How does the speed (of vehicles) measuring device used by the traffic work ?

Chronometers are used to determine the speed of the vehicles with a hand held stopwatch. No radar, no loop just a plain and simple digital stop watch, and two points of a known distance apart. Start and stop switches are to be manually actuated when the vehicle crosses the points.

The speed will be displayed instantly. The second method is by using a Piezo Sensors, this method uses set of 3 rubber strips with a specific distance apart are build across the road or buried in the road. The time between comparisons is measured to give the resulting speed of the vehicle. If the speed exceeds the camera associated with the system will photograph the vehicle.

In another method two beams of invisible infrared beams are emitted across the given traffic lane. The elector optic head directs the beams to two retro-reflectors mounted in the road surface.

Every vehicle passing between the electro optic head and the reflectors breaks beams and triggers the unit computer to measure the speed. If a vehicle exceeds speed limit, the high resolution camera photographs the front or rear of the vehicle including its license plate.

In another method, infra red diodes are pulsed at certain frequencies and to create radar beams. This beam is emitted via parabolic reflector at 15 degree spread and directed towards traffic. The emitters are shielded to prevent external interference.

They normally use K band in a frequency 24 GHZ. The associated computer will measure speed and triggers camera to capture the image of erring vehicles.