found this on the assocation british drivers site

theres also a responce from the manufacturer tele traffic uk!
he said they are accurate but bbc have discovered they arnt!!

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?

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

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

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.

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.