When I was living in Illinois in the late 1960s and early1970s, I think the Illinois State Police tried this aircraft assisted speed enforcement on Illinois highways. The plane would fly a pattern over a stretch of road. The airborne observer would spot a vehicle and mark the moment it passed a particular spot on the road with a stopwatch. She then halted the stopwatch when the vehicle passed a second particular location. A quick distance divided by time calculation revealed vehicle speed. Significant speeders were reported to state troopers hidden off the main road. The airplane could stay up for a couple of hours, land to refuel, and resume monitoring. I think it was used mainly to catch those who thought 100mph or more was the speed limit on Illinois highways.
Back then it was much less expensive to use a small and simply equipped, but relatively fast. airplane to conduct these speed enforcement operations during good weather in the daytime.