The billboard industry likes to proclaim its small, local nature.  When a state or local government considers enacting tougher billboard controls, industry lawyers and lobbyists appear at hearings and speak long and loudly about how the government is threatening another small business with extinction.  The reality, however, is quite different.  Nationwide, three companies, Lamar, Clear Channel, and Viacom [now CBS Outdoor] dominate outdoor advertising.

Additional Resources on the Billboard Industry:

Industry Tactics to Circumvent Regulation
Nature of the Outdoor Advertising Business
Billboard Industry Myths & Facts
Public Service Billboards: Signs of an Ulterior Motive
Fast FAQs: Sign Code Shakedown