When you buy radio or TV you buy it based on the audience that you’re trying to reach.  You look at the audience demo; you look at the audience size; you look at where that audience is.  Oftentimes that message reach is half an hour to an hour away from your location.  Outdoor Advertising is one of the best reinforcement media out there.  Billboards serve a twofold purpose:  they can act on their own to get your message to the masses and at the same time serve as back up for your other media.  By increasing the recall value of that media, billboards can drive home your message.