| There are two main types of speed camera commonly seen on UK roads,
TruVelo Combi S, a camera unit that can monitor both speed limits and red lights It looks like a cross between the grey GATSO box and a traffic light. The main pole is grey, with the outrigger painted blue. The camera box has two circular holes in the front of it and faces oncoming traffic. The red/purple coloured 'hole' is actually an infra-red flash which means that it flashes, but you don't see it because your eyes aren't tuned to see infra-red. (The flash has a power of 360 Joules and can cycle in half a second - which means 2 pictures per second). The camera film, on the other hand, is very sensitive to red in an image, and the reflected red light from an IR flash provides more than enough image detail. The system uses Truvelo's MPC (Multi Piezo Circuit) speed measuring instrument and a 35mm camera. One system, installed in temporary or permanent sites, can photograph speeding vehicles or vehicles jumping a red light. A single front photograph, taken just after the vehicle has crossed the piezo sensors used for speed measurement, includes all the secondary check information needed for reliable operation. Piezo sensors have the distinct advantage over radar or other speed measuring equipment, in that they are very efficient in high traffic volumes and cannot be detected by any of the detection devices currently on the market. The Multi Piezo Circuit can either be buried in the road, or laid across it as rubber strips for a temporary setup, so watch.
The other main type is the GATSO which is named after the 1953 Monte Carlo Ralley winner and Indonesian immigrant to Holland, Maurice Gatsonīdes, who invented them. They are currently manufactured by Gatsometer BV., also in the Netherlands. The speed of a vehicle passing the unit is measured using a wide band Ka radar emitted from the front of the camera (the rectangular plate) in a 5o beam across the road, at an angle of 20o to the road. Ka-band radar (also known as photoradar) emits in a frequency range of 33.4000-36.000GHz. It measures your speed and if you're travelling above a pre-determined limit, trips the camera which in turn takes two photos of the back of your car - hence two flashes. It takes two so that the speed information superimposed on the resulting photographs can be manually double checked by calculating the distance the car has moved between pictures. As the pictures are taken half a second apart, and there are markings on the road at 2 metre intervals then the speed can be calculated.
This method is also used to confirm which vehicle is the target. If you contest the allegation on the grounds that "someone faster was overtaking me", a quick check of the photos reveals relatively which car has travelled further between them.
The actual radar unit takes between three and four hundred readings of a single vehicle as it passes through the beam. Within these readings, the variation in measured speed must not be more than 2mph. If it is, the radar unit aborts the test. The unit can also differentiate between large vehicles and cars by the amount of radar return, so a lower activation threshold can be set for large goods vehicles and coaches. This explains why sometimes you see a truck that is doing 70mph on a dual carriageway trip a camera when a car at the same speed does not - it has registered that the truck was going too fast (trucks are limited in top speed).
Most GATSOs are tuned slightly above the ACPO guidelines This is a very rough estimate, but it means that theoretically a GATSO in a 30mph zone will only register vehicles travelling at 35mph or more. Of course this is all tuneable by the officer who sets the camera, but those are the guidelines. This is to take account of the number of cases that would otherwise come back with people complaining that "my speedo said 30mph" and suchlike.  |