It might be us (SPC). We are under contract to count cars at a few hundred locations on State Roads for PennDOT each year. I'll check our schedule to see if it is us.
If not, it could be part of any number of ongoing studies/investigations. The two that come to mind most readily are the Allegheny River Green Boulevard Project, and the effort to re-assess Washington Boulevard in light of the flooding/deaths earlier this year. But, those are just guesses.
As for how they work, perhaps Ryan can speak in more detail, but they are basically pneumatic tubes. They measure the passing of a tire over the tube, and the distance between axles to determine the type of vehicle (car, truck, etc) and the number of vehicles.