There are two big things to remember when writing copy for billboards: keep it short and make it simple. With outdoor advertising, your target audience is focused on something other than your message—the road.
Most likely you have only a few very short seconds to do your job, usually the time it takes for a car to fly by at 55 miles per hour (unless of course you’re lucky and there’s a traffic jam!). That gives you enough time for a quick headline, a catchy picture, and a phone number or web address—maybe. It’s possibly the purest form of advertising there is.