From Drudge:
"Billboards have tiny cameras that gather details about passers-by -- gender, approximate age and how long they looked at the ad... Developing..."
I imagine this is both legal and effective, but it is also pretty creepy. I don't spend much time looking at billboards anyways, but now I think I will deliberately avoid making eye contact, knowing that some creep is watching recorded video of me.
Furthermore, aren't a good deal of billboards owned by that massive company Clear Channel?