There's a simple rule to effective outdoor advertising: less is more. Try to stick to one benefit or promise. Unless you're speaking to a target audience that is stuck in traffic, assume they have only a few seconds to read your message. Simply put, If you try to say everything, you'll communicate nothing.
DESPITE POPULAR BELIEF,
A PICTURE IS NOT WORTH
A THOUSAND WORDS.
When I started writing collateral for Nissan, car brochures were traditionally nothing but glorified picture books with very little body copy. As a copywriter, I found that a tad insulting. I told the client to imagine potential car-buyers sitting at their dining room tables. In front of them, lay all the brochures for all the cars in which they're interested. What would solidify their car buying decision? Big pictures with little information? Or a more detailed, copy dominant approach that actually educates the buyer on relevant benefits? Needless to say, pretty pictures didn't fair as well as smart words. A fact that was not only noticed by competing car companies, but by the International Automotive Advertising Awards who presented Gold, Silver and Bronze awards for work that would shift the paradigm of automotive brochures.