The rules of a great billboard (great = cost effective) are as follows:
1. "Next exit"
2. EZ off and on
3. Free parking
4. Major thoroughfare
5. Take action now
6. Credible promise
Billboards are useful for building brand trust, but only if you run them for a very long time and over a wide area. In other words, it's an act of faith.
This billboard, on the other hand, probably started paying for itself before they finished painting it.