The LA Weekly, after a legal battle with ClearChannel and the City of Los Angeles, obtained a court order which required ClearChannel to provide a list of the locations for each one of the 1,600 billboards it claims to own in the city. ClearChannel turned over a list of 300 billboards, with incomprehensible "addresses" like "Lincoln ES 0.6mi S/O Washington F/N-1."
To quote the print article:
Los Angeles officials suspect that 40 percent of the estimated 11,000 billboards in LA are illegal.
A lot of the talk around graffiti depicts graffiti as a conflict between criminals and/or artists (depending on who frames the debate) vs. corporations and/or governments. It's very important to realize that corporations are in fact sources of graffiti just as much as graffiti artists are.
Illegal visuals are illegal visuals. In terms of the legality of their public displays, corporations are frequently every bit as criminal as graffiti artists are. An honest debate around the meaning of graffiti must acknowledge that.