How do portable speed cameras work?

Police forces in England & Wales sometimes use portable laser speedmeters to detect excess speed. Two commonly used devices are the LTI 20.20 and Kustom Pro Laser III.

A speedmeter measures and displays the speed of a moving vehicle and the range at which the speed was measured. Laser speedmeters calculate speed by determining distances - by measuring the time of flight of short pulses of infrared light. 

A laser speedmeter may be hand-held:

Here is the same device:

The device has detected a vehicle moving at 51 mph at a distance of 184.2 meters. 

The same technology may be used from the inside of a so-called Safety Camera Van:

Here is the inside of the van:

The above image shows a laser speedmeter attached to a tripod, screen and video recorder. The operator has detected a motorist travelling at 119 mph, from a distance of 267 meters. The video may be kept for evidential purposes, and can be used to produce stills. Here is an example still:

 

Images by North Yorkshire Police and Surrey Police.