This AdRants post says the best practices for billboard advertising better than I ever could. What is the expression, "The more you add the more you take away"? (Or something to that effect.) I wish more billboard advertisers would heed the sage advice of AdRants imparted in the last paragraph:
It's unfortunate most agencies and marketers don't realize the billboard medium is at its most powerful when it's used for simple, direct messaging. Two to four words and a simple visual is about all the medium can take. Too many marketers and agencies try to cram the entire marketing plan onto a single 14 X 48 foot space which is usually seen for less than three seconds. For billboards, simplicity rules. In creating this McDonald's billboard, Leo Burnett clearly understands this.