Cyclists who are new to riding in traffic often have a morbid fear of being hit from behind by a car. Don't bother, it almost never happens. The odds are very high that any problems you have will involve hazards coming at you from the sides... or from your own bike!

Besides the common types of collisions described here, I know several bike commuters who have been run into a curb or squeezed off the road by a car driving too close, or involved in a collision with another cyclist. But by FAR the most common source of injuries from my personal experience have been single-cyclist accidents. Common, preventable mistakes -- riding over train tracks incorrectly, the lower pedal clipping the ground while turning, catching a garment in the chain, losing balance while messing with something on the bike -- account for a lot of these. Major mechanical failures -- I know one guy whose bike literally sheared in half while he was riding! -- account for most of the rest.

I know it's hard to relax on the bike when you're worried about being hit from behind, but try to let go of that "target painted between my shoulderblades" feeling... and concentrate on dealing with the things you can see to the front and sides of you.