We have all got used to seeing those speed camera boxes at the side of the road, but until a speeding fine arrives in the post, does anyone wonder "How They Work"?
I asked a policeman, and he said he was not completely sure but thought this was how they worked. Because there is no way of actually knowing how fast the cars are going until they have gone past, a clever probability system is used. At least 80% of drivers admit to speeding, and the chance that one of them is speeding at this point on the road system is calculated to be .3%. So one in every 300 cars going past the camera is probably speeding. If 75% of those see the speed camera and brake, then one in 900 cars will still be speeding.
Once a car has passed the camera, the relative speed of cars can be picked up by the white markings on the road surface. These are made of electrically conductive paint and spaced at sub-logarithmic intervals. Each car will touch a sequence of lines as it passes, and if the correct pattern is detected (we think an electric circuit is linked up between the various lines through the body of the car) the detection circuit is triggered. This identifies the one car out of 900 which is going the fastest.
In order that a record can be made of any speeder, the camera constantly films the passing cars. However it cannot tell which are the speeding ones, so inside the speed camera box it displays the video of passing cars on a small video screen. When the detection circuit is triggered, a Polaroid instant camera mounted inside the box takes a photo of the video screen, which at that moment will be showing the speeding car. This photo accounts for the flash that can sometimes be seen in the rear-view mirror by speeding cars. These photos are sent via underground tunnels to the local police station where any pictures of police officers or politicians are removed. Then the fines are sent out in the post.
Apparently, each speed camera also contains a small radar emitter so that drivers who have invested in a speed camera detector can tell when a speed trap is coming up. They also have an invisible laser that points straight upwards so that SatNav satellites know where they are.
In this way enough fines are generated from drivers who don't know about speed cameras and "How They Work", to pay for the system, and for the huge PR exercise which is needed to constantly prove that the cameras are a safety feature.