I believe the way they argue around it being a speed trap, is that the aircraft paces the car, and the pilot uses the stopwatch to determine the gound speed of the aircraft.

This sort of skirts one of the essential elements of a speed trap, where the offenders vehicle is the one being timed.

In an aircraft, the claim is, that the pilot is able to line up his eye with the wheel of the plane and the auto. While maintaining this view, he paces the offender and measures the time it takes for the wheel of the plane to cross from one, to another, mark on the road. The mechanics of all this just doesn't seem to make for accuracy when measuring the true ground speed of the aircraft.

Any way, the courts have accepted this fiction as precedent, so there's not much use fighting it. Although, If you could find some other record of the aircraft's true ground speed that differed with the pilots testamony, something like a radar trace. Well....