If you’re having a big sale, unveiling a new business or backing a political candidate a billboard is a great medium to get your message seen by a lot of people. A billboard is especially useful for drawing attention to problems in a community like not wearing seatbelts, preventing forest fires or rescuing the water supply from out-of-control pollution.
Billboards also have an advantage of repeating a message. Every time people drive by a billboard they get one more exposure to the message. You can try to look away, but somehow you can’t resist looking just before you pass by. You know it’s there.
Whether people act on the message is a different story. People will often ignore an advertisement until they need that something being advertised. In the case of wanting clean air and clean water, people will probably only act when they feel they can’t get it anymore. By that time it will be too late.