There is a new technology, that counts the eyeballs that are viewing a billboard. And not just the ones close to it:

Xuuk eyebox2 is a $999 portable device with a camera that monitors eye movements and automatically detects when you are looking at it from up to about 35 feet away. Until now, Vertegaal says, such eye-trackers have been ineffective beyond 2 feet, required people to remain stationary and cost more than $25,000

Another example of classical media becoming measurable. Even better than the internet, where you still can’t measure eyeballs, only pageviews – yet you don’t know if the person really saw your ad.
(found at here)