We made our own AIR door counter, composed of a few distance sensors. It can tell us in which direction a person is passing, and gives a distance profile that we process to distinguish individual people. Technically, we measure your circumference to some degree, but you might agree that is far from enough to compromise your anonimity.

AIR being active infrared as opposed to passive (or a particular waveband)?

Anonymity is a more interesting problem than "can distance sensors uniquely identify a passerby among 500M North Americans" as your reply implies. What if you are the roundest or tiniest person in town? All the sudden you are uniquely identifiable.

You're early in development, and I bet whoever did the circumference estimation has more in mind. I imagine you could make a good estimation of a person's height from your data: whether the profile sees knees hips or hands. Can you identify the asymmetric waist bulge of a CHL carrier? Does your infrared band penetrate polyesters but not cotton? I hope the reader's feature-vector blood is flowing at this point.

Your page needs to have way more formalization around the concept of anonymity (outside of the registration required area) for me to feel that you are appreciating the problem from an engineering perspective and not a marketing one.