How accurate are speed cameras?
Speed cameras are officially described as being calibrated to an accuracy of
two per cent. However, in recent years some well-publicised court cases have
hinged upon alleged inaccuracies in the speed camera evidence; the
reliability of the cameras, and their correct use, has become fertile ground
for lawyers seeking flaws in a prosecution case. Is this just legal
hairsplitting, or are the cameras really untrustworthy?
Fixed speed cameras
Oddly enough, the fixed speed cameras with which we are all familiar are
named after a racing driver, Maurice Gatsonides, who invented the camera
(now known as a Gatso) to measure his speed when practising on a race track.
The first fixed speed camera in the UK was installed in Twickenham in 1992,
but in those days it was intended to catch only the very worst offenders, so
drivers were allowed a very wide margin of error before prosecution took
place; up to 20 mph, it is claimed by the former police officer who was then
responsible for the camera.
Nowadays, Gatsos are sometimes referred to as 'scameras' and are widely seen
as a money-spinner for the local authorities who share in their revenue;
there are reports of motorists fined for travelling at just 32 mph in a 30
mph limit. It follows that the accuracy of the cameras can be of crucial
importance to drivers who are caught by them. The Gatso camera works by
taking two flash photographs of the car travelling over road markings. The
camera itself gives a speed measurement, but a court will rely on a
technician's calculation of the distance covered over the ground, which is
estimated to be accurate to within one mile per hour. Successful challenges
have been made on occasion, however.
Mobile speed cameras
Unlike Gatsos, mobile cameras employ laser technology to detect the speed of
vehicles, so no flash is visible. They are generally installed in stationary
police vehicles, and are said to be accurate at distances up to a kilometre,
using the Doppler effect to measure speed. Any inaccuracies are likely to
result from errors in calibration or usage; drivers challenging a
prosecution are entitled to see the operator's statement and the calibration
certificate, and this has led to acquittal when the paperwork could not be
produced in court.
Red-light speed cameras detect and record the speed of a vehicle, either by
vehicle tracking radar or electronic detectors embedded in the road’s
surface. These detectors accurately measure the speed of the vehicle, and if
it exceeds the speed limit, a digital photograph is taken of the offending
vehicle. Again, any errors is likely to result from faulty calibration.