Here’s a link to a terrific Paul Suggett article in About.com on Six Basic Rules of Billboard Advertising. The rules are designed to help your customers to place effective billboard ads.
1. For billboards, six words or less is ideal. Drivers don’t have time to read complicated messages.
2. Get noticed but don’t make you billboards a huge distraction. Don’t cause accidents like the Eva Herzigova Wonderbra ads.
3. This is not the time for direct response. Drivers don’t have time to scribble down a phone number or a web address. Not sure if I agree completely with this especially if the phone number or web address is easy to remember.
4. Be smart, but not too clever. And never be boring.
5. The more billboards the better.
6. Don’t say it, show it. Consider a 3D or moving billboard which is eye-catching and memorable.