Green Street Media is a British ad agency that emphasizes eco-friendly advertising. One medium that it uses consists of applying stenciled designs and a power washer to dirty London sidewalks. The company estimates that an ad should remain legible for 3-5 months.
Link via DudeCraft | Company Website | Photo: Core77
http://www.flickr.com/photos/alexandreorion/
http://www.wired.com/underwire/2007/10/dirty-trick-cau/
http://www.google.com/search?q=alexandre+orion+tunnels
It is still defacing public property for commercial purposes and should be prosecuted.