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i've actually been up in the airplane they use for this. We were doing a story because they'd just gotten the plane. A cop (usually a state trooper because city cops 1) can't afford an airplane and 2) don't have any roads where it would be practical) sits in the plane looking down at the cars. When he sees one that's speeding, he times how long it takes the car to go from one of the white marks to the other. He has a chart on his knee that tells him the speeds for each second of time it takes. He does this 2-3 times and then computes the average of those 3 readings. If they're too far off from each other he tosses them and tries again. (this means it's theoretically possible to beat VASCAR if you radically change your speed every time you pass a white hash mark). Then he radios to the cop on the ground with the description of the car. As the ground unit closes in the plane stays in contact with him, telling him when he's behind the right car and confirming that the right car is pulled over. He then tells the ground cop what the speed of the car was, and goes off to spot more vehicles. There are often many ground cops working with one plane. It's a very efficient system but it has one significant weakness - -- BOTH cops have to testify against you in court, which can often be difficult to arrange. If you get a VASCAR ticket, it's a very good idea to fight it - you might get off on the cop's failure to appear.
Helicopters are rarely used because they're a lot more expensive to run (and buy), their pilots are a lot more expensive to pay, and they have to land more often to refuel.
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