Go to www.outdoorbillboard.com for a whole bunch of information on this topic. I know a bit about this business.
First off, there are a handful of companies that completely DOMINATE the markets all over the country which are Lamar, Clear Channell, Viacom, CBS, and a couple more companies probably control close to 90% of the supply of boards.
The biggest thing you need to know is that digital billboards make a RIDICULOUS amount of money. Example friend of a friend constructed a full size digital board down in Atlanta ((just picture a really gigantic plasma TV over 40 feet accross)) they cost a few hundred thousand dollars as well, but what's mind boggling is the revenue they can produce.
Each ad displayed on a digital board is called a "Loop" you can fit anywhere from 6 to 8 loops on one screen.... here's the crazy part though.... depending on traffic counts you can charge big advertisers like Coca Cola, Hilton etc as much as $5,000 to $6,000 per ad per month. Multiply this by fitting 7 companies per month X the ad price and you're talking some big money.
The hardest part is getting permits approved iin high traffic areas, as most of them are already taken by large outdoor advertising companies. Obviously the further out you get into rural areas your ad revenue per board drops but youd be banking on the development of the city to boost the traffic counts over time and having your ad revenue increase as well... similar to land supply constraints in real estate markets when rents rise higher.
Bottom line, the most profitable arena in this game for the next 20 years is to get into digital boards and tri-vision boards. Less than 1% of the existing supply of billboards are digital... its estimated over 20 boards a day are converted to digital, one of them could be yours..??