alan s 

What is the world coming to? The word "nuts" could refer to insane people or to male body parts, both of which are highly offensive to some people. It could also be making fun of people with nut allergies.

Perhaps the graffiti artist (if you could even call such a person an artist) should more clearly state his or her meaning. If the so-called artist is referring to bike enthusiasts, then just say so. Is it that hard? "May contain bike enthusiasts." If the "artist" is referring to mentally challenged people, such as many of our homeless, try saying "May contain mentally challenged people." If this artist is making light of people with serious nut allergies, which can be deadly, especially to innocent children, well, that is over the top!