The system uses microphones to capture the sound of a passing car, and then filters out all but the sound made by the engine. By measuring the doppler shift of the engine sound as it moves past the speed trap, the system can calculate the vehicle's speed.
While the sound detector is unobtrusive and completely passive (therefore undetectable by a conventional radar/laser detector), identifying speeders will require something a little less stealthy, like a traffic camera. Of course, other early detection systems are possible, like the subtle warning in the accompanying picture.
Hmmm. Sounds like better muffler technology is in order.
[Source: New Scientist]