Billboard is the least time of all advertising mediums. The viewer is often on the move and looks at your ad for only a few seconds. So the golden rule of outdoor design is ruthless simplicity: single visual, single message, few words.
Remote readability determines everything. Large and clear typography, a strong contrast and a breathing gap; details that work on small screens are completely lost here. The question 'Does it read from a car in 4 seconds?' is the only test of a good billboard.
When designing an outdoor ad, I take the text out of the sentence, down to the core idea, because there are few, always many in the open air; every extra word reduces the likelihood that the message will be read.