Art is art. Whether it be on an overpass, on the side of a building or on a canvas. Most street ART is considered vandalism. Banksy, Shepard Fairey are great examples of street artists whose work can also be considered vandalism. They're very successful commercially, but they still go out and bomb a building every once in a while. Just because you or the government or the city workers who have to clean it up don't like it, doesn't mean it's not art. I guess that proves the point: art stirs emotion and stimulates reaction.