How did I miss this thread? I did a sociology semestral paper on graffiti some 11 years back. Had a boyfriend from Belgium who was in Graffiti youth group - combining the art with social outreach and leftist politics... It was fun. I traversed Vienna, Prague, and Brussels - bridges, underpasses, etc... collecting photographs. Still have them at home somewhere, though not digitalized.
Brussels has a ton of impressive graffiti art, there is one particular highway overpass that's famous for its panels. I got a tour of the city's graffiti - just like other folks go see buildings, museums, parks... these youth group takes you around to see the best graffiti in town. They probably still do that.