This weekend's Guardian has an article on ad blindness, using a newly-developed pair of spectacles to monitor what ads the wearer looks at, for how long. A round of questions then links that up to what brands and products you actually remember, and surprisingly (or perhaps not) they find that 99% of adverts have no effect.
Note that this is probably quite contextual - when bombarded with 130 adverts in 45 minutes, the brain is probably quite effective at filtering the whole lot out. If you saw 2 or 3 in the same amount of time, they might stand out more. Still, the point is that an entire landscape of massvertisement really doesn't help much. What good, then, does all that billboard budget do?