So that's a really novel idea. It would be incredible to tell a first responder how many people we think are inside... accurately.

We just do installs at each entrance and then normalize.

This would be a reallly cool application if PIR's were capable of counting "horde" foot traffic, like a giant mass of people streaming out of a door during an alarm. I don't think that something like a PIR could handle that number of people, since it's just the single beam.