Using eyecatching colors and copy can make your Billboard memorable, burning your company name and image into the minds of your audience. Many boards on highways reach hundreds of thousands of people every day. That's a lot of different people seeing your ad every day. Most advertisers want the audience to remember the name of their company and what the company does as most important. 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.