traffic light cameras

[interele]

How do cameras that catch you jumping lights work ?
Specifically I was think this ....Imagine a big ish junction

a) I drive across a set of lights on amber at 40mph and I just clear the junction before they go red
b) I drive across a set of lights at 20mph I will be only half way over the junction when they go red

Is there some sort of calculation they do and set a timer or something ?

M

[Fitzer]

Jumping a light is when you cross the near white 'stop' line rather than when you cross the junction, as such the cameras are (I believe) set to trigger as soon as the light turns red and they detect anything then crossing the white line. People have been done for just going half a car's length across the line rather than all the way across the intersection.

[buildersmate]

The cameras do not detect the vehicle, they only photograph it.
If you look at every junction with a camera you will find a rectangular box outline in black tarmac about 60cm long and the width of the carriageway located just beyond the white line. Under the tarmac outline is a loop of cable which does the detecting. Immediately the lights turn to red, the loop is set to sense any new vehicle over the top of the loop. The implication is that, once the lights turn red, any vehicle sensed by the loop has crossed the white line after the red signal appears. And you're nicked.

[jno]

now that the technology is available, the ambulance should show up in the photo - either the photo of your car, or in its own photo.