A major row is breaking out because of claims of 'errors' with one particular laser speed meter - the LTI 20.20. (say L.T.I. Twenty Twenty) This equipment is used by Police and camera partnerships to measure the speeds of vehicles and provide legal evidence of speeding offences. It's easily the most common device of its type and is used in virtually all speed camera vans. (At the time of writing we're not aware of any other equipment being used inside a camera van anywhere in the country.)

The critical error is called 'slip effect'. It means that the device can record and display an incorrect speed reading. Because the error is frequently undetected it is impossible to be sure that the error is absent in any particular speed reading. It is therefore impossible for speed readings from this equipment to be 'beyond a reasonable doubt'.

If the speed reading is not 'beyond a reasonable doubt', then the standard of evidence required by British courts has not been met and all convictions are unsafe.

Safe Speed estimates that at least 5 million motorists have been convicted by this device over the last five years.

The authorities are presently suggesting that because the device has been subjected to Home Office testing prior to being awarded type approval, it is utterly trustworthy and does not make mistakes. But official papers seen by Safe Speed confirm that the Home Office never even tested for 'slip effect'.

LTI 20.20 laser speed meters are imported and supplied by Tele-Traffic UK.
 

BBC TV: Inside Out (28th February 2005)(BBC web page)
Numberwatch web site (article)
BBC TV: Inside Out (12th September 2005)(BBC PR)(web page+debate)
The Daily Mail (first article)(online version)
The Daily Mail (follow up)
ITV: Tonight with Trevor MacDonald (4th November 2005)

Safe Speed PRs:
PR175, 28th February 2005, BBC will reveal laser speed meter errors
PR239, 12th September 2005, Laser Speed Meters Untrustworthy
PR251, 4th November 2005, Motorists: you must see Tonight tonight
 
 

The LTI20.20 is a laser speed meter. Laser speed meters don't measure speed directly. They work by taking a rapid series of distance measurements then calculating speed from the rate of change of distance. In the case of LTI20.20, 40 distance measurements are taken over about one third of a second.

These distance measurements are assembled into a table. Up to five of them may be discarded and the speed is calculated by fitting a straight line to the series of distance measurements. If the straight line fits within an acceptable tolerance the device calculates and displays a speed.

Distances are measured by flight time of pulses of infra-red laser light. The device 'flashes' and times how long it takes for the flash to be reflected from the target and returned to the device.

In practice this device may be hand held or used on a tripod. When used by Police officers it is used stand alone. When used by camera partnerships it is coupled with video recording apparatus. Either way the actual speed reading is under operator control. The device is aimed via a sighting scope, then the operator pulls a trigger to initiate the measurement cycle.

LTI 20.20 isn't a speed camera, it just measures speed. When connected to video recording apparatus (Typically Lastec local video system) the system functions as a speed camera.

Imagine pointing one of these devices at a wall. If you sweep the beam along the wall during the measurement period, it sees a changing distance. If the distance changes smoothly, the device must accept the changing distance as a speed.

In practice is it entirely possible for the aim point of the device to slip along the side of a vehicle or perhaps over the sloping surface of a bonnet. This results in a speed reading based on the speed of the vehicle +/- the 'virtual' speed resulting from the change in distance of the aim point. The error can be in either direction (i.e. it can exaggerate or underestimate the speed of a vehicle).

The device has no way to distinguish between a smooth and linear slip and a steady speed. From the point of view of the device they are absolutely identical. Similarly the device cannot recognise that it has seen a combination of slip and speed.

Slip errors in use may occur for any of the following reasons:

• With a fixed aim point, the target vehicle may move across the field of view
• The operator may accidentally pull the trigger too soon and sweep a wall or road surface while bringing the device to bear on the target vehicle.
• The operator may 'wobble' the aim point during the measurement cycle
• When the device is used in a camera van, the van may be buffeted by wind or the slipstream from passing vehicles.
• Squeezing the trigger in itself is highly likely to alter the aim of the device. This problem is very well known to marksmen and photographers.

