That s how long your viewer will have an opportunity to see your ad. Remember, if your ad is on a highway, cars are traveling at an average of 55 mph or more. Instead they will ignore the ad altogether as too hard to read. It is generally recommended to use no more than 7 words.
Simplicity is key.
FONT SIZE Notice whether the copy lines are instantly readable at 300ft for posters and 500ft for bulletins. Better creative typically has sturdy letters with even spacing. Make sure your text does not crowd the letters together, resulting in confusion. The eye typically moves from the upper left corner to the lower right corner of a billboard. Consider this when creating your artwork, including how the images and text are juxtaposed.