How Speed Cameras
Work
What do they do? (Gatso Cameras)
They are designed to photograph speeding motorists, thus freeing up valuable
police resources to drink tea, etc.
They measure motorists speed and if they are exceeding the speed limit, they are photographed (currently from behind) giving the authorities your vehicle registration mark.
Firstly, they have to measure your speed. This bit can be done in two ways. One way, by using piezoelectric strips embedded in the road; two such strips at a known distance apart will detect your wheels over them. The time it takes between the two strips gives the speed. (Speed equals distance divided by time.) Another way is to use radar. By emitting suitably high-frequency radio waves, they can bounce these from the vehicle, and the rate-of-change of distance is detected by a frequency shift from the reflection. If this rate-of-change is higher than the rate-of-change for the speed limit, you're caught.
Next, they have to photograph the vehicle. This bit sounds easy but is not so straightforward. The photographs taken will be both normal and infrared. (The infrared will 'see' your registration mark when the vehicle is caked in dirt.) Also, to collect the proof, two photographs are taken in quick succession, so that the distance travelled can be seen. Obviously the "quick succession" is a known time: Again speed can be calculated. To determine the distance, white square marks are usually painted in the gutter of the road and again on the crown of the road. These are a known distance and the two photographs will show the distance travelled.
Again, in order. If there are piezoelectric strips, there is nothing practicable that can be done. Radar on the other hand is quite another thing.
For the boffs amongst you, current technology uses X Band radar (which is really microwaves). The frequency is about 10 Gigahertz, which (take it from me) yields about a 30 hertz frequency shift for every one-mile-per-hour travelled. If you are travelling towards the source, the frequency is added (and goes up) and goes down if you are moving away by the same amount. 30mph gives a bit over 900 hertz shift. If another (similar) radar source were to mix its frequencies with the reflected ones from the camera, you would confuse the living daylights out of it. It might record your speed as something very erratic and probably impossible!
The photography bit would be interesting, too. For the infrared part - simply drill some small holes in the vehicle registration plate to take infrared emitting diodes. A number of them permanently emitting infrared would fog an infrared film. Secondly, the normal photography: This requires that when the 'flash' from the camera gets you, something flashes back. Preferably two flash sources either side of the vehicle registration plate (to fog the film again). However, the cameras might not flash in daylight.
Then again, they also use video camera's. Have you noticed the rather large circular or square white painted blobs in the road? They are 1/10th mile apart (or 2/10ths on motorways).
