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How Enforcement Cameras Work

Fixed Mobile and Red Light Cameras


In Dorset we enforce the speed limit using different types of cameras depending on what sort of enforcement is being carried out.

  • Fixed Camera enforcement - using Gatsometer Type 24
  • Red Light Camera enforcement- using Gatso Red Light
  • Mobile camera enforcement - using either Gatso mobile, Prolaser 3 Laser or LTI Ultralyte 1000 Laser

The speed enforcement measuring devices are required by the Secretary of State to be Type Approved for enforcement use. To obtain Type Approval, the equipment is independently tested to extremely high standards to measure the accuracy of the device. The devices are very accurate for measuring speed and that is why Dorset Road Safe use them for enforcement.


Fixed Cameras

The Gatsometer Type 24 speed camera is a radar activated speed measuring device and is loaded inside the fixed camera housing box for speed enforcement at fixed sites. Once in use, the device has a built in calibration check which is activated every time the camera is loaded or unloaded with film. This calibration check is shown on the film batch recording sheet and may be produced in Court to uphold an offence.

The Gatsometer Type 24 speed camera can work in either direction and can distinguish between small and large vehicles. The threshold, which is the set speed at which offences trigger the camera, is programmed into the system to detect different vehicle types and levels.

The device, which includes the camera control unit, radar and connector leads are sent to the manufacturer on an annual basis for a calibration check as required by the Home Office. Once the checks are complete, the device is returned with a new calibration certificate.

How The Radar Works

 secondary speed check

 The camera sends a radar beam across the road at an angle of 20 degrees from the kerb. As a vehicle enters the radar beam, the beam is reflected back to the camera. The frequency of the reflected beam is changed by the movement of the vehicle through it – known as the “Doppler Effect”. From this frequency change the camera can calculate the vehicle’s speed.



   secondary speed check2


If the speed of the vehicle is above a set threshold the camera will take a photograph as the vehicle leaves the radar beam.  This speed is the primary evidence



 secondary speed check3

 Another photograph is taken half a second later. By comparing the two photographs and using the white road markings painted on the road, the distance the vehicle has travelled in the half second can be worked out. A time over distance speed calculation is then carried out as a secondary check to confirm that the camera operated correctly. This secondary check speed has to be within +/- 10% of the speed from the radar.




Red Light Cameras

Red light cameras are used at signal controlled junctions to reduce the risk of collisions caused by drivers’ non-compliance to the red light signal. A red light camera will only activate when the traffic signal is red.

The red light camera is linked to two induction loops set in the road, one just before the stop line and a second one a short distance past the stop line.

When the traffic signals have changed to red and a vehicle moves over the first loop, the sequence is initiated. If the vehicle continues to cross the second loop (past the stop line) the camera will take a photograph, followed by a second photograph one second later.

The photographs will record the amount of time elapsed since the red light was illuminated, lane identification, the date, the time and the speed of the vehicle.


                                           How red light works



Mobile Cameras

Mobile speed enforcement is carried out using either the mobile Gatso camera which is the same camera as above but is mounted on a tripod at the side of the road or laser camera speed enforcement which is operated from within the mobile camera vans.

How the Laser Works

A laser speed meter works by using a pulsed laser beam directed at an upright surface on the front or rear of a vehicle. Ideally this will be the number plate as it has a retro-reflective surface, but any suitably reflective part of the vehicle can be used. The vehicle can be travelling towards or away from the speed meter.

How Laser Works

Hundreds of pulses per second are sent out but as each travels at the speed of light its reflection is received back from the vehicle before the next pulse is generated.

The speed meter measures how long it takes for each pulse to travel to and from the vehicle and from this time can determine the distance. By using the distance change between each pulse it can then calculate the speed of the vehicle. A number of matching calculations have to be made before the speed meter will display a speed reading, this usually takes around 0.3 seconds.

The laser beam only spreads to approximately 3 feet wide at a distance of 1000 feet. This enables individual vehicles to be picked out of a stream of traffic. With the equipment currently used* by the Dorset Road Safe camera teams, the targeting of the vehicle and the speed reading obtained is recorded onto either video tape or DVD.

*Equipment used:

 For information about enforcement thresholds, please click HERE


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