FOI release

Speedmeter Type Approval

We have a received two requests under the Freedom of Information Act 2000 for the following: Request 1: Laser speedmeters interpret the …

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We have a received two requests under the Freedom of Information Act 2000 for the following:

Request 1:

Laser speedmeters interpret the speed from the difference of a number of range (distance) measurements. The rate of change of range giving the speed.

  • When it is used at an angle the cosine effect is mentioned but not the associated sine and tangent effect. While the cosine effect reduces the speed reading the sine and tangent effect increase it; and the increase is always greater than the reduction due to the cosine effect. At 10 degrees angle the speed is inflated by 2%; at 20 degrees by 6%; and at 30 degrees 15%. How is this inflated figure compensated for?

  • Does Type Approval ignore this error?

  • The sine of the angle gives the relative lateral movement of the beam across the target surface and the tangent the consequent difference in distance of the reflecting points. I can supply drawings if it needs to be made clearer; but I assume as a scientific branch of the Government you are fully conversant with trigonometry involved.”

Request 2:

I am looking for advice and help to answer some technical questions regarding Laser speedmeters.

  • The first question relates to calibration.  The HOSDB speedmeter handbook requires the measuring accuracy to be assessed by “Measurements at different speeds up to the maximum stated by the manufacturer shall be made. Simulated speeds may be used for speeds above 120 mph.” This would make sense as the equipment should be assessed/calibrated for the range of speeds over which it is expected to be used.

  • What is the status of equipment that hasn’t been calibrated to this standard and is then used in excess of its calibrated range?

  • When it is used at an angle the cosine effect is mentioned but not the associated sine and tangent effect. While the cosine effect reduces the speed reading the sine and tangent effect increase it; and the increase is always greater than the reduction due to the cosine effect. At 10 degrees angle the speed is inflated by 2%; at 20 degrees by 6%; and at 30 degrees 15%. How is this inflated figure compensated for?

  • The HOSDB guidelines require less than + 2mph, or +3% error over 66 mph, if not I assume it fails its type approval. At a relative angle of 10 degrees or more the device is reading more than 2% over and does this not mean that the device shouldn’t be used over this angle? Or at least some further time/distance checks made to corroborate the reading? Can you confirm whether Type Approval require this measuring accuracy?

  • Where does this error come from?

We released the following information on 30 July 2007.

Date: Mon Jul 30 00:00:00 BST 2007

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