When I hit a goalpost it was only "as fast as I can run" which may have been in the range of 15mph after doing a little research into human footspeeds (which is an odd looking plural to say the least, but I'm pretty sure I don't mean feetspeed)
I also managed to jump my head into a 2" wooden beam in college, which was quickly followed by hitting the tile floor. I'm betting that's not very unusual, though. For a best answer to this question I'd have to ponder it for a pretty long time.