From an article in the NY Times about billboards that look back at viewers...
"In advertising these days, the brass ring goes to those who can measure everything — how many people see a particular advertisement, when they see it, who they are. All of that is easy on the Internet, and getting easier in television and print.
Billboards are a different story. For the most part, they are still a relic of old-world media, and the best guesses about viewership numbers come from foot traffic counts or highway reports, neither of which guarantees that the people passing by were really looking at the billboard, or that they were the ones sought out.
Now, some entrepreneurs have introduced technology to solve that problem. They are equipping billboards with tiny cameras that gather details about passers-by — their gender, approximate age and how long they looked at the billboard. These details are transmitted to a central database. full story - free subscription required to view