Speed radars are good for speed traps, but not so great to prevent speed related incidents. They share the same fatal flaw with all other single-point speed meters: You can just slow down for a minute, while you pass by the speed trap, and then speed up for the rest of your trip. Instead, a 2-point speed meter (i.e. license plate readers every few miles, measuring time between matching reads) system is superior becau…
If you've traveled around the country, you've invariably come across those "speed is monitored by aircraft" signs, and what you described is how they do it (tracking how long it takes cars to go from point A to B)
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.