1. A method of counting persons comprising: (a) producing signals from sensors adapted to detect vibrations made by persons crossing a floor under an area connecting at least three passageways; (b) evaluating the signals with a machine learning system trained to detect patterns of movement of persons between the passageways from vibrations in the floor; (c) determining from outputs of the machine learning system produced on the basis of the signals that a person has entered the area by an identified passageway and that the person has exited the area by an identified passageway; and (d) producing a count of the number of persons that have passed through the area using the combination of passageways identified by the outputs.