Legal i

Radar Speed Meters

Last month, we looked at the technical requirements that the prosecution have to observe when using technological aids in evidence, regardless of the specific device. Let us now consider this in relation to radar speed meters.


These devices appear to be very accurate if working under good conditions and operated by experienced police constables. Where the courts have dismissed cases reliant on radar speed meters, it tends to be due to failings in the police evidence, rather than in the device itself. Such cases include those in which the police witness had failed to read the meter correctly, and where the reading given in evidence could not actually be proved to relate to the defendant’s vehicle.


Radar traps are set so that one constable is positioned by the radar set, observing the speed of the vehicle as it passes through the radar beam, with another constable stationed further down the road to stop an offending vehicle. That constable is usually told by radio by the first constable the vehicle registration number and type of vehicle, and sometimes errors are made. Cases have been dismissed where the court has not been satisfied that the correct vehicle was stopped, or where the first constable made an error in identifying the vehicle, or where the constable stopping the vehicle made a mistake.


Hand-held radar guns may be operated by a single constable. The guns work on the doppler effect, by which the gun is pointed at the moving vehicle and a radar beam strikes the vehicle, and the frequency of the beam reflected back to the gun is changed in proportion to the speed of the vehicle. The gun measures the change and displays the speed of the vehicle on a screen for the operating constable to view. This reading can be saved in the machine, and the guns have a range of 500 yards or more.


There are at least five ways in which a gun can give a false reading: through low batteries, poor contact through car lighter sockets, radio or similar interference, reflection of the beam off a metal object (such as a lamppost) onto some other moving object, or measurement of the speed of another vehicle in the wide beam. A test on a smaller, nearer vehicle might pick up a reflection from a larger, more distant one behind. Usually, in such instances the reading would not be steady and would jump from one vehicle to another.


A Home Office study, Measurements on Police hand held radar speedmeters, did not indicate any likelihood of false readings from aircraft, birds, insects, power lines or a movement behind the radar gun. The training manual recommends that the device is not used within a quarter of a mile of powerful VHF radio or UHF TV transmitters, within 100 yards of high voltage overhead power cables, near large rotating fans or signs, near large rotating radar equipment, or within 30 yards of smaller transmitters. The Home Office study recommends that to operate the gun, the constable should select a position in the road with a clear view and that he should check the battery indicator, if there is one. The range switch should be pointed at the sky and turned slowly through 360 degrees. During this test, the display should be examined for several seconds to see that it is clear and that there is no radio interference, and that the meter is working and the battery is not flat. This procedure should be repeated at several intervals between speed measurements. Any test buttons should also be checked. A tuning fork should then be struck and vibrated in front of the aerial. The reading on the machine should correspond with the reading on the fork. A written note should be kept of the speed of the vehicle to test the radar gun.


When the constable considers that a vehicle is exceeding the speed limit, the gun should be pointed at the vehicle and a reading taken. The reading should be observed for at least three seconds, and during that period the reading should be steady. An erratic series of numbers would indicate that an erroneous reading had been taken, and that the measurement was invalid. If the reading is considered correct, the trigger of the gun should be squeezed and the speed reading fixed on the display. Although the reading can be shown to the motorist, there is no legal requirement to do so. When the trigger is squeezed again, the reading is lost and the meter is ready to make another measurement. Any challenge to the evidence might therefore query when the reading was recorded in the constable’s notebook – was it done immediately after the reading was displayed, or noted down some considerable time afterwards?


The courts have become increasingly reluctant to accept technical challenges based on whether or not the radar gun was operating or operated correctly, and such challenges would usually not succeed in the absence of compelling evidence to introduce significant doubt about the reading produced by the gun. There could, for example, be evidence from witnesses in the vehicle to prove that the speed limit was not exceeded and that there were lots of fast moving vehicles around and behind the vehicle in question. Each case will turn on its own evidence.


Next month we will consider laser speed measuring equipment.


Designed by solicitors, tested by barristers and available around the clock, Road Traffic Representation is an online legal system that allows people accused of a motoring offence to get free advice on how the law will be applied in their case, and referral to a telephone helpline and representation by a barrister in court if required. Practising solicitor Martin Langan spent two years designing the system and creating the data repository which allows the software to analyse road traffic offences with the same authority as a solicitor.

Twitter: martinlangan

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