I'm guessing someone was off camera. He adusts the camera so that it's pointing to where the person just was and switches it to IR when the residual heat from the person that has just moved away is fading out.
I'm of two minds to this (and this as someone who's lost their licence from speeding fines)...
It's meant to be a deterrent for sure, but if he's putting a sign up people will slow down for that stretch and then speed up again, no real long term change in driving habits, where if someone gets caught enough they should change their driving habits long-term.
But yeah not like he was breaking any laws either :/