Eli5: How does Speed Camera or Radar work?


How can the radar capture a specific vehicle moving at a specific speed while there are other vehicles moving alongside?

In: 11

Speed radars use the Doppler effect: the frequency of radio waves bounced off an oncoming vehicle is higher than their original frequency, and the frequency of radio waves bounced off an vehicle moving away is lower than their original frequency. The amount of change in frequency is proportional to the vehicle’s speed. Since the radar “knows” what frequency of radio waves it send a out, calculating speed from the frequency of the received waves is pretty straightforward.


A speed camera is triggered when some kind of sensoric system determines that an object is moving quicker than it should.
Some systems use radar, which has already been explained. Others use two loops of wire under the road. When a metal object such as a car moves over one, a small amount of electric energy is generated in the loop. This starts a clock on the first loop, the second stops the clock. The distance between the loop is known and the speed is then calculated by dividing the distance by the measured time. Too quick? Snapshot.

This method is also used to check the plausibility of a hit by taking two pictures. Using markers in the environment, the law enforcement clerk can measure the distance traveled between the photos. The time between photos is fixed. This has been especially useful if multiple vehicles are present in the photos to determine which have been speeding.

The advent of cheap computing power has ushered in more sophisticated methods. For a while now LIDAR sensors have been used to build a spatial image around the camera, a computer can detect moving objects in this image and calculate the speed.
The bleeding edge tech employs video cameras and image processing to track vehicles in the video feed, but as far as I know they’re not that widespread yet.