Sorry for being a buzz kill. I once considered doing something like this. I had the means and the opportunity. Then I had the following thought process.

1. Make funny sign.
2. Some idiot pays too much attention to the funny sign and gets in a car accident.
3. Someone is seriously injured or killed as a result.
4. Authoritarians, offended at your appropriation of their sign, make a big deal about the fact that sign's purpose is for safety.
5. Police figure out that you were the one who made the funny sign.
6. Judge throws you in prison for fifteen years.

Local governments are full of totally vicious authoritarians who have absolutely no sense of proportion with regard to their reactions when their power is undermined. This stop sign case involved witness intimidation. Stealing a stop sign is obviously dangerous, and there ought to be consequences, but these particular consequences were way, way the hell out of whack with the real culpability that the defendants had for the tragic events that unfolded. However, punishments that are totally disconnected from reality seem to be the general course of business for state and local governments.