Safe Speed Forums

That's not what you were saying, just a few posts ago is it?



Definitely it can be applied to the car.



And unnecessary. You need the secondary target to act like scotchlite, not the first.



Or hit something at a shallow angle for a specular reflection. Incidentally the beam divergence gives a beam width of about 0.26 metres at 83 metres - which is around 2 feet not 1.



Then you are missing the whole issue with the LTI 20.20. The thing is the chances for ANYTHING to go wrong are very small, but with so many samples being taken, errors are inevitable.

Slip errors are rare. So too are reflection errors. Most errors are caused in the first instance by misalignment. Misalignment also leads to slip and reflection errors being more likely.

The bike test illustrates whan can happen with a misaligned device and as such is the most likely scenario of the three illustrations to occur. A complete contradiction of it being "misleading" and "invalid".

The fact that it looks so outrageous is another irony of the LTI 20.20. Obvious "wrong readings" are easy to pick out. It's the small errors that are more dangerous to justice. For example would an operator detect an error that gave him 36mph instead of 30mph?

Yes and to obtain a reading of 44mph at that distance (63.6m) you also have to physcially MOVE the device - which I see as an external factor influencing the reading. Think about how far the laser beam must move along that angled wall to get a reading of 44mph.

Of course at much great distances this becomes much more of an issue, because jerks and wobbles at the device lead to much greater movements at the target. That's why the device takes many samples, validates them for acceleration and takes an average. That doesn't mean that slip at distance in this way doesn't happen. It just makes it extremely rare.

Yes, we keep on hearing that the chances of this and that are very small but where's your evidence for such a statement. This is especially so as a majority of persons just send off their 60 quid and licenses. The scamera partnerships including the operators clearly don't check for inconsistancies, even when they receive a 03 error.

The more people that challenge this device, the more errors will be found. Regardless of that fact, if this device is being used to prosecute it should be error free and reliable.

What we all appear to be agreed upon, is that there are errors, or at least this device is susceptible to errors or innacurate readings which means that people will continue to dispute this device.

Also, have you taken into consideration parallax misalignment and it's effects on allignment. BD?

I had already seen this on the BBC website. It is clear that Smeggy understands the general principle of the LTI 20.20, but he's not spot on. Here's an example:



In actual fact the intensity information is retained. It is part of the data stored for each sample in a dataset. One of it's key uses is the detection of panning errors. As well as detecting accelerations (by looking for deviation from a constant, predicted speed curve), the LTI 20.20 can detect panning errors by finding changes in returned signal strength. The assumption being that if you pan the device, sooner or later you'd get a marked change in reflectivity.

Of course this method cannot eliminate all panning errors, just some of them, but without it, panning errors would be VERY EASY to produce.

I genuinely can’t believe you are still going.


I gave a reason against, you found another path, I poopooed that too. My underlying point is still correct.


Like I said, this is extremely unlikely given the number of retro reflectors on the bike.


Can you explain that to me please? I thought 3mrad would result with 82.7m/333 = 0.25m


No. For whatever reason, you are missing my whole point. The Mail claimed to have “recorded the bike at 66mph” implying that slip must have occurred, but I am saying is that they could not have been measuring the bike at all (for whatever reason).

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

The returned power level is managed by means of pulse width measurement from the avalanche diode to ensure it’s not under or overdriven. Intensity is roughly inferred, but it’s certainly not measured, (and we agree that it plays no part in the measurement, so the point is still valid?).

Give me your source and I‘ll give you mine.

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That's just splitting hairs though. They attempted a reading from the bike in the same way an operator would, and the speed, which was apparently from the bike, was recorded as 66mph. So in the only sense that is important, the bike was recorded at 66mph.

does this mean that a driver who sees a scamera and jumps hard on the brakes will generate an error on the device?

My poor head!

Right I'm a chartered engineer but in the field of mud and concrete etc so to all intents and purposes a layman when it comes to lasers and the like.

Can I attempt to clarify the issues with the dodgyscope.

Three fundamental issues

1) Slip error, when the gun and sights are aligned correctly but due to movement along the target a 'speed' reading is induced.

2) Laser deflection/scatter (call it what you will) whereby the reflected bean is deflected by a angled surface/reflective surface/roadsign etc the effect of which is an innaccurate reading.

3) Misalignment of the gun and the sight whereby you could be aiming at a vehicle but the laser would actually be targetting something else.

Of course there could be compound errors where 3) is combined with say 1). Say the bike in the Mail article. Aiming at the bike but a misaligned sight causes the laser to point at a passing vehicle. But because the passing vehicle is unsighted there must be a strong possibility of 'slip error'.

How does that sound? A reasonable synopsis?

I think so. Assuming you are decelerating at a feasible rate of 1G, you could have scrubbed off up to 6.6mph during the measurement period, resulting with your position being off by 44cm from where it should be. I can’t believe the software wouldn’t detect that.

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

The implication that "slip must have occurred" is your assumption and nothing more.

As I said in a reply to SafeSpeed, the 22mph reading from the car was not caused by slip either.

The three photographs show three different effects. All three errors are perfectly plausable, valid and not misleading.

And please remember, the article is aimed at lay people, not advanced Engineers.

Exactly right. And in this respect the bicycle is no different from the other two examples.

Thank heavens that somebody else can see what I'm trying to say. I was just climbing into a straight jacket!

Exactly right.

Knowing this, one might argue that the LTI 20.20 encourages dangerous driving.

Very reasonable to me. The only thing I would say is that for your "deflection/scatter" there are really two distinct categories. One is effects caused by reflection of the beam. The other is caused by the pattern of the beam and the fact that the LTI 20.20 always acquires the closest target.

Again compound effects are possible as you rightly say.

Good,

I was trying to bring things to some sort of consensus.

We need to be able to communicate this sort of information to our families/friends collegues etc etc etc and in order for it to sink in we have to keep it simple and succinct.

So we know there are three categories of error that the Lti 20-20 can produce and also that combinations of these errors can occur.

Where do we go from here?

We've got some silver bullets but who is going to fire them?

Return signal power does play a part in the speed measurement process. First of all you need to monitor it and make some decision as to when to "stop the clock". Secondly, as I have already said, the power of each returned signal is stored and the software algorithms operate on this dataset to attempt to detect slip.




Sadly that is not possible - for very good reasons. But I will PM the moderator who can make a general comment on my sources. Hope that is OK.

:rotls



Ernest you nearly caused the coffee to spluttter all over my desk, , screen and keyboard here////

A vertitable tonic after a hard day's work....

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I've had a PM and BD appears to be 'well connected'.

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Paul Smith
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Hi BD.

It has come to light that other forum users don’t feel our debate to be particularly useful, hence I have PMed you my response offline. I assume you wish to conclude this, so I look forward to your (offline) response.

Steve

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

May I suggest a 'technical' forum or area?

we need to have this rigour in our debate and we need to encourage experts. there must be merit in creating an environment where complex technical matters in order to unerpin Safe Speed campaigns and PR.

Other people are more concerned with the machinations of the SCP's, legal aspects etc.

There are subject areas but we seem to be blurring them.

Nothing wrong with airing grievences about SCP's either.

But we need to start focussing our efforts.