My city has a designated graffiti wall, Patricia. It’s a block or so long and runs along a 4-lane road that connects the NW and SW sections of town. The sidewalk in front of the wall has trash and recycle containers specially for the paint cans and paint brushes used for the artwork. What people paint on the wall is fascinating, and because the wall is never static and always changing, the postings are continually new and interesting.
The graffiti wall has also cut down on graffiti in other places of town. Once you make it legal in one spot, the thrill of painting illegally somewhere else is diminished.