> My house is two miles away from the office. I still need a car: biking to work in the winter is just crazy.
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).
For me the biggest part was having a place to change. I commuted rain/snow/shine for almost 2 years straight in Chicago, but I was only able to make it happen because I had a gym membership in the building. Some places around here are (supposedly, so I hear) getting better about having more of a locker room style bathroom to allow for people who ride to work and not be a sweaty mess when they get there.
For about four years I did bicycle commuting year-round, rain or shine (no snow here). It was only possible because I had an indoor place to store my bike, a place to hang my clothes to dry, a place to change and wash up a bit, and space at work to store things like dry shoes and indoor jackets. This allowed me to just accept the weather and completely change between biking at work.
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 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.