. On a bike you've got ... a helmet.
Now, I don't have that luxury. I don't have a good place to store things, I don't have a good changing/cleaning area. I'm dreading the rain.
I just plan for the bike to be ruined and ride a crappy bike that I replace every so often.
The scrappy-looking dependable workhorse also has a side benefit of you not having to worry as much about it being stolen vs. something that still shines.