From Wikipedia:
QuoteWoodblock graffiti is artwork painted on a small portion of plywood or similar inexpensive material and attached to street sign posts with bolts. Often the bolts are bent at the back to prevent removal. Documented as early as 1995 in New York,[1] woodblock graffiti has seen a resurgence in Chicago in 2007 as a reaction to the city's Graffiti Blasters anti-graffiti program.
Street Signs
Most street signs have standardized holes drilled in them that have been designed for signs to be attached to them by the a City's Bureau of Signs & Markings.[citation needed]
Style
The ability of artists to produce these blocks at their own leisure allows a fine level of individual detail to be used on each piece. This makes for art that is not easily dismissed as vandalism by general pedestrians
Here is a sampling of this type of street art:
(http://upload.wikimedia.org/wikipedia/en/c/ca/Woodblock_on_Milwaukee_Ave_Chicago_2007.jpg)
(http://upload.wikimedia.org/wikipedia/commons/3/3d/North_Ave_Chicago_Woodblock_Graffiti_.jpg)
(http://upload.wikimedia.org/wikipedia/commons/1/16/Woodblock_Enter.jpg)
(http://upload.wikimedia.org/wikipedia/commons/d/dc/Photo_-169.jpg)