The next time you see a billboard on the side of the road, it may also be scanning you.

A geolocation-tracking feature on billboards owned by Clear Channel Outdoor gives the company new ways to target advertising and measure its effectiveness. The service has caught the eye of privacy advocates, who worry that the so-called Radar tracker will be able to collect massive amounts of information from smartphones in cars driving past.

Radar will collect mobile data from three Clear Channel partners, including AT&T. “When you pull all of that together, you have lots of granular information” about a person, said Jeffrey Chester, executive director of digital privacy group the Center for Digital Democracy. “There are serious privacy issues here.”

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