They're really not effective you know. When I'm driving, if I suddenly think 'oh I better slow down a bit', it's because I've spotted a sign, or children walking, or whatever - not because I suddenly remember that advert.
And Dan is obviously talking about government spending, which is relevant - commercial adverts obviously are irrelevant here as they aren't funded by the government. Sometimes I wonder why you even bother trying to argue with Dan :P.

