Speed cameras are fallible in some circumstances when the radar beam is deflected, it emerged today.

The little-known phenomenon was blamed when a bus driver was threatened with prosecution after being clocked at 81mph. His tachograph showed that in fact he was proceeding at a sedate 29mph, and his bus was fitted with a limiter that prevented it exceeding 62mph.

The incident happened on the A140 at Earl Stonham in Suffolk, when Trevor Martin, 43, of Ipswich, was photographed by a camera operated by Suffolk SafeCam.

The effect, where the radar is reflected from a large slat-sided vehicle like Mr Martin's bus and is then bounced back from a vehicle moving in the opposite direction, is the same principle used by the American Air Force to disguise its Stealth bomber.

Terry Marsh of Suffolk SafeCam admitted that it was generally known in the industry that speed cameras could make mistakes, but insisted that there was no need for the cameras to be changed as a human operator usually picked up any anomalies.

"It is known that it is possible for a camera to record a speed inaccurately under sets of very rare circumstances," he said.

"I think all camera operators have been aware of this since cameras were introduced. What can happen is that on large slat-sided vehicles the radar beam emitted from the camera can be deflected. If a similar vehicle is travelling in the opposite direction the beam can bounce off that vehicle and then back to the camera, giving an inaccurate reading.

"That is what seems to have happened in this case in the first instance.

"It’s difficult to be exact but it can probably affect vehicles from the size of a large estate car upwards. But I would stress that there has to be a very particular set of circumstances with vehicles travelling on the other carriageway at a certain speed for this to happen. It’s very rare.

"There’s a similar principle with the Stealth bomber designed by the American Air Force. That is designed to hide itself from radar. It does that because it is made up of a series of slat sides which deflect radar beams in many different directions throwing the reading. That can happen by accident in certain circumstances to a speed camera."

The Institute for Advanced Motorists immediately called for an overhaul of checking procedures by speed camera operators to ensure that more motorists were not being wrongly prosecuted.

It said that camera operators had an obligation to ensure that the system was "robust" because of the potential consequences for motorists.

But Mr Marsh said that this was not necessary, and that the case had been a rare one. He said: "Every speed camera is double-checked by a member of staff who views the footage and calculates the speed of the lorry by measuring the distance it has travelled in a certain time.

"And in this case the operator has made a mistake. Whether he fell asleep when doing the checking or whether he accidentally clicked the button marked ‘prosecute’ when he meant to click the button marked ‘no further action’ I don’t know.

"I’ve taken the view that it was not going to be possible to establish exactly what had happened in the checking process. We know who the operator was but he doesn’t know why he made the mistake or has any recollection of it.

"All operators have been reminded of the responsibility they have to ensure the checks are right but I don’t feel there is any need to review general procedures.

"It’s a very rare mistake and we’ve been completely open about it. I would want everybody to have full confidence in the system and I think they can do."

A spokesman for the AA said that the admission was "absolutely not" a lifeline for speeding motorists facing the prospect of losing their licence.

"Most people who think they have a good case, don't," said Paul Watters, the head of roads and transport policy at the AA Motoring Trust.

"The system is fairly foolproof. It is a pity that this notice of intention to prosecute was sent out, as it lessens the credibility of the technology a little bit. We know the cameras do have some problems with larger vehicles, but usually these things are nipped in the bud by checking the photos for the distance the vehicle has travelled."

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