Evidence gathering
While digital cameras can be used as the primary means of speed detection when combined with automatic number plate recognition (ANPR) average-speed camera systems, their use is more commonly restricted to evidence gathering where speeding offences are detected by various other types of sensors such as such as Doppler radar, piezo strips, infrared or laser devices.
Photographs are typically time-stamped by a high resolution timing device so that a vehicle's speed can be checked manually after the fact if necessary using the secondary method of calculating its speed between a series of calibrated lines (known as "Dragon's Teeth") painted on the road surface.
The change from analogue "wet film" to digital technology has revolutionised speed cameras, particularly their maintenance and the back-office processing required to issue penalty notices. Images from digital cameras can be uploaded in seconds to a remote office over a network link, while optical character recognition software can automate the "reading" of vehicle registration numbers.
(Source: Wikipedia)