Answer:
Counting human traffic may be achieved with the MAXQ7667 ultrasound distance detector.
When mounted over a door, the movement of persons in or out of a room may be detected and if set at an angle, the ongoing presence of persons in the room may be determined.
This would be especially useful in a fire safety application where it would be useful to determine if a room is clear of personnel.
The device uses ultrasound to detect range and therefore where a person walks into the 'beam' a return will be detected which with suitable processing may show if the target is approaching the sensor (leaving the room) or moving away from the sensor (entering the room) By recording the waveform profile of the 'clutter' in the room, it would be possible to determine if a person was present even if there were no motion, merely by seeing additional returns where previously there were none.
Such a scheme would work in the dark or smoke filled room and could be a useful addition to a fire safety system.