These Billboards Know Where You've Been

Clear Channel Outdoor Americas, a massive billboard company controlled by iHeartMedia Inc. with thousands of displays across the country, is debuting a new kind of consumer tracking technology the company is calling “RADAR.” The system taps into mobile phone signals to track how well ads are doing their jobs, answering questions like who’s really buying a product, looking up a brand, or talking about it with friends after they see an ad.
http://fortune.com/2016/02/29/clear-channel-billboard-tracking/