I thought it wouldn't be long before this turned up on
Numberwatch (3rd article down) since John Brignell was involved in the program. (My bold.)
Quote:
The camera does not lie
When you have been involved in an activity such as monitoring the frauds and errors involved in the abuse of numbers in the media and politics it is easy to convince yourself that you are unshockable. So it was with your bending author until he became involved in a BBC investigation of hand-held speed cameras. These devices have often been mentioned in Number Watch and they are probably the cause of more impassioned correspondence from all over the world than any other topic. We have observed before that they are a fruitful source of income for governments, and are protected as a random and secure form of taxation, but the degree of official obfuscation, Orwellian misrepresentation and sheer bureaucratic bullying involved in the UK is quite startling. The BBC programme demonstrated quite clearly that with a tiny judicial movement of the camera you could record a stationary car as going more than thirty mph. The Managing Director of the importer of the instruments, apparently the regular police expert witness in such cases, stated that such an error would not occur if the car was moving.
Who is this guy? What are his academic qualifications? Are they relevant? What institutions have admitted him to their fellowship? Apart from making money out of importing boxed instruments, what does he actually know? Is he likely to give a disinterested opinion?
To a simpleton such as your bending author, zero is just as valid a speed as any other. At what speed does the transition occur between a thirty mph error and a zero error? Does this managing director actually understand the algorithms that are implemented in his goods? Is he aware of the problems of fitting a straight line to a data sequence? How does he decide whether a sequence is linear or non-linear? Incidentally, if you too are confused at this point, the instrument does not work on the Doppler principle, but calculates a sequence of distances by determination of time of flight of the infra-red beam, over a period of about one third of a second. The data are only rejected if they are not in a straight line. Who decides what is a straight line? How straight is straight?
Some poor victim is going to lose his licence, possibly even his livelihood, because the programmer of a bit of microprocessor firmware writes a few glib lines of code. Does that programmer understand the theory of quantisation noise, the z transform, the limitations of the method of least squares, the degree of tremor in the human hand, the propensity of human subconscious to achieve the result that is expected and desired or the ruthlessness of the Establishment in obtaining funding for its overweening bureaucracy?
The behaviour of the Home Office in this saga is quite extraordinary and menacing. Perhaps, when one has spent twenty years being a professor of industrial instrumentation (possibly the only professor of industrial instrumentation), one might have developed an exaggerated respect for one’s own appreciation of the basic rules of the subject. Nevertheless, it would not seem unreasonable to propose that, in evaluating an instrument, one of the essential elements is to determine and investigate the possible failure modes. Not only did the Home Office fail to do this, but they have assiduously prevented anyone else from doing it either. Furthermore, according to the coda of the recent BBC programme, the Home Office have threatened that anyone who uses the information coming from that programme in his defence is likely to receive a higher fine.
There was a time when the level of fines was determined by the judiciary. What have we come to when you are punished for daring to defend yourself against a dubious charge? Turning the police into random tax collectors causes a great deal of resentment and damage to their ability to tackle real crime. Police in North Wales, the fiefdom of the egregious Richard Brunstrom (who generated our Number of the Month for June, 2005) are given targets for the number of arrests of motorists, regardless of the actual number of offences. It can be shown that people are able to learn subconsciously to achieve desired results in measurement. All it needs in the case of the hand-held speed camera is a movement of the front of the camera of the order the thickness of a human hair. With a little acquired skill you can beat the algorithm that flags an error for non-linearity and become the constabulary champion of North Wales. Among others, Jones the Burglar will love you for it. Time was when Home Office scientists were fiercely independent of politics and widely admired, while chief constables were appointed for their skills as thief takers, not their political correctness.
Testing an instrument is like testing a scientific theory – you try to break it. It is clear that the Home Office only tried to demonstrate that the device worked in favourable circumstances. It is evident that the whole concept of the instrument is seriously flawed. Rather than exploit the Doppler Effect, it takes a series of readings over a period of about a third of a second. The speed is calculated by fitting a straight line to the data sequence. Only if the data do not form a straight line is an error flagged. Just fitting a straight line is fraught with difficulties; determining whether the line is actually straight is a whole new ball game. The major potential source of falsity is “slip error” in which the beam rakes the side of the vehicle.
All individuals to some extent experience normal hand tremor. The frequency is reduced but the amplitude increased by carrying a mass (such as a speed camera). In addition, the process of aiming is a feedback process involving vision, nerves and muscles, resulting in behaviour such as lag and overshoot. If this seems nit-picking, note that the distance between the number plate and the edge of a car subtends an angle of a fraction of a degree at the distances usually involved. This means considerably less than 1 mm of motion of the front of the camera relative to the back. Try focussing a pair of powerful binoculars on a moving number plate a couple of hundred metres away, or try taking a photograph of a moving object with the shutter speed set at a third of a second.
There is no reason to suppose that slip error will necessarily produce a non-straight line of data. The only reasonable conclusion you could come to in view of all these caveats is that thousands of motorists have been wrongly convicted, some of them losing their livelihood as a result.
This is one of those cases in which it is difficult to understand why the fundamental design decision was made. Laser Doppler velocimetry has been successfully applied to the measurement of speeds of everything from blood corpuscles to rolled steel. Why throw it aside for a mechanism with such obvious contradictions?
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