Okay, you have a good point.
10% off the price if the sticker is aesthetically pleasing or witty.
When you say sticker, you mean a sticker that we send you right? How big of a sticker will fit there? Would you be willing to put up a large sticker advertising domain names for dating, escort service, etc.?
Also what is the traffic like. Slow moving or fast?
During rush hour (really about 3-4 hours each work day), the traffic goes 5-10 mph on that stretch of highway.
As for size, the limiting factor is how long it takes me to firmly attach it. I want to do it in a few seconds, so perhaps a 1 foot by 1.5 foot sticker is the largest I'd consider... also, pick an adhesive that will stick to rough concrete.
Advertising anything is fine.