How do variable speed cameras work, specically how do they read and transfer data ?

Answer 1

Almost all speed cameras are digital.

The radar detects the cars speed and if above whatever trip point is currently set by the central control room, the (digital) camera is triggered.

Two photos are taken, with the fixed time delay between the 2 shots allowing the distance traveled to be used to calculate speed and thus back up the reading from the radar.

Automatic Number Plate Recognition software then extracts the vehicle reg. number and an on-line computer look-up via DVLA Swansea allows automatic printing of the fixed penalty notice (or, if the car has been reported stolen or the reg. number does not exist, automatic notification to the nearest Police car)

The only bit not fully automatic is the extraction of the fine from your Bank account (however I'm sure they are working on that too, especially as London Congestion Charge has already shown the way :-) )

Answer 2

variable display speed signs that can be varied as conditions change. It is intended that the initial introduction of variable speed signs will be on the M4 motorway. Variable speed limit technology will improve the safety and efficiency of major motorways and freeways in a number of ways. Firstly, in the event of a major incident such as a traffic accident, or if weather conditions deteriorate, variable speed signs will enable speed limits to be adjusted to reflect those conditions and ensure a safe speed limit is maintained. In morning and afternoon peak congestion periods, speed limits will be adjusted to achieve the most efficient traffic flow.