In terms of total people volumes, a company called shoppertrak has been providing technology to do this for at least 10 years.
This used a hidden camera, and the people appeared as fuzzy blobs; it was relatively easy to see how long individuals are dwelling at a particular shelf or display. Camera technology has definitely come a long way in the last 10 years, so there are probably many more features available today. I've been out of that sector since May 1997.
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Oh Lord, won’t you burn me a Knoppix CD ?
My friends all rate Windows, I must disagree.
Your powers of persuasion will set them all free,
So oh Lord, won’t you burn me a Knoppix CD ?
(Missquoting Janis Joplin)