Billboards offer a lot of bang for the buck! Regardless of medium you use, repetition is what makes advertising work. People are creatures of habit and have daily schedules that cause them to drive up and down the highways to and from work, taking the kids to school and picking them up, and daily shopping trips.
Strategically located billboards can give businesses a lot of visibility for just a few dollars per day. The repetition of your potential customers seeing your message every day develops awareness of your business or service creating what is known as, "top of the mind awareness." When your customer has a need for your business or service, your name is fresh on their mind, giving you a shot to earn their business.