I added one for Highland, MI. I'm not 100% sure if it's legal... but it's just a huge wooden wall in the woods on some trail in the park. It doesn't have any purpose of being there. People paint on it all the time. I've never ever been hassled by any passer byers.
i hate legal graffiti, i feel like its so much less impressive doing something that you can take your time on without having to worry about getting caught then doing something that you have to do fast because there's a chance you might get caught. i think part of graffiti is the illegalness of it. as banksy said "IF GRAFITTI WAS LEGAL IT WOULDNT CHANGE ANYTHING"