Legal i

Legal i – VASCAR, Police Pilot and electric trip wire equipment

We conclude consideration of the technical requirements that the prosecution have to observe when using technological aids in evidence, by looking at VASCAR, Police Pilot and electric trip wire equipment.


Some police cars are equipped with VASCAR (Visual Average Speed Computer And Recorder). These are extremely accurate devices which record the average speed of a vehicle over the distance recorded. Like radar meters, it is extremely difficult to defend a speeding charge on technical grounds where VASCAR is used. The degree of training for a police constable using VASCAR is high and most forces require a constable to pass a stringent test programme before using VASCAR as evidence in prosecutions, so enquiries about the officer’s training might be a possible ground for challenge. The constable has to operate switches accurately and be able to satisfy a court that there was no mistake in identifying the offending vehicle and no misjudgement of the exact moment the vehicle passed the relevant landmark used during the operation of VASCAR.


The Metropolitan Police use Police Pilot, which measures average speed, similar to VASCAR. It can be used while the police car is in motion or parked, in daylight or at night and in all weather conditions. If the police car is in motion, it does not have to be travelling in the same direction as the offending vehicle, and neither is it necessary to follow the vehicle at an even distance or pursue it. No signal is emitted from Police Pilot, and it does not interfere with radio transmissions or reception. Only an electrical failure in the vehicle carrying it can affect its working or other accuracy. The equipment should be recalibrated once a month and checked daily, either by driving over a set distance and comparing the readout with the known distance travelled, or by comparing the speed readout in standby mode with the vehicle’s certified speedometer.


Electric trip wire equipment is set up with two wires stretched across the carriageway 1.5 metres apart. The wires consist of coaxial cable, which are sensitive to pressure. When a vehicle is driven over each wire, an electrical pulse or charge is created and these are relayed to a computer, which calculates the time taken between compressing each cable, and therefore the speed of the vehicle.


Although a theme has run throughout this series of articles, warning of the difficulty in challenging technological devices used in a prosecution, where the facts of the case raise questions or doubts, it is still possible. In a case in 1982, an appeal court decided that even though the trip wire equipment in that case appeared to be in working order and operated correctly, the motorist was acquitted, because the age of the lorry and the fact that it was driving around a bend, going uphill, cast doubt on the accuracy of the trip wire data that the lorry was speeding.


The cost of challenging this kind of evidence, and the small chances of success, dictate that it should be considered only if there is other evidence to cast real doubt on the accuracy of the equipment or the propriety of its operation, or if your licence and livelihood are at stake.


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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