In California the CHP (California Hwy Patrol) plane has to do the same thing the CHP car would have to do: match your speed and then note its own speed. Measuring time between points is forbidden no matter what the observer's vehicle is. Radar is also used from the ground but was not available to CHP when they started using aircraft. The planes are on the inexpensive side so sometimes they have to dive to achieve the speed. Also recently CHP got to use radar statewide so they don't use aircraft so much.