Clear Channel Outdoor, one of the world’s largest billboard companies, will in coming days roll out technology across Europe capable of letting advertisers know where people go and what they do after seeing a particular billboard.

Sounds creepy, no?

Well, brace yourself. Clear Channel has been quietly using this technology in the United States for the last four years, including in Los Angeles.

“They’re spying on you in your own neighborhood,” said Jeff Chester, executive director of the Center for Digital Democracy.

“You don’t know it’s happening,” he told me. “You don’t know who they’re sharing the information with.”

Chester and other privacy advocates said Clear Channel’s system is an example of how private companies are building out commercial surveillance networks right under our noses.

“The scary thing is that there are so many companies handling different pieces of this, the ecosystem is enormous,” said Alan Butler, interim executive director and general counsel for the Electronic Privacy Information Center in Washington, D.C.

“All this data is being collected and we have no idea how it’s being used,” he said.

Clear Channel isn’t alone in developing what’s known as “out of home marketing” — a decidedly benign term for such a potentially invasive practice.

Different companies are rushing to install similar systems in malls, subways and other crowded venues. The aim is not just to see where you go and what you do but also to prompt impulse purchases at nearby merchants.

If you’re like me, the image that comes to mind is that scene from Steven Spielberg’s “Minority Report” where Tom Cruise is recognized and marketed to as he passes a series of digital billboards.

Current out-of-home marketing technology isn’t like that — yet. But experts say it’s just a mater of time.

“We’re already used to being tracked online,” said Lori B. Andrews, director of the Institute for Science, Law and Technology at the Illinois Institute of Technology. “Now it’s bleeding into the real world.”

Clear Channel is an especially powerful force in this field because its more than 500,000 print and digital billboards worldwide provide a far-reaching foundation from which to track passers-by and share data with marketing partners.

The company calls its technology Radar. The system, Clear Channel says, “leverages anonymous, aggregated mobile location data to help advertisers understand consumer mobility, behavior and true campaign impact.”

An animated video for Radar appears to depict people on foot and in cars passing a Clear Channel billboard and connecting automatically via Wi-Fi, providing marketers with “highly customized solutions” to help them “connect with the right customers at the right time and place.”

That’s a bit misleading.

Jason King, a Clear Channel spokesman, acknowledged that the company “does not equip its billboards with technology aimed at tracking individuals.”

Rather, Clear Channel gathers location and tracking information from multiple sources — apps, data firms — and then analyzes the info for insights about how people behave after passing a Clear Channel billboard.

The idea is to be able to tell advertising clients that a consumer is likely to visit the client’s business after being exposed to a billboard touting the client’s products or services, or to market to that consumer based on their location.

King said Radar “helps advertisers understand what happens after someone sees their ad.”

Wireless companies for years have been using “geolocation” data from smartphones to bolster advertisers’ marketing campaigns.

Basically, if you carry a phone, your whereabouts are known to your wireless provider every second of the day — and the companies make money selling that info to others.

Clear Channel is taking this capability up a level by creating a bridge between a consumer’s location and their exposure to an outdoor marketing pitch.

Now advertisers can go beyond just passively plastering a message on a billboard. They can follow you after you’ve seen the ad, and watch where you go and what you do.

Clear Channel is being disingenuous when it insists all data collected as part of Radar is anonymous, privacy experts say.

Kyle M.L. Jones, an Indiana University assistant professor who focuses on data mining, said that for a company to target you with advertising, it has to know who you are and have an idea about your personal tastes.

Even if you’re identified only by a number affiliated with your phone, rather than by your name, it’s not difficult to extrapolate from there if a more robust marketing profile is desired.

“Enough of a mixture of geographic, behavioral and demographic data will almost inevitably open up opportunities for re-identification,” Jones said. “It’s hard to know what their privacy-protecting practices are, but their practices have risk.”

Although Clear Channel’s King played down the “Minority Report” implications of Radar, the company’s chief executive, William Eccleshare, told the Financial Times that the September introduction of Radar in Europe will create a host of eye-opening opportunities for advertisers.

“We can follow your movement to a store,” he said. “We can follow what you purchase. And yes, we can look at your viewing habits that evening if you pass an ad for a Netflix show.”

For businesses, that’s pretty exciting.

For consumers, it should send a shiver down your spine.

Nanda Kumar, an associate professor of information systems at New York’s Baruch College, said “lackluster privacy laws” are partly to blame for companies feeling free to monitor consumers as they go about their daily affairs.

Many out-of-home-marketing businesses “take individuals’ privacy for granted and collect information from them opaquely without providing consumers any reasonable ways to control the flow of their data,” he said.

I wrote last week about how difficult some companies make it to opt out of data sharing. Clear Channel is no exception.

The company’s privacy policy says it’s up to individual consumers to “refer to your device’s or browser’s technical information for instructions on how to delete and disable all or some cookies, and other tracking tools, as available, including how to reset your advertising identifiers and limit advertising tracking.”

Yeah, good luck with that.

The privacy policy also acknowledges that even though Clear Channel primarily relies on “de-identified” personal information, it does in fact disclose identifiable info to business partners.

This can include your name, address, purchase history, online behavior and “inferences drawn from any of the foregoing to create a profile about a consumer reflecting the consumer’s preferences, characteristics, psychological trends, predispositions, behavior, attitudes, intelligence, abilities and aptitudes.”

Inferences about people’s intelligence, predispositions and psychological trends?

Not so benign after all.

“When they made ‘Minority Report,’ it wasn’t science fiction,” said Chester at the Center for Digital Democracy. “That scene was based on what they knew was actually coming.”

And here we are.