It's just a quick-and-dirty mockup using PhotoShop and my favorite font for sarcasm - Comic Sans.
Lemmiwinks pointed out in comments that it could offend a motorist, who could buzz by too close in order to "teach you a lesson." And although he's right, he's also assuming that motorists are capable of (1) reading at speed and (2) comprehending what they've just read and (3) deciding to act on in in the second or two it takes to pass a cyclist. I'm thinking it's far more likely they'll be 50 yards past the cyclist before the brain kicks in and says, "What?"
Of course, it's still way better than this: