Originally Posted by John E
3) To return to your original question, speed undeniably kills. Your odds of surviving a collision with a motor vehicle traveling 15 to 20mph are decent, but they drop to almost nil if you are struck with a car going, say, 50mph. (In San Diego County, we just lost a very experienced and talented cyclist when he was struck from behind on a 55mph road in Poway.)
That said, bear in mind that the wrong head hit, just falling of a bike not even moving, can kill too. A lot really depends on how you are hit. But indeed the higher the vehicle speed, the more energy it will impart into the human it hits.