According to a study by the League of American Bicyclists, the largest cause of bicycle fatalities from 2011 thru 2013 was rear end accidents where the bicyclist is struck from behind.
Some 628 fatalities occurred in the above time period and fully 40 percent were hit from the rear.
Most of these accidents happened on urban arterial roads.
A frequent scenario is when a bicyclist moves into the road to make a left hand turn. A vehicle coming from behind may speed up and strike the bicyclist.
Bicyclists should be aware that a left turn merge situation is extremely dangerous. Never assume the other vehicles on the road see you. If you cannot be certain it is safe, it may be better to stop your bike than try to make a merge decision on the fly.
Take your time. Look in every direction. It helps to wear bright or florescent colors but never assume you are seen.
A bicyclist is usually at the mercy of the 3,000 to 4,000 pound vehicles they share the road with.