Van Wagner, the outdoor advertising company, has created billboards to advertise advertising on billboards. Catch that? It's kind of meta.
When a billboard doesn't get sold, its owner can keep the old ad up, thus devaluing the space, leave the board blank or put a placeholder on it in the meantime. Van Wagner chose to go with the latter, but didn't want a simple black-on-white "Your Ad Here" sign.
Instead, the company is now into its second round of posting its own curious ads on unsold billboards in New York and Los Angeles.