They can measure 10' X 40' or 14' X 48'. They are either painted or covered with a flexible vinyl sheet that fits over the front of the board like a tablecloth and is tightened in the back. As you go by those big boards you may see little wrinkles at the corners that indicate the use of vinyl flex. Each board will have a sign at the bottom edge of its frame telling you which outdoor billboard company owns it. Ask about the cost of production as well as the monthly cost of renting the board.
Paint stays beautiful for about one year.
Ask for your vinyl at the end of your contract. Store it yourself so it doesn't get lost or painted over. Color on these signs is critical to attract attention to them. Ask about the cost of production as well as the monthly rental charge. Have a few more than the actual number of posters you need printed in case of vandalism or wear and tear by inclement weather.