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Post by Deleted on Mar 9, 2019 0:47:48 GMT 1
A bit of reading material;
How do Gatso speed cameras work?
Gatso speed cameras use radar technology to measure how fast a vehicle is travelling and to trigger the camera into action. If a motorist is driving above the road speed limit then two photos are then taken in quick succession. The Gatso uses a powerful flash to show the rear of the vehicle, its registration plate, and white painted calibration lines on the roads surface.
Gatso speed cameras are always rear facing. The reason for this is that the speed cameras 'flash' will not blind oncoming motorists. However, this also means that the speed camera may not be visible until the last second (as pictured right - Gatso sited behind road traffic sign).
It is a legal requirement to have a secondary measurement for speed. This is why at every Gatso speed camera location there are white lines painted on the road. The distance between each line represents between 5mph so there can be no dispute over how fast you were driving. If there is any dispute over whether the radar technology captured the correct speed of the vehicle that was speeding the white lines are there as a secondary measurement.
Gatso can differentiate between different speed limits for different vehicles. For example cars, caravans and HGV's have different speed limits and the camera will measure the vehicles length and impose the correct speed limit for each vehicle.
The fixed Gatso uses a reel of film to record photos to. The film can soon run out in busy areas.
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Post by valhalla on Mar 9, 2019 0:58:49 GMT 1
That is why you can be charged with speeding in an urban area irrespective of which direction you are instantaneously driving at the time, otherwise you would always have the get-out-clause of "I was driving at right-angles to the traffic sign, so I was only doing 0mph".
Are you saying that the camera used to show your vehicle is not positioned in a given known direction, therefore the vehicle speed is not known as a vector?Precisely. A fixed road camera might, retrospectively, have a precise bearing quoted, possibly inferred by the positioning of two or more cameras at static positions along a kerbside. But....
consider two clear examples of where a vector makes no sense;
1) The road has a bend in it, and the speed is measured by "average-speed" calculation of time between those two cameras (such as on a motorway). The velocity, quite literally, has no bearing on the calculation, just the length of the road between the two cameras, divided by the time taken to traverse the distance between them = your average mean speed.
2) A police officer can hold a radar-gun in any direction within an urban area, in order to enforce the speed limit implied or indicated for that area. Again, he does not measure the angle of the measurement path relative to a magnetic or true North, he simply takes a scalar measurement of the Doppler interference in the reflected beam of his gun. No court case (AFAIK) has ever quoted the angle of measurement to the nearest degree relative to North, the direction is airily quoted as "in a Southerly direction" or similar.
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remmington
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Post by remmington on Mar 9, 2019 21:47:10 GMT 1
Not sure if it is still the same? - but Police traffic cars used to have "calibrated speedos" - IE the "measured mile" test - so the they could nick you for speeding by following you at excess speed.
This is speed measured by a speedometer fitted to dashboard of a car - in case I am mistaken.
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Post by studabear on Mar 9, 2019 22:32:04 GMT 1
Not sure if it is still the same? - but Police traffic cars used to have "calibrated speedos" - IE the "measured mile" test - so the they could nick you for speeding by following you at excess speed. This is speed measured by a speedometer fitted to dashboard of a car - in case I am mistaken. They do well at least in the traffic cars. A mate of mine bought a Mondeo ST and completely missed the special speedometer.
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Post by studabear on Mar 9, 2019 22:33:01 GMT 1
And the calibration stickers on the A post
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remmington
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Post by remmington on Mar 9, 2019 22:47:12 GMT 1
That thing in the instrument display that you look at to assess how fast you are going. valhalla should know different As it happens, I don't know any different!! In all my Engineering history, I have never called the instrument that measures speed of the vehicle by anything else other than a speedometer. I think that is why the speed measuring device is called a Speedometer in the Testers' Manual under Section 7 (Other Equipment) paragraph 8, and continues to be misappropriated as a "Speedometer" throughout the text. Maybe the DVSA team who put this together were not familiar with the other terms for a speed measurement instrument? Pitot-tube pressure? Ticker-tape timer? I'm intrigued, really..... Thank you Valhalla, my bubble remains un-burst.I was beginning to wonder? - if the speedometer in every car I have ever been in - has morphed into something else? I have been wrong about many things in my life - but I am sure a vehicle speedometer measures the speed the car is going in MPH or KMH. A more interesting point is - we fit Taxi meters and calibrate them on a measured mile - pulse feed meters - odometer readings. You would not believe how inaccurate some vehicle speedos are against a measured mile (if I am using the right word for a speedo).
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Post by Deleted on Mar 10, 2019 13:20:53 GMT 1
As it happens, I don't know any different!! In all my Engineering history, I have never called the instrument that measures speed of the vehicle by anything else other than a speedometer. I think that is why the speed measuring device is called a Speedometer in the Testers' Manual under Section 7 (Other Equipment) paragraph 8, and continues to be misappropriated as a "Speedometer" throughout the text. Maybe the DVSA team who put this together were not familiar with the other terms for a speed measurement instrument? Pitot-tube pressure? Ticker-tape timer? I'm intrigued, really..... Thank you Valhalla, my bubble remains un-burst.I was beginning to wonder? - if the speedometer in every car I have ever been in - has morphed into something else? I have been wrong about many things in my life - but I am sure a vehicle speedometer measures the speed the car is going in MPH or KMH. A more interesting point is - we fit Taxi meters and calibrate them on a measured mile - pulse feed meters - odometer readings. You would not believe how inaccurate some vehicle speedos are against a measured mile (if I am using the right word for a speedo). You'll never sit in the driving seat of any vehicle and look into the instrument display and photograph your finger pointing to the clock that has written upon it 'Speedometer' or 'Speedo'. EVERY instrument display you look at will always say 'MPH'. This really is just telling the driver 'How Many Miles in the Hour' you have driven, yes for simplicity they tell everyone it is speed, even as valhalla says it is 'averaged', but primarily back in the 1970's our road signs were supposed to be changed to the correct standards but our government opted out because they knew, 'Common sense' and all that the public would become very confused, so it was left as the current system, which everyone accepts, and all the case laws have been concluded, which is why the street camera was updated using radar technology. Speed is a scalar quantity only so if a vector analysis were never required then the radar would never have been introduced.
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Joepublic
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Post by Joepublic on Mar 10, 2019 13:37:48 GMT 1
That sounds like a complicated way of boiling an egg?
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Post by Noberator on Mar 10, 2019 22:49:21 GMT 1
That sounds like a complicated way of boiling an egg? What started off as a question about an MOT speedo checks has gone a bit of track IMO so don't egg him on.
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