Safe Speed Forums

Ernest,

may I point you to the direction of this thread:

http://www.safespeed.org.uk/forum/viewt ... 8&start=40

See my lyrics and all will become clear

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

You DID Black Douglas, but in each one, you yourself explained that it was NOT the bicycle which was being measured, but something else targeted in error.

As I read it, Smeggy was pointing out that although the Mail said the bicycle was doing 66mph, it was invalid - ie the Mail had allowed a reading OTHER than the bicycle to influence the reading - which you illustrated.
It's just a perception of how you should word the desciption of the Mail's process - flawed/invalid/false/wrong/incorrect - any of these would be correct!
Which is why Paul wished us all to move on, rather than argue over symantecs!

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Time to take responsibility for our actions.. and don't be afraid of speaking out!

Maybe. We shall see.



A very possible scenario. Which is why I don't understand why you think the 66mph bicycle is "misleading".



66mph does not equal stationary. What you said in your original post was that you thought the beam had missed the bicycle and slipped over a stationary object instead. This is indeed possible, however if an object moving at 66mph intercepted the beam at the time the reading was taken and was closer to the device than the bicycle then the LTI 20.20 would have acquired the closest target and displayed the speed of the CLOSEST target. This is more likely to happen at greater distances where the spread of the beam becomes important, and therefore perhaps not that likely in this case.



This statement is wrong. You do not fully understand the workings of the LTI 20.20.

First of all you have made the assumption that there will be a direct reflection from the bike. There may not be. Depending on the angle that the laser strikes the bike and what it strikes. It might be the case that the laser strikes the bike so that the surface acts as a specular reflector. In this case very little light will be reflected back directly.

Of course small amounts of reflected light will be reflected back, but the LTI 20.20 won't accept returns that are too weak - otherwise it would never work. The air is full of particles that reflect soime light back to the device. You therefore need a significant return before the device can acquire a target.

Therefore the "earlier" bit can be FALSE if not enough light is reflected back from the first reflection. The "stronger" bit is irrelevant (because the LTI 20.20 always acquires the closest target), and the "correct direction" bit is also a red-herring because Scotchlite (tm) like surfaces will reflect the light back exactly frmo when it came causing it to arrive from exactly the same direction as the primary reflection.

In other words if the first reflection was primarily a specular reflection and the secondary reflection was from Scotchlite, then this scenario is VERY VERY possible.

In fact it is EXACTLY how the reading of the car doing 22mph was obtained.





Could you point me to the bit in the article where that claimed to have got a sweep result from the bike please?

But that's the case in the 22mph reading for the car too!

In fact in all cases something other than the intended target had influenced the reading. Otherwise you'd get the correct speed every time.

That's the WHOLE POINT.



The LTI 20.20 can give the wrong readings because "other things" can influence it's results.

Two more cases:

The Daily Mail also said they measured the speed of a stationary car as 22mph. They didn't - probably they thought they did. Nobody has said that is misleading? Why the difference?

The Daily Mail also said they measure the speed of a brick wall as 44mph. They didn't - probably they thought they did. Nobody has said that is misleading? Why the difference?

Ahh, but they did - gun, wall, point, measure = 44mph. Nothing 'external' involved. That seems to be the point you're missing.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Hi again BD.

I can’t believe you are still questioning my post.
I think Paul would prefer to keep this offline considering the length of your posts.


I didn’t see anything closer than the bike in the picture (line of sight of the gun). Also, the reference to a ‘quiet industrial estate’ would strongly suggest there are no vehicles in the vicinity travelling at 66mph (there certainly isn’t anything else in that picture).


You might just be surprised.
I agree that the bike could reflect a relatively great amount to a secondary surface (very unlikely given the number of retro reflectors required on bikes) but, and most importantly (my point), the receiver diode will be similarly collimated as the transmitter (it’s view will be wider, but by an insignificant amount), hence the receiver will be focussed only on the primary target, hence secondary reflections from other sources away from the point of incidence won’t fall onto the diode.

