Check out this great animation on lane control from cyclingsavvy.org

Driving in the middle of the lane actually protects cyclists against the most common motorist-caused crashes: sideswipes, right hooks, left crosses, and drive-outs.  A bicycle driver’s top safety priority is to ensure he or she can be seen by motorists with whom they might potentially be in conflict, and bicycling in the middle of a lane is one of the most effective ways to do that.  Most overtaking crashes involve a motorist who attempts to squeeze past (illegally) in a lane that is too narrow to share.