Radar Detectors.

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Radar Detectors and other speedtrap detectors are designed to detect radar and normally also laser signals that the motorist may come across while driving. To understand what they are used for and why, it first of all needs an explanation about the devices that they detect and that is the touchy subject around motorists speeding, bad drivers, boy racers, police traffic patrols, speedtraps, radar guns laser guns and a whole host of other topics that can be brought into the debate.

OK so we will deal with the basics and maybe discuss the other aspects of road safety revenue raising etc. etc. at a later date.

The majority of speedtraps on British roads are of the Gatso type, these are the large yellow boxes set normally at the side of the road(but sometimes in the middle of a large road) on a thick grey post facing in the direction of travel for that lane.
The box consists of a radar operated speed detection device working on the doppler effect(explanation later) and a camera which is aimed at a section of the road. Normally you would also see a number of white lines painted in the same section of road that the camera is aimed at.
The radar is constantly on and when a vehicle passes through the 'beam' of the radar the beam will bounce back towards the radar transceiver, because of the motion of the car which is normally moving away from the camera at that point the wavelength of the radar is changed, it will be lengthened, this difference in wavelengths between the outgoing radar signal and the incoming radar signal can be accurately measured and that measurement gives a reading in terms of a speed in miles per hour or kilometres per hour.
If the speed reading is over a certain threshold that has been set by the Police or local authority then it will activate the camera. The camera will then take 2 images of the back of the car and this is where the white lines on the road come in. Because the car is moving over the white lines which are set at a certain distance apart, the white lines will give an accurate distance that the car has travelled over the period of time that the camera has taken the 2 pictures.
Speed is calculated as distance travelled over a given period of time so the 2 photographs can accurately show what speed the car is travelling at. So now we have 2 pieces of evidence, we have the radar speed calculation using Doppler effect and we have the speed camera pictures which show distance travelled over time(mile per hour).
The 2 pieces of evidence will be enough for a speeding offence prosecution.

If you want to be alerted to the radar speedtrap ahead then you can either use a GPS type database speedtrap device which tells you where exactly the cameras are located or you can use a radar detector which can detect the radar emissions coming from the Gatso camera.

Radar detectors can pick up a variety of radar frequencies and some are more sensitive than others and you usually get what you pay for. The radar frequencies used can vary in different types of speedtrap and the radar detector will be tuned to those frequencies but in order to avoid false alarms it needs to have circuitry which will filter out other signals that could be classed as a false alarm. Common sources of false alarms would be anything that uses a microwave signal(radars are in the microwave range), such as automated shop door openers, mobile phone masts and traffic sensors at pedestrian crossings.

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