I think VRKELLEY brings up an important point about having a back-up. I started carrying one of those mylar emergency blankets just in case. Dressing for the ride, even five miles through a dense urban area, means that if I stop when it's very cold, I may well be hypothermic before I can change a tire, get on a bus, taxi, or other 'bail-out' option. Don't forget a little extra $$ to buy your way out of trouble too.