Fwiw,
California's new toll expressways do this - counts people in the car at speed, and charges different rates based on that - so the technology already exists.
In the San Francisco Bay Area, they require toll transponders, some having a switch saying whether you have one, two, or three or more occupants. I don’t know how they would catch cheaters, if at all.
Edit: Apparently the highway patrol is notified of carpools they should watch in passing.
California's system only cares about 1, 2, or 3+ passengers. The counting is primarily done with a transponder that is set by the driver (and a display is shown so the highway patrol can catch violators). At toll booths, they also snap photographs from the sides to catch violators. It is not only far from perfect, but wouldn't be useful if you actually need to know the exact number of passengers in (say) a van or bus.
Edit: Apparently the highway patrol is notified of carpools they should watch in passing.