In our city graffiti is allowed on a couple select bridge pillars and retaining walls. Those often have excellent artworks that anyone can appreciate.

Not that illegal graffiti can't produce great art too, and the illegality is a part of what fuels the culture. But I've come to the conclusion that if a place is likely to be "vandalized" by graffiti anyways you are better off just allowing it and see what people do without the time pressure of avoiding arrest.