Someone asked me for the best advice I could give someone training for a race, be it cycling or triathlon. Here it is...
1. ALWAYS wear a helmet.
2. ALWAYS carry your ID and health insurance info. (If you don't have health insurance, GET IT! If you ride enough, one day you'll need it.)
3. ALWAYS carry a cell phone for emergencies, when riding.
That's my best advice. It may not sound like much, but if you follow it, it may just save your life, or someone you ride with, one day.