Clear Channel and its partners — AT&T Data Patterns, a unit of AT&T that collects location data from its subscribers; PlaceIQ, which uses location data collected from other apps to help determine consumer behavior; and Placed, which pays consumers for the right to track their movements and is able to link exposure to ads to in-store visits — all insist that they protect the privacy of consumers. All data is anonymous and aggregated, they say, meaning individual consumers cannot be identified.

Still, Mr. Stevens acknowledged that the company’s new offering “does sound a bit creepy.”

But, he added, the company was using the same data that mobile advertisers have been using for years, and showing certain ads to a specific group of consumers was not a new idea. “It’s easy to forget that we’re just tapping into an existing data ecosystem,” he said.

In many ways, billboards are still stuck in the old-media world, where companies tried to determine how many people saw billboards by counting the cars that drove by. But in recent years, billboard companies have made more of an effort to step into the digital age. Some billboards, for example, have been equipped with small cameras that collect information about the people walking by. Clear Channel Outdoor’s move is yet another attempt to modernize billboards and enable the kind of audience measurements that advertisers have come to expect.

Privacy advocates, however, have long raised questions about mobile device tracking, particularly as companies have melded this location information with consumers’ online behavior to form detailed audience profiles. Opponents contend that people often do not realize their location and behavior are being tracked, even if they have agreed at some point to allow companies to monitor them. And while nearly all of these companies claim that the data they collect is anonymous and aggregated — and that consumers can opt out of tracking at any time — privacy advocates are skeptical.

“People have no idea that they’re being tracked and targeted,” said Jeffrey Chester, executive director of the Center for Digital Democracy. “It is incredibly creepy, and it’s the most recent intrusion into our privacy.”