The problem with physical advertising is that there's no way to measure its effectiveness. For example, I live in Boston, and currently, H&M is advertising at the T-stop near my house. The last time I was at the stop I was thinking "how the hell do they know if that ad is actually sending customers to their stores"? This idea would solve this problem using two small hardware widgets.
The first would be on or near the sign at the T stop. It would pick up, and identify unique cell phone signals within 20-50ft. If it finds a new cellphone, it would record the visitor.
The second widget would be at the H&M store in town (or at all of them!). Whenever a new phone walks into the store, the widget would cross-check the unique signal with a DB and determine wether the user saw an ad this week/month/year.
With years of data, one could measure which ads are effective and which ones are not.
Thoughts?
P.S. There are two assumptions* I made: 1. Each cellphone emits a unique signal. 2. This unique signal can be picked up and detected with fairly straightforward hardware.
*I confirmed these assumptions with a friend who's a hardware engineer at the FCC.