What generates recall more than anything is the number of times someone sees an ad. Many people drive to and from work or other places by the same route everyday. Put the billboard in their neighborhoods. You can pick any audience and in most large cities you can target them with huge, bold ads that they will notice and remember. If you want to pay less for every person that you reach then choose to advertise on billboards.
Billboards are the most efficient and lowest cost advertising found in the traditional media mix.
You need to remind them of who you are. Don't let your customers forget about you and get used to going somewhere else. Billboards are the best medium available for frequency, unless you have millions to spend to reach the same number of people as frequently on TV or radio. If you are trying to drive people to your establishment, what better way than a billboard just before your exit. Savvy advertisers everywhere say that the goal is the hit the audience with an ad as close to the point of purchase as possible.