I was checking out Washington state laws on an unrelated issue (actually e-mail spam) and decided to do a search for their laws on street spam. Here's the law on it:
RCW 46.61.075 Display of unauthorized signs, signals, or markings.
(1) No person shall place, maintain or display upon or in view of any highway any unauthorized sign, signal, marking or device which purports to be or is an imitation of or resembles an official traffic-control device or railroad sign or signal, or which attempts to direct the movement of traffic, or which hides from view or interferes with the effectiveness of an official traffic-control device or any railroad sign or signal.
(2) No person shall place or maintain nor shall any public authority permit upon any highway any traffic sign or signal bearing thereon any commercial advertising.
(3) This section shall not be deemed to prohibit the erection upon private property adjacent to highways of signs giving useful directional information and of a type that cannot be mistaken for official signs.
(4) Every such prohibited sign, signal or marking is hereby declared to be a public nuisance and the authority having jurisdiction over the highway is hereby empowered to remove the same or cause it to be removed without notice. RCW 70.54.090 Attachment of objects to utility poles.
It shall be unlawful to attach to utility poles any of the following: Advertising signs , posters, vending machines, or any similar object which presents a hazard to, or endangers the lives of, electrical workers. Any attachment to utility poles shall only be made with the permission of the utility involved, and shall be placed not less than twelve feet above the surface of the ground. Of course, this brings to mind the possibility that people are nailing rogue vending machines to utility poles twelve feet off the ground, in an attempt to skirt the law. I don't think I'll try to shark them.