Originally Posted by mrmojo77
I didn't even know, that the speed is measured that way.
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Well, there are different methods to measure the speed with different speed cameras. One is as described above(*), you also have the classic electromagnetic/radio waves reflection out of which the speed can be calculated thanks to the doppler effect. 'New' on the market are speed cameras with induction loops in the ground. And some governments also use (hand held) laser guns. And another method are anonymous road cars which follow you for a time and register an average speed with a calibrated speedo.
I know in Belgium static speed cameras use radio waves or induction loops. And if I'm not mistaken they also take 2 photos because Belgium legislation dictates the detected speed has to be verified. They lay a transparent grid on top of the photographs so they can verify the speed, in the same way it's done with those unreliable UK speed cameras in the article above.
(*) of course those speed cams have to be triggered when a speeding car passes. This can be done via many ways.