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.
I've written a lot of print ads in my career, some have gotten awards, but all have effectively communicated a message. And isn't that really the point? Some advertisers forget that little tidbit. Be certain, I won't.