The Roads Policing Unit has a variety of speed detection equipment available to enforce speed limits and reduce the casualty rate.

The most commonly used devices are either hand held lasers or 'in-car' computers. All such devices in use by the police are approved by the Home Office.

These devices are supplied by a number of manufacturers but all work in a similar way. In simple terms lasers determine speed by measuring the time of flight of very short pulses of infrared light.

Since the speed of light is constant, the time it takes the laser pulse to travel to the target vehicle and back is directly proportional to the distance between the device and that vehicle. By firing two pulses, a known time apart two distances can be calculated.

The change in distance divided by the time interval between the two pulses gives the speed of the vehicle.

When switched on, all devices go through internal checking systems. If the internal checking system detects a fault, an error code is displayed and the device cannot be used. It must be returned to the manufacturer for repair.

Before using the equipment, officers carry out the 'set up' procedure checks as listed in the manufacturer's manual. The alignment of the laser beam with the sighting-scope can be checked to ensure the laser will strike the selected target.

The device can be used at night in the same way as in daylight. Weather conditions do not affect the use of the equipment other than a possible reduction in the range it can cover.

In-car computers

These devices are operated by an officer from within a moving or stationary patrol car.

The Officer checks the speed of the target vehicle by measuring the time taken for that vehicle to pass between two identified points on the road. The distance between the two points is measured by driving the patrol car between them.

The device calculates the average speed of the target vehicle, dividing distance covered by time taken.

How do I know the equipment works?

All devices are a type approved by the Home Office. Laser devices are calibrated annually by the manufacturer. Officers carry out other checks prior to enforcement activity.

How do I know the equipment has been used correctly?

Only authorised officers are allowed to use the equipment. These officers have been trained either by the manufacturer or an approved trainer. Training certificates or authorities for use can be produced in court proceedings.

What are the rules that govern the equipment use?

The ACPO National Manual of Guidance and the Manufacturer's User Manuals provide the rules for using the equipment.

Can I have a copy of the ACPO National Manual of Guidance and the Manufacturer's User Manual?

These documents are copyright protected but are available either from ACPO or the manufacturers, addresses available through the Home Office.

www.homeoffice.gov.uk

For more information about speed cameras including technical information please visit the following link:

http://www.homeoffice.gov.uk/docs/speed.html