Motoring Discussion > Speed camera sensitivity Miscellaneous
Thread Author: sherlock47 Replies: 7
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New Tue 16 Mar 10 12:28 
 Speed camera sensitivity - sherlock47
A friends business has 2 vans one a luton and the other a standard transit. His 2 employees use them interchangeably on a per job basis. They are both convinced that the 'luton' with its large alloy panelled frontal area triggers a number of the local speed cameras at speeds well below the 30mph threshold. (They believe it is because the van is painted yellow;)!).

Now we all know that the trigger is doppler shift based - so why should it trigger at low speeds?

Yes, we have checked that the speedo is not under reading (using satnav). Any bright ideas please?
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New Tue 16 Mar 10 12:39 
 Speed camera sensitivity - Zero
Its because the flat front of the Luton pushes a big wave of air in front of it. The pollen swirls around faster on the bow wave of the air triggering the camera. And the fact that yellow light is faster than white light,
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New Tue 16 Mar 10 12:44 
 Speed camera sensitivity - Dave_
On the single-carriageway A5 near Nuneaton there's a GATSO in a 50mph limit that ignores everything travelling at under 50mph except the 7.5t truck I used to drive... It would flash (twice) at the back of my truck at any speed from 45mph up to 50mph (checked by TomTom).

I assumed there was some mechanism within the camera to detect HGVs passing at more than their single-carriageway limit of 40mph, and it wrongly assumed that my puddle-jumper was one of the big boys?


>> yellow light is faster than white light

I was told this years ago by a Renault salesman, in explanation of why the 5 GT Turbo had yellow front fog lights...
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New Tue 16 Mar 10 12:40 
 Speed camera sensitivity - Focusless
>> Now we all know that the trigger is doppler shift based - so why should
>> it trigger at low speeds?

Reflections of cars coming the other way, with their effective speed equal to their own plus yours?
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New Tue 16 Mar 10 12:44 
 Speed camera sensitivity - sherlock47
Reflections of cars coming the other way, with their effective speed equal to their own plus yours?

I like that - the geometry fits as well, with the speed camera displaced up and to the left, the resulting reflection would be directly towards oncoming traffic.

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New Tue 16 Mar 10 13:01 
 Speed camera sensitivity - Focusless
>> Reflections of cars coming the other way with their effective speed equal to their own
>> plus yours?
>>
>> I like that - the geometry fits as well with the speed camera displaced up
>> and to the left the resulting reflection would be directly towards oncoming traffic.

Thinking about it (or trying to...) will the effective speed be the oncoming car's plus double yours? Eg. if you throw a tennis ball at a wall moving towards you at 10mph, it comes back at 20mph, doesn't it? (approx)
Last edited by: Focus on Tue 16 Mar 10 at 13:01
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New Tue 16 Mar 10 13:33 
 Speed camera sensitivity - Zero
Alas the reflections thing is a good theory, but they would be out of phase cancelling out the other reflections - effectively making the transit "stealthy"
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New Tue 16 Mar 10 15:08 
 Speed camera sensitivity - sherlock47
>> Alas the reflections thing is a good theory but they would be out of phase
>> cancelling out the other reflections - effectively making the transit "stealthy"
>>

No reason that I can think of that they should be out of phase (any more than with a single vehicle).
I think that there will be 2 frequency components to the return signal, the normal return with a shift from the speed of the van, and a second component where the doppler shift will be as a result of the sum of the speeds of the 2 vehicles. IYNWIM.

Last edited by: pmh on Tue 16 Mar 10 at 15:12
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