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

Yes there is - the MOVEMENT of the device. If there was nothing external to influence the result, you would have got much nearer to 0mph.

Don't get me wrong, I know what you're saying here. It's just that I believe you aren't being consistent. Just stick with it for a bit I'm pretty sure we are getting there now.

So what you are saying is that the 44mph wall test is "valid" and "not misleading" because the laser was pointed at the wall and actually hit the wall right?

Now what about the 22mph car? Give me your argument for this test please.

Movement of the device isn't 'external' - it's an integral part of the measurement process.

The car is the same as the wall.

The bike is different because PHYSICAL external object(s) must have been involved. hence misleading.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Here's my take

I have gleaned from discussions that. to get a non-"error-flagged" reading from the dodgyscope, one needs to get a linearly decreasing (or increasing if vehicle is receding) time between the exit of the incident beam and the earliest reflected beam of substance coming back, with a number of readings (at least three) over 0.3 secs.

To get a speed reading of 22mph, in 0.3 seconds, the hit object has to advance/recede/slip by

22mph X 1760 y/m X 3 f/y X 0.3s = 32 ft
60min/h X 60 s/min

If this was all got from the car, and it was not moving, and the angle of reflection was not totally grazing (ie there was also a cosine effect, meaning the car had to be even longer...) Not too many cars over 25 ft, let alone 35 ft.

EITHER - this also is an "invalid" reading (same definition as bike reading being invalid, whatever that is) OR the statement earlier of 0.3 sec inclusive is excessive.

Can anyone verify the 0.3s?

Very interesting. but I think you got the math wrong...

It's 22mph*1.46667 = 32ft per second

32 * 1/3rd second = 10.7 feet.

The Lti2020 takes 43 readings in 1/3rd seconds.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Ooops forgot to multiply by the 0.3 didn't I? Just as well I showed my workings Right - I retract "invalid" from the statement above. Apologies to all for the interjection!

So, forgetting for a minute the cosine issue, a good rule of thumb for the speed error for slip is, in round figures, +/- 2mph per foot of slip. Right?

Following from the above, at a distance of say 500 yds, and a sloping car with a long bonnet front with an angle of, say, 30° downward to keep the maths easy, to get a full slip of 10' horizontally, the vertical slip at the car needs to be 5' vertically, with a slip angle at the dodgyscope of under 0.2°.

I can't even hold a rifle that still when prone and using a sand bag. What hope does a scameraman have with one of these handhelds? There is something fundamentally wrong with allowing these things to be used over such great distances.

Bingo.

WRONG.

NO the car is NOT the same as the wall. A PHYSICAL external object must have been involved there too. You see, the 22mph reading for the car was NOT obtained via slip and therefore it must have involved an external object. You can tell that by looking at the picture.

See the distance on that reading - 31.2m. That car is clearly closer to the device than that. So it is clear that the distance reading (and also the speed reading) were not off that car, slip or otherwise. The 22mph reading from the stationary car is an example of a REFLECTION from a secondary (and very external) target.

Are you starting to see it now?

The mistake you and Smeggy made was that you assumed that the Daily Mail was claiming all three photographs were produced via slip. The Daily Mail article makes no such claim. In fact the three photographs illustrate three different effects.

The 22mph car demonstrates a reflection effect.
The 44mph wall demonstrates a direct slip effect.
The 66mph bicycle demonstrates a misalignment effect (leading to unintended slip).

I submit they are all perfectly valid and none is misleading.

Are you surprised by that? Surely anything moving at 66mph would have moved off picture by the time the operator moved out of the way of the device so a photograph could be taken.

(I do incidentally agree that in this case beam interception was extremely unlikely).



You might be surprised.

Humour me a little. Why don't you draw this scenario and have a think about it.

Laser beam leaves device and strikes a surface A such that surface A acts as a specular reflector. The laser beam is therefore reflected to a second surface B. Consider now that B is a Scotchlite like surface such as a registration plate of a car. Scotchlite is made up of tiny beams and so light striking it is reflected back exactly the same way from whence it came. So our laser beam is reflected from B back to A. It will strike A on the way back at exactly the same point and with the same angles as it struck A on the way out. Therefore another specular reflection takes place and the laser beam is reflected onwards from A back to the laser device - at exactly the right angle it to fall onto the diode.