Multiple reflections

Alignment

Parallax alignment error
 
 
 
 
 

MCN have been complaining that the device was never tested on motorbikes before being given type approval. Three particular features of motorbikes may affect the ability of the device to accurately measure speeds. We should certainly be concerned that testing procedures did not include motorbikes given the additional potential errors.

No front number plate

In typical use laser speed meters will get a good strong reflection from the front number plate of vehicles (due to the glass bead 'retro-reflective' surface). Since no such retro-reflective surface exists on motor bikes it is hard to be confident that a single reflective surface will provide the primary reflection during the measurement cycle. This may well provide even more common slip errors as the gun picks up different points on the bike.

Large exposed wheels

On some bikes the most reflective surface might be the rim or spokes of a wheel. Clearly the top of the tyre on an oncoming motorbike is moving towards us twice as fast as the bike itself. It is unknown what effect this might have on the measurement process. The time over distance method would appear to be tolerant of the problem, but since it has not been tested we do not have anywhere near enough confidence to say that no error will take place. 

'Lumpy', 'glinty' and 'see-through' structure

Cars and vans are box-like shapes, while many motorbikes have no bodywork at all. It might be possible for the laser to see-through the bike to a swept surface beyond or different components of the bike may glint back at the device during the measurement cycle leading to slip errors.
 

The authorities appear to be 'in denial' about the observable problems with LTI20.20.

At present they appear to be offering three lines of defence:
 

It has Home Office Type Approval, therefore it is beyond doubt.

Historically this has proved to be a fair position with most Type Approved equipment, and we are not surprised that magistrates are happy to accept it. Unfortunately awarding type approval in this case appears to have been a mistake. 

The errors do not take place with official approved UK equipment.

We have been expected to believe that approved UK market equipment is somehow superior to equipment supplied to other markets.  It's clearly rather bizarre to suggest that a better version exists, but non UK purchasers are uninterested in it. Safe Speed agrees that different versions exist for different markets, but has been unable to substantiate the suggestion that UK versions are in any way superior.

If you challenge the evidence of this device it will cost you more.

An outrageous intimidation tactic - no more and no less - contained in statements issued to BBC Inside Out and Tonight with Trevor MacDonald.

They have been offered a number of opportunities to bring current 'in use' Police equipment along to testing, (for example with Tonight with Trevor MacDonald). They have consistently refused to do so. We are left to assume that they have reason to believe that their devices would fail the test.

If you are being prosecuted
 
 
 

If you are a concerned member of the public

Consider writing to your MP. 
 
 
 
 
 
 
 
 

Body text

The LTI 20.20 is a series of devices in a family.
 
 

Mr. Lyons: To ask the Secretary of State for the Home Department how many police forces are using LTI 20–20 laser cameras in mobile speed patrols. [183038]

Caroline Flint: This information is not recorded centrally. It is for chief officers of police to decide which speed enforcement devices to purchase from the range that have been type approved and that are commercially available. 
 
14 Jul 2004 : Column 1165W

From Hansard ggg
 
 
 
 
 
 

18th November 2005

Replies to letters from MPs include phrases such as:

"I have taken this issue up with the appropriate Transport Minister and will write to you again when I have a reply."

"I have forwarded a copy of your email to the Transoprt Secretary, Alistair Darling MP, to ask him what checks were carried out to test the accuracy of the LTI 20.20 light weight hand held speed camera before it was approved by police forces. As soon as a response has been received, I will forward a copy to you."
 
 
 
 

Paul Smith, founder of the Safe Speed road safety campaign said: "I was present at the filming carried out by Tonight with Trevor MacDonald. I had to see slip effect with my own eyes and I wasn't disappointed. I saw slip effect time after time after time. I am left in absolutely no doubt that the LTI20.20 device is unsuitable for gathering legal evidence according to the standards required by UK law. It should never have been granted Home Office type approval.

I urge anyone prosecuted on the basis of an LTI20.20 speed reading to challenge the evidence in court.
 

"

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