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Mobile speed camera
Mobile speed guns are gaining in popularity with the police

Mobile speed cameras are increasingly being used by the police to enforce speed limits, but how accurate are they?

We look at these cameras and see if their claims of accuracy are themselves accurate.

A recent report by the RAC shows that nearly two-thirds of all drivers admit breaking the speed limit on a 30mph roads.

It's not surprising then that the amount of speeding tickets we are all getting are on the increase.

But we discover that some of the equipment used by the police may not be as reliable as they like to think.

In the last year the numbers of mobile speed cameras hidden on motorcycle, police van and cars have risen by more than a third.

That means there are just under 3,500 mobile speed units in the country.

In 2003-4 speeding fines generated 112 million. Of that, 92 million was ploughed back into installing and operating the cameras.

A lot of this revenue is now created by the mobile cameras. It is predicted that by the end of the year they will be as many mobile speed cameras that they are fixed roadside cameras.

But are those mobile cameras as reliable as the police would like to think?

Case One: Paul Cox

Paul Cox
Paul appealed against his speeding conviction and won

Paul Cox was driving towards Plymouth on the A303 Dual carriageway when he was stopped for speeding.

The car he was driving was fitted with cruise control, which he had set to just below 70mph - the speed limit on that stretch of road.

He passed by a marked police car that was carrying out speed checks and was asked to pull over.

The police told Paul that they had clocked his speed as being in excess of 90mph.

He was confident that he had not broken the speed limit and contested the case in court.

Former policeman Paul Cox appealed against his conviction – and won. The court found there were discrepancies in the speed gun evidence used against him.

Paul had the confidence to contest his case, but many simply accept the fines even though they feel they are in the right.

Home Office approved

All the speed guns used by the police and the camera safety partnership must first be approved by the home office. Several type of laser devices used in the UK but they all work on the same principle.

The devices work by sending out a beam of infra red light. Ideally this should be targeted at the number plate of a car, because number plates have a special reflective coating which bounces the beam straight back to the machine.

As the car moves the devices quickly take a series of distance readings, and from those works out the speed of the vehicle.

However the accuracy of these devices has been disputed.

To see how accurate they are we have invited Dr Michael Clark, a leading expert in laser and traffic control, to test some of the government approved mobile speed guns.

Erroneous distances

Diagram showing the reflection effect
A wing mirror and a road sign doubled the distance recorded

The machine relies on the laser beam being reflected back at the gun.

However Dr Clark demonstrated what happens when that beam of light is deflected off another object before returning to the speed gun.

He set up a situation where the laser beam was hitting the wing mirror of a stationary car. He explains;

"What's actually happening is the device is sending out a laser beam that is hitting the wing mirror on the car, then it is being reflected onto the [roadside] sign … it's then coming back off the sign, back onto the wing mirror again and back into the receiver."

As the devices use a distance measurements to work out the speed of a car, Dr Clark believes that such reflections could cause erroneous speeds readings.

The slip effect

diagram showing the slip effect
If the laser doesn't focus on the same area you can get the slip

As the gun calculates speed by measuring the changing distance to a car, if the beam of the gun is moved along the car while taking a reading, this could affect the results.

As Dr Clark explains; "This instrument doesn't know if it [the speed gun] is moving. So it started measuring a little bit further away down the vehicle, now it's a bit closer so it thinks there's a speed reading".

He then pans the speed camera down the side of a stationary car and clocks it doing 4mph.

"This is of course very relevant. If a policeman is pointing at a vehicle going by and he moves it across [the vehicle] then he will get an increased, or indeed a decreased, speed reading."

Dr Clark says that all laser speed guns suffers from the same problem so we thought we would give it a go on a wall with one of the latest guns used by the police: an LTI 20.20.

speed camera showing a reading of 58mph
We clocked a stationary wall at 58mph - now that's motoring

By aiming at the wall and pulling the trigger whilst panning with the device we managed to get a reading of 58mph from the stationary wall - enough to get three points and a fine in urban areas.

Dr Clark has only been demonstrating the speed guns on stationary cars to us, but he says the problems could be worse in real-life situations;

"Because the car itself is moving they have to hold it very very closely on the same point on the vehicle otherwise they will get an erroneous speed reading."

