They paint billboards on the wings, so as to hide behind them.
I’ve seen this happen in Southern VA once, near Bristol on I-81. They were getting people as they came off the highway. I’d still like to know how they can use the little white lines on the road. I know that if you used the front of the car for both lines, but what if you used the middle of the car or the back of the car then you’d get different readings. During my air photo class we had to determine the speed of the car over two different photos and they told us it was an ok method of getting a speed but not very accurate over all.
I have also heard, though not seen, them doing the same thing in Maryland near I-81 and I-70, and in the Great Lakes area.
Reminds me of this hilarious image:
Speed Limit Enforced by Aircraft
If the link breaks, do a Google search on the filename:
“rte81speed_enforcement.jpg”
There are special lines painted on the shoulder here that airborne troopers use to time your speed. And at http://www.state.oh.us/ohiostatepatrol/FAQ.html#e5 there’s a brief explanation of the setup and how it’s legal in Ohio. There’s no radar; it’s all done with a stopwatch from the plane. I can tell you from experience that on many summer Sunday afternoons they do in fact stand with one foot in the traffic lane on I-75 and flag people over at a steady pace. You can even pay the ticket then and there with a credit card. We are very good at collecting funds from travellers in this state. It works so well that we have 14 OSP pilots and 15 aircraft, according to http://www.state.oh.us/ohiostatepatrol/office/fieldops/aviation.html
I’ve always wanted to sneak over to the highway and paint in extra lines just to mess with the works but I’m too old now to carry it off…dang it.