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PostPosted: Mon Sep 10, 2007 00:37 
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Well, I'm not sure this post belongs in here because I'm not being prosecuted...

...not yet, anyway! :wink:

I went under a bridge with a scamera van on it the other day whilst (possibly) exceeding the limit somewhat! I saw him while a good distance away (I guess the best part of half a mile) and braked fairly gently.

My question is twofold:

1. How long does it take the equipment to register a speed? (I guess it's a split second)?

2. If the vehicle is changing speed at the time (braking or accelerating), what does the equpiment do? Does it need to see a steady speed for (x) seconds (miliseconds?) before deciding what speed you're doing?


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PostPosted: Mon Sep 10, 2007 01:20 
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I'll let the experts answer your questions, but look on the bright side: at least the SCP in your area isn't too aggressive. :gatso2:

Nice places though they are, I'm never even going to consider going to Cumbria or Wales until the thieving stops.

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"The freedom provided by the motor vehicle is not universally applauded, however: there are those who resent the loss of state control over individual choice that the car represents. Such people rarely admit their prejudices openly; instead, they make false or exaggerated claims about the adverse effects of road transport in order to justify calls for higher taxation or restrictions on mobility." (Conservative Way Forward: Stop The War Against Drivers)


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PostPosted: Mon Sep 10, 2007 01:49 
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1. 0.3 seconds or thereabouts (plus time taken "forming a prior opinion" and aiming)

2. If the change is too great it should register an error. The greater change the greater chance of triggering an error. Hard breaking seems to have saved me so far.

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PostPosted: Mon Sep 10, 2007 09:00 
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At such a distance the gun is at great risk of giving E03s, so the operator may need several attempts to get a good hit.

Braking gently, say 0.3G, over 0.30 seconds (including allowed removal of outlier samples) would put your displacement from the ideal, as determined during the acquisition phase, at 13cm. I know that would be well within the limits of the older LTIs; I’m not sure about the newer versions.

Fingers crossed!


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PostPosted: Mon Sep 10, 2007 15:57 
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smeggy wrote:
...I know that would be well within the limits of the older LTIs; I’m not sure about the newer versions.

But surely they all have to work the same or would require separate type approval?

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PostPosted: Tue Sep 11, 2007 00:17 
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According to Mr Garratt, TeleTraffic, at the Preston Crown Court Appeal of Brian Wiltshire, the LTI 20.20 error trapping
allows a deviation from linearity of +/-20mm. i.e. 40mm overall.

There are about 40 distance readings in 0.3 seconds to get your speed, and it is allowed to drop 5 outliers (exceptional readings).

Considering the situation relative to the middle of the speed measuring interval, +/-0.15 secs,

but allowing 6 outliers in 40, this reduces to +/-0.1275seconds

X = 0.5 A T^2

X is total deviation from linearity (40mm)
A is acceleration
T is time (0.1275s)

hence A = 2 X / T^2 = 2 x 40E-3 / 0.1275^2 = 4.92m/s^2 = 0.5g

but don't believe all the claims made for the tolerances of the error trapping.

I understand modern cars can achieve braking of 0.8g, or more, but fairly heavy braking would be required.

Hence if you keep braking as soon as you see the camera van it should produce an error reading if they zap you.

You can see what amount of braking , A, in metre per second squared, is required for this by measuring your stopping distance, X, in metres, from some known speed, V, in metre per second, then;

A = 0.5 V^2 / X , divide by 9.81 to get the answer in g.

Given a speed of 30mph = 13.4m/s
and stopping distance of 18m
A = 4.99m/s^2 = 0.51g

However they often have the speed cameras about 400m after a left-hand bend in the road, or some obstruction such as a sign mostly on the left, so as to get your speed before you realise they are there, but they can't read your number plate at that distance, so they track you until you come close enough to read your number plate.

If you can, stop before you get within about 200m and then wait to follow a large passing lorry, or if possible turn around and drive away, or turn off, before you get close enough for your number plate to be readable. The video cameras used have fairly limited resolution, even with the powerful telephoto lenses used.

Obviously this doesn't help if you are going away, but you should have seen the camera van by then, but they may be concealed to make then difficult to spot.


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PostPosted: Tue Sep 11, 2007 01:08 
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Thanks all - it wasn't in Cumbria but Scotland (M74) and interestingly, it WAS just after a left hand bend but that worked FOR me I think! I saw the roof of the van from behind some trees a fair way before I had a line of sight to the scamming window. Unfortunately, I didn't brake at 0.5g - or anything LIKE it! (I'd have probably caused an accident and as these things are supposedly there to enhance safety :wink: , I guess that wouldn't have helped matters)!

I'm not desperately worried, my licence is currently clean and I wasn't going THAT fast! There were other cars passing me at the time so he could have had richer pickings in the other lanes.

Oh well, fingers crossed for the next couple of weeks then!


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PostPosted: Tue Sep 11, 2007 09:03 
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Ziltro wrote:
smeggy wrote:
...I know that would be well within the limits of the older LTIs; I’m not sure about the newer versions.

But surely they all have to work the same or would require separate type approval?

Yeah, I should have said the American versions. For some reason I subconsciously associate greater age with those.


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 Post subject: BOMBUS
PostPosted: Sun Sep 16, 2007 12:09 
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Bombus your ealier post has a cartoon type picture of a man engaged in the illegal act of vandalising a GATSO, What kind of example is this??


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 Post subject: Re: BOMBUS
PostPosted: Sun Sep 16, 2007 12:52 
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safetyman wrote:
Bombus your ealier post has a cartoon type picture of a man engaged in the illegal act of vandalising a GATSO, What kind of example is this??

That's a man?

I've seen a great many cartoons broadcast by the BBC which clearly shows animals being physically abused by a woman - what kind of example is this?


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 Post subject: SFATTYMNEANNAM
PostPosted: Sun Sep 16, 2007 19:51 
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safetyman wrote:
Bombus your ealier post has a cartoon type picture of a man engaged in the illegal act of vandalising a GATSO, What kind of example is this??

I'm not answering any questions of yours until you make an effort to answer at least some of ours.

I'm not holding my breath.

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Paul Smith: a legend.

"The freedom provided by the motor vehicle is not universally applauded, however: there are those who resent the loss of state control over individual choice that the car represents. Such people rarely admit their prejudices openly; instead, they make false or exaggerated claims about the adverse effects of road transport in order to justify calls for higher taxation or restrictions on mobility." (Conservative Way Forward: Stop The War Against Drivers)


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