In theory, this means that when doing a speed check, if the operator lets the measuring laser move across the side of a car during the speed check, then the length of the car could be added to the distance the machine uses to calculate the car's speed.

Laser guns typically take their series of measurements in about a third of a second. If a slip effect adds an extra couple of metres onto the distance you travel in a third of a second it can increase the speed registered by anything from an extra one to 30 mph.

The manufacturer's response

Frank Garratt
Frank Garratt says that his devices are accurate

But Tele-Traffic, the UK manufacturer of the LTI 20.20 reject the possibility of getting erroneous speed reading from a moving vehicle.

Frank Garratt, Managing Director of Tele-Traffic, says that his guns are fitted with a technology which will detect any slip effect from a moving vehicle.

If it detects any slippage it will display an error message instead of a speed.

Mr Garratt says the device "traps out any panning error."

He insists that on moving objects errors of more than 2mph are highly unlikely. He says the system could display speeds out by "no more than 1mph, if at all, but in any event 2mph is well within the target parameters".

Case Two: Michael Hall

So far Dr Clark has been involved has an expert witness in 5 court procedures, one of them being his own.

In 4 occasions the prosecution dropped the case. Michael Hall who got clocked by an LTI 20.20 in Southampton was one of them.

Michael Hall
Michael Hall escaped losing his license when speed camera evidence was withdrawn

Michael recalls the events; "I am just convinced that I was at the most 30 [mph] because I checked my speed.

"When I got the summons the police said I was doing 41[mph]".

With Dr Clark's help, Michael managed to have the evidence in his case dismissed.

Michael has his own view on why this happened;

"I think they did that because the video evidence proves that their machine wasn't working properly.

Looking at some bits of the video there were clear errors in what the machine thought what distances were, and if it can't work out a distance it can't work out a speed".

Video evidence

Inside Out got hold of one of the very few police speed check videos which has been released.

We showed the recording from the South Wales police to Dr Clark.

He pointed out instances where the camera recorded speeds indicating the vehicle was travelling in the opposite direction to the way it can be seen going on screen. Dr Clark explains;

"If there is a minus sign in front of the reading that means the target has been measured as going away.

"In this case it wasn't. And that is typical of the errors you will get.

"Here we have negative speeds for vehicles coming towards us - It's a nonsense".

Tele-Traffic commented on the video: They say that even though the video does not represent the event accurately; the laser gun itself was always working properly.

Mr Garratt, the Managing Director, says;

"In that particular case there's no doubt in my mind that, in overall terms the officer did not set up the video element as well as he might have done, and certainly made some operational procedural errors in the way he did that".

Growing concerns

Following a successful court challenge in Scotland in February 2005, the Home Office is now considering reviewing the approval of another type of laser gun.

But as far as the police is concerned, it is the home office who decide what equipment they should use. Superintendent Lawrie Lewis from the Avon and Somerset Constabulary says;

Superintendent Lawrie Lewis, Avon and Somerset Constabulary
"If they [The Home Office] have confidence in them, I have confidence."
Superintendent Lawrie Lewis, Avon and Somerset Constabulary

"The Police Scientific Development Branch and the Home Office have type-approved this equipment.

"They've gone through extensive testing - If they have confidence in them, I have confidence.

"If the Home Office decides for whatever reason that the confidence is no longer there then they will withdraw the equipment".

The RAC say it's important the police get it right when clocking drivers. Paul Hodgson from the RAC says;

"I think it's important for the police, as well as motorists, to know that the cameras are working.

"They need the trust of the motorists, so if a motorist's caught - they need to think they've been caught fairly and squarely.

"If the technology's not working .. then those findings need to be fed into the home office review".

Dr Clark says, "I think that these instruments, or instruments of this type should be reviewed, both in their use, and in the capability of the technology to perform the task that is being asked to do.

"We talk of I think it's in excess of 2 million prosecutions using electronic devices - if only 1% of those prosecutions are incorrect that's 20,000 incorrect prosecutions, and that cannot be right".



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