A good billboard ad can be the difference between 15 interested customers and 150 interested customers. So what's the best artwork or content you should put on a billboard? Well digital billboards are far different from a magazine or newspaper, so the graphic made for it must be different too. Billboard artwork should be vivid, brief, and use contrast to it's advantage. When an ad on a billboard has too many things to read and too many colors or pictures on it, it ends up having too many design elements competing for visual importance.
A good billboard most of the time will have 3 strong design elements. The first element is the most important one which gets the viewers attention with a "call to action", product image, service, or logo. The second design element will consist of support for the first design element and gives definition or detail to it. The third element will provide relevant information that tells the customer who, where, or how to contact to get more information to the first and second design elements.
So let's say a car company wanted to create an ad for a billboard. The first element might be giant text that says "financing available" or "$200/month". then the second element would be an image of one of their vehicles. Lastly would be either the companies name, website, phone number, etc to give the customer an idea of who they are and how to get ahold of them. The financing available would get the passer-by's attention, then the second element, the vehicle picture, would show them what they're financing, then the third is giving them a way to get the financing for the advertised vehicle. This example shows an ad that not only gets peoples attention, but quickly gets it's message to the potential customer.