It was a surprise to me when I learned it on a motorbike safety course, but it's useful to know that an impact with a solid object at 35mph is only 50% survivable - a blow to the torso at that speed will tear the aorta. It's even worse if the object is something small in profile like a lamp or sign post.

Here's a forum post by a guy who was lucky to survive a crash at 30mph (some nasty post-op pictures - may be NSFW): http://www.bikechatforums.com/viewtopic.php?t=253168