Billboards across the country will soon begin to spy on the behavior of passers-by and sell that data to advertisers.

Clear Channel Outdoor Americas, which owns tens of thousands of billboards nationwide, is on Monday announcing plans to use people's cell phones to allow its billboards to track the behavior of everyone who walks or drives past the ads.

"People have no idea that they’re being tracked and targeted," Jeffrey Chester, executive director of the Center for Digital Democracy, told the New York Times, which broke the news on Sunday. "It is incredibly creepy, and it’s the most recent intrusion into our privacy."

The marketing behemoth is partnering with AT&T and other companies that track human behavior to collect data on viewers' activity, which advertisers could then use to create hyper-targeted ads—similar to how websites track visitors through their browsers and sell that data to online marketers.

The problem, say privacy advocates, is that most people when out in public will have no idea that their every move is being recorded and analyzed and sold for marketing purposes. When similar ads that used smartphones to track behavior were installed in phone booths in New York City in 2008, there was loud public outcry and the billboards were quickly removed after a Buzzfeed investigation.

Indeed, even Clear Channel Outdoor Americas' own spokesman conceded to the New York Times that the company's new service does "sound a bit creepy."

Critics also note that the use of smartphone data to track the behavior of unsuspecting passers-by poses specific risks to children, who are more susceptible to advertisements, studies show, and who are also using mobile phones at younger and younger ages. Fifty-six percent of children ages eight to 12 have their own cellphones, a 2012 study found.

Facial-recognition technology is also increasingly used by advertisers to track behavior in public spaces, and many people remain unaware of it. The February 2016 issue of Consumer Reports drew attention to the growing phenomenon, and listed a few examples of how the technology is being put to use:

In Germany, the Astra beer brand recently created an automated billboard that noted when women walked past. The billboard approximated the women’s age, then played one of several prerecorded ads to match.

Retailers can use facial recognition systems to see how long people of a particular race or gender remain in the shop, and adjust displays and the store layout to try to enhance sales.

Using related technology, some high-end retailers in the U.S. have experimented with "memory mirrors" that perform tricks such as storing images of what shoppers tried on so that they can be revisited, or emailed directly to friends for feedback.

Public tracking techniques such as facial recognition are "largely unregulated," the magazine observed.

"People would be outraged if they knew how facial recognition" is being developed and promoted, Alvaro Bedoya, the executive director of Georgetown Law’s Center on Privacy & Technology, told Consumer Reports. "Not only because they weren’t told about it, but because there’s nothing they can do about it."