Legal i

Laser Speed Measuring Equipment

In continuing to consider the technical requirements that the prosecution have to observe when using technological aids in evidence, we will look at laser speed measuring equipment in this month’s issue.


There are very many laser speed measuring devices and it would require a small book to review them all, so let us look at one such device as fairly typical, the hand-held LTI 20.20 TS/M. This works by measuring the time of flight of very short pulses of infrared light. The time it takes the laser pulse to travel to the target and back is directly proportional to the distance to the target. By firing two pulses a known time apart, two distances can be calculated and the change in distance divided by the time interval between the two pulses gives the speed of the vehicle at which the device has been fired. The device has two lenses on its face, and these transmit the laser beam and receive the return signal. A sighting scope is mounted on the top of the device and the scope contains a red dot within it, the dot being aligned with the beam that is aimed at the target vehicle. The operator presses the trigger to activate the burst of pulses.


Around 60 pulses take a series of measurements in as many milliseconds. The device software then compares the measurements to ensure consistency and, if consistent, the device will provide a speed and range reading within 0.3 seconds of its actuation. Any error detected will cause only an error message to be displayed, so no erroneous reading will be shown. And in order to avoid mistakes caused by, for example, a shift of the aiming point between the two pulses, the device applies independent tests to the pulse data and a single failure in any of those tests will also cause an error message to appear.


The beam produced by the device is very narrow (120 centimetres at a range of 400 metres), so it is particularly accurate even in moving traffic in multiple lanes. Ranges up to one kilometre can be captured, and an operator with average skill and experience can capture targets at 700 to 800 metres.


The device is not susceptible to interference, as is the case with radar equipment, and neither can it be fooled by rotating or vibrating objects. It can also detect the difference between approaching and departing objects.


As has been stated previously in this short series of articles, the courts are becoming ever more impervious to arguments seeking to cast doubt on the accuracy of speed detection devices, and if you are faced with a prosecution based on a device such as this, you will have an uphill struggle when seeking to exclude the evidence.


Next month, we will conclude this review of speed detecting technology with a look at Vascar, Police Pilot and electric trip wire 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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