Reported two days ago on CNET:

[Entrepreneurs] are equipping billboards with tiny cameras that gather details
about passers-by—their gender, approximate age, and how long they
looked at the billboard. These details are transmitted to a central
database [… ] The goal, these companies say, is to tailor a digital display to the
person standing in front of it—to show one advertisement to a
middle-aged white woman, for example, and a different one to a teenage
Asian boy.

Paolo Prandoni, the founder of the company offering this technology, is assuring the public that the imaging is all done anonymously. But concern about the secret use of cameras continues to grow.