I'm sure if you draw this out you'll get it. I have produced exactly this effect with various real LTI 20.20s and I'd be prepared to bet you TEN MILLION pounds that the 22mph reading illustrated in the Daily Mail article was obtained in EXACTLY this way. Slip was nothing to do with it.

Why would you try to move the device to measure the speed of a wall?

You have to point it at the wall.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Your path theory is of course correct, but you made some far assumptions (my reference remains with the bike test, but perhaps this could be applied to the car):

1) The beam did not strike a retro/Scotchlite like surface on the bike. This is hugely unlikely. The beam divergence at 83 meters = 1 foot, so will likely strike the pedal reflectors or the main front reflector (or perhaps even the wheel reflectors), otherwise the beam must completely miss the bike.

2) The secondary surface is retro/Scotchlite, given that the beam did not strike such a surface on the bike.

I don’t like the odds…..

_________________
Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

On reflection ( ) I don't see how the bike could have played any part at all in the 66mph reading. Bikes are far too wobbly to set up a reflection, and far too knobbly to set up a slip.Bikes are also fairly see-through and the most likely basis for the reading would be slip effect on a surface beyond the bike.

I'm unconvinced that the 22mph car was anything other than slip effect. I don't think you can make a worthwhile distance judgement from the photo without knowing the approximate specification of the lens in use.

I'm also bored with this - I can't find any substantial point of disagreement.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Sorry, had to post this, this is Smeggy's reply to SC regarding some technicalities. Doesn't mean a great deal to me but is nevertheless interesting. It appears to have stopped all other posts on the BBC inside out forum,,,,well apart from Earnest of course

"Smeggy
Dear Steve C. Why wouldn’t you concede that my technical argument couldn’t be faulted? You must have a good technical reason if you are to justify that comment. My answer may have been simple (it was for everyone to understand), but it was not an interpretation. The Lidar gun isn’t compex (at least for me), but in the interest of completeness: It sends out variable power laser pulses (ensuring the receiver isn’t saturated or under-driven, by means of pulse width measurement). The reflections are received by the avalanche diode (adjustable bias) which can only measure intensity with time, not Doppler shift or any other effect; hence it cannot distinguish reflections from static or swept targets. This signal is sent to a comparator (reference level adjustable for background levels and countermeasure noise), which removes the intensity information to give ‘clean’ digital pulses, which are in turn is sent to the gating (for predictive ‘lock on’), interpolation and timing circuits. The rising edge delay for each returned pulse is measured against an internal reference pulse (processed in much the same way). Therefore, measurement of a moving target and sweeping a stationary continuous surface (not necessarily flat, depending on hand movement) will result with the same digital pulse train from the comparator, each consecutive pulse having a uniform rate of change of delay with time. So the question remains: how can the rest of the gun differentiate between the results of these two measurements? It can’t, the hardware does not support panning detection, instead some software algorithms have been cobbled together in the hope that it will cure the problem. Let’s forgo the bizarre claim that LTI would supply the American Police with a version of the gun which is much more prone to this ‘needless’ error. The acceptance windows may be tighter for the UK models, but this will only mean that the window of opportunity for a false reading is reduced by the same ratio, but it won’t be eliminated. If it’s possible to get a speed reading, it’s also possible to get a sweep reading. Not so ‘poorly conceived’ now huh? It's not just plausible - it’s inherently possible. I will agree with one of your points, perhaps your deliberate attempts of sweeping were not done correctly. What about vehicle mounted units? Do you lock out the suspension when carrying out speed enforcement? The mass and the dampening components of the van will ensure a smooth motion is applied. Interestingly, I emailed Frank Garratt almost two weeks ago asking much the same thing. He has put himself in a tough position by admitting that false speeds are produced from stationary targets, even from UK models; I’ve still had no reply. Cheers"

PS, hope you don't mind me posting this Smeggers!!