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You’ve seen thousands of billboards, but have you ever been moved by one? Responded to a call to action? Chances are you can count the ones you remember on one hand, the ones you’ve liked on a couple of fingers. So, is outdoor advertising effective? When used appropriately, yes.

Outdoor advertising has its own strengths and weaknesses, and its own set of rules. Here are some guiding principles to help you get the most out of your billboards:

Remember the 4-second rule. On average, drivers have four seconds to digest the information on an outdoor board. Four seconds. Short headline, visual, logo. Time’s up. Try it tonight on your drive home. At the point you can clearly see a billboard, begin counting: One-Mississippi, two-Mississippi. Stop when you’ve passed the board.

Keep it short and simple. To make an impact, the information on the board must be a single concise message, ideally conveyed in 7 words or less. Unless your phone number is somehow repetitive or easy to remember, leave it off. Most drivers don’t have a pen and paper to write it down, and even if they did, they shouldn’t attempt to do so while driving. Leave the www off your url; it’s unnecessary. Boil it down to one short, impactful statement.

Make it big and clean. You can’t just resize your print ad and expect it to work. The font must be simple and large enough to read. The more copy, the smaller the font, the less likely drivers will be able to read it. Clean, san serif fonts 10″ to 24″ high on a contrasting background work best.

Test your concept. Don’t hesitate to ask your creative firm for proof of concept prior to production. If you’re producing your own creative, the Outdoor Advertising Association of America (OAAA), Lamar Outdoor and Clear Channel all have simulators to test your creative from a street-level view.

Billboards build awareness. They’re a great way to reinforce your print or broadcast ad, or simply get your name and logo in front of commuters on a daily basis. But they are not a targeted medium. Unless a billboard is solely directional, it must be used in concert with other media efforts to be effective.