You bike in this weather? is a question Chicago cyclists constantly hear during this time of year. For some reason folks are astonished that people ride their bikes when it is cold outside, but hardly bat an eyelash at the idea when it is in 95 degrees and humid. As someone who rides all year 'round, I can tell you that riding when it is 20 degrees is much more comfortable than when it is 95 degrees.
I have personal limits. For the record, I did not ride to work today. I took the "El". It is -15 (-30 with the windchill). That does not make it too cold to bike. That makes it too cold to just be outside for an length of time, period.
When considering whether to bike in the winter there are safety issues to consider.
Snow. Snow is not scary by itself. Yeah, depending on how much there is, snow can make for a slippery ride. But if you remember to stay seated to keep weight on your rear tire, and take it slower than usual you will be fine. If there is a substantial amount of snow, reduce the air pressure in your tires to increase traction. You will be surprised at how much grippier your tires will be when running at very low pressure.
The real problem that snow can present is decreased visibility, by which I mean the ability of drivers to see a cyclist on the road. In the snow, turn on your bike lights. Illinois does not require bicyclists to use lights in bad weather (some other states do), only at night. But the smart cyclist should use them in crappy weather anyway. Also, do not hesitate to "take the lane" when riding in wintry conditions. Illinois law generally requires cyclists to ride to the right on the roadway. However, the law contains many exceptions, and permits cyclists to abandon the right when safety necessitates doing so. Often in Chicago, the ride side of the road, including bicycle lanes, will be a bumpy, icy mess in the winter. It will often be unsafe to ride on the right side of the lane in these conditions. Understand that you are within your rights to move left into the main travel lane when the conditions require it.
Ice. Hitting a patch of ice, particularly ice that cannot be easily seen, is scary. Again, substantially reducing your tire pressure can help you stay upright. You will want to reduce the pressure far more than you think. When it is very icy I will ride with my tires nearly flat, with perhaps 15 psi. The grip bike tires provide at that low pressure is amazing. There are downsides to riding with your air pressure that low. For one, you are at an increased risk of getting a "snake bite" flat, so do not leave then house without all of the equipment you need to change a flat, spare tube/tubeless tire, tire levers and pump/CO2. However, I have found that if you are riding slowly the risk of getting a pinch flat is low and worth taking given the traction benefits. Running at low pressure will also increase your risk of the tire coming off the rim of the wheel. This risk is lower the fatter your tires are. Also, tubeless tires are at decreased risk for encountering this problem because they are glued to the rim. Again, if you ride slowly, the risk should be minimal.
The other thing you can do to better your ability to stay upright in icy conditions is to ride with studded tires. Several companies make tires with small metal or composite studs that grip extremely well on ice. Lots of bike shops in Chicago sell these during the winter. The downside to studded tires is that they just plain suck on hard, dry pavement. They slow you down considerably and make you feel like you are pedaling in loose sand. They can turn a long commute into quite a slog. But, when there is a lot of ice, they are awesome.
Sun. Ever since the beginning of time man has yearned to destroy the sun. Okay, not really, but on the rare occasion that it shows itself in the winter, it can be a real bitch. With ice and sun on the road producing a mirror-like effect, glare from the winter sun can be blinding. This means than when riding into the sun, drivers may have a harder time seeing you. You may appear as a back-lit shadow to an approaching motorist, if you are seen at all. To increase your visibility, ride with your lights on. Also, make sure that you lights have fresh batteries so they operate at maximum brightness. When turning, remember to use arm signals to indicate your intent, as is required by Illinois law. In sun glare conditions be extra vigilant about doing so. Wave your arms like a crazy person if you have to, just do your best to be seen.
Dark. Here is something you already know: It is dark a lot in the winter. If you are a bike commuter, you will often be riding to and from work in the dark. The law, and good sense, requires that you ride with a front facing headlight and at least a red rear reflector. In addition to those requirements, I suggest riding with a bright, red rear light. You should also consider riding with something, reflective tape on your bike or clothing, or lights on your wheels, that increase your visibility to motorist from the side. Some manufacturers even make bike tires with reflective sidewalls.
Equipment problems. The CTA and many airlines were experiencing equipment problems today due to the severely cold temperatures. I am guessing that there were more than a few Chicagoans who had cars that would not start this morning for the same reason. Bikes, of course, are much simpler devices than planes, trains and automobiles and less prone to breakdown. However, equipment problems can happen. I have had derailleurs freeze on me. I tend to see this happen when I ride in wet conditions, then leave the bike outside during which the temperature plummets causing wet parts to become icy parts that do not work very well. Plain filth can cause equipment problems too. Looking at the muck and grim that accumulates on your bike after just a single winter ride you may be amazed that so much nasty junk exists in the world. A build up of grim can cause parts to fail. To reduce the possibility of bike equipment failure in the winter, store your bike indoors whenever possible. Also, clean your bike way more than you do during the rest of the year. After riding in snow, take a few seconds to wipe your chain with a dry rag. This will keep it cleaner and decrease rust build up. A reasonably well cared for bike should get you around town all winter long without a hiccup.
I am not aware of any data that demonstrates that city cycling in the winter is more or less dangerous than during the rest of the year. Like in the warmer months, the key to staying safe when sharing the road with cars is being as visible as possible. Do that, and winter riding will be safe and a lot of fun.