Writing billboard headlines is one of the toughest test for a copywriter. People drive past a billboard at 60 kmph. They have to be seen and read at a glimpse.
The letters have to be big so they can be seen from a distance, so you learn that the headline has to be short.
Ideally, four to six words, any longer and you have already driven past, on to the next billboard.
There is no room for copy. The whole message is just a headline, picture, tagline and a logo. The headline has to tell you everything you need to know.
Those same lesson apply to any headline you write, whether it is a headline on a banner, a Facebook ad, an email subject line or a YouTube video. People are flying past every message they see. Clear and easy to read headlines work best.