I am trying to envision the position in which you oufnd yourself hitting a Caution sign with your shin . . . .all I get is you, doing a Jackie Chan-like flying kick at the poor sign, which had somehow angered you.
It's a far less interesting but longer story than that, unfortunately (but almost entirely not my fault). I work as a bicycle race mechanic, often jumping from the pillion seat on a moto (see video below). At the Tour of California this spring, my driver, who has been flawless for the past 12 years, got chatting with a racer and didn't see the sign sitting low on a tripod next to the road. I yelled soon enough for him to barely get the bike around the sign, but the corner caught my leg. http://www.youtube.com/watch?v=dKN8RXHKtdU