When I was 13, and I was riding on the back of my Mom's car for a laugh, while she was driving down the street (she didn't know I was on the back) and I decided to jump off. For some reason I thought since I was no longer moving, I would suddenly stop. Big mistake, and I ended up with 3 broken fingers, a broken wrist, and cracked my head open.
Keep in mind, she was only driving at around 30kph, so I'd hate to see someone's body after doing that same thing at 100+.
*shudders*