speedometer inaccuracies

[muddyboots]

As well as the level of tyre wear affecting rolling radius, tyre pressures and vehicle load could presumably make even more of a difference ?

Worn tyre at low pressure carrying a heavy load

vs.

New tyre at high pressure carrying a light load

....could be a good few mm difference in radius.

I checked my Yeti accuracy a while back, it's somewhere between 5-10% out, I just factor that in where necessary.

[BillBham]

Speed limits are exactly that - a maximum speed limit.

There is no minimum so long as you are making due progress.

I keep it simple, I drive at what my speedo says. So in a 50 I do 50, going by the speedo.

I know it's about 3% out but this allows for a margin of error.

As for people behind me who have issues with me doing 48 in a 50, that's their problem! And really, are those extra few seconds you save really worth the stress and all it brings?

[gumdrop]

Why do people drive 5-6mph slower than the advised speed limit,

it is so annoying. OK conditions may require it at times but it seems

to be a national disease at the moment and can cause frustration.

[Urrell]

And they slow down when passing a speed camera even if doing 50 in a 60 limit

[Frenchtone]

^^ try life here in france!

[richardg8jvm]

Muddyjim

the link supplied is over simplified,  and makes no allowance for distance travelled of the target vehicle.

Older X band microwave systems needed a longer measurement interval as the target area needs to be greater than a quarter wave length,aprox 10mm, and the incident angle

needs to be 0 degs. its also subject to Faraday rotation..

The use of infrared lasers with a wavelength of typically 1000 nm requires a very small area where the incident angle is 0 deg , as car bodies a curved there will always be an area where an incident angle of 0 degrees is present;  hence one of the reasons why stealth aircraft use many flat surfaces so as not to provide an incident angle of 0 degs to radar

so the measurement period is very short, and limited by processing speed a few 10's of milliseconds.

With the older X band microwave systems the police were well aware of its short comings, with IR laser based systems, which the manufacturers claim to be far more accurate, they still 

have to be used correctly.

Ryeman,

don't get confused with the Mercator projection of maps, you live on a spherical object, albeit in your case an upside down one

Thanks to those who read the original question and  provided the answers to that question

[Ryeman]

Flying directly towards a DME (distance measuring equipment) in still air, the instrument readout in the cockpit is effectively a radar gun in reverse (nothing to do with map projections) and it will give a rate of closure ie a speed readout as well as a distance-to-run.

As you get very close, the speed declines (if overflying) to zero then increases........it never shows a speed higher than the true rate of closure.

In fact it will never show the true speed unless you are aiming to hit the station.

Are you suggesting that the radar gun is maintaining a running plot and calculating tangential tracks?.

Why have such a complex system to simply calculate a rate of closure?.

[richardg8jvm]

No I'm not

#

but you said

"If I fly to a DME sight via a curve flight path I will get there sooner than a straight one apparently and my GS will be higher also?

Sounds like a way of getting to Mars much quicker. "

 

Just pointing out that what appears to be a straight line on a Mercator projection is not .

The great circle line which will appear as a curve on a mercator projection is true shortest route A to B

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[Ootohere]

Life is so simple.

When you know your speedo reads high, (or low) it is simple maths for people with maths skills / knowledge to figure quickly what the real speed is.

Someone who has done GPS checks knows, as the OP here knows.

 

Stay at the NSL for where you are or lower and then Safety Partnership Camera Vans, Police operators or overhead or roadside fixed cameras accuracy is not of concern.

No NIP's arrive, and if they do then others will be getting them wrongly as well so the inaccuracy will easily be proved by the dozens or hundreds getting ticketed.

 

PS

As to ACPO Guidelines, forget that in Scotland because Police Scotland introduced their scheme on those over the speed limit and stopping 

to educate and formal warnings or prosecution.

Not heard much since the pilot scheme was started and if it was a success and continues.

Average Speed cameras on many Scottish Trunk Roads as well as Safety Partnership Vehicles do really make a diffferance though,

no idea if they are set as ACPO Guidelines.  Having received enough points and fines means i don't try to find out..

Edited 24 November, 2016 by Offski

[muddyjim]

Muddyjim

the link supplied is over simplified,  and makes no allowance for distance travelled of the target vehicle.

Older X band microwave systems needed a longer measurement interval as the target area needs to be greater than a quarter wave length,aprox 10mm, and the incident angle

needs to be 0 degs. its also subject to Faraday rotation..

The use of infrared lasers with a wavelength of typically 1000 nm requires a very small area where the incident angle is 0 deg , as car bodies a curved there will always be an area where an incident angle of 0 degrees is present;  hence one of the reasons why stealth aircraft use many flat surfaces so as not to provide an incident angle of 0 degs to radar

so the measurement period is very short, and limited by processing speed a few 10's of milliseconds.

With the older X band microwave systems the police were well aware of its short comings, with IR laser based systems, which the manufacturers claim to be far more accurate, they still 

have to be used correctly.

 

All this is very interesting reading, but it does not get away from the fact that you will ALWAYS get a lower speed reading than the true vehicles speed...... Unless of course the vehicle is heading straight for the speed reading device - in which case the reading will be accurate...

 

Jim

Edited 24 November, 2016 by muddyjim

[richardg8jvm]

I beg to differ Jim

That site is  over simplified and is two dimensional.

time has not been factored in . Velocity is the relationship between distance travelled and the time taken to do it.

 

If you travel in a direct  line towards the measuring device the recorded velocity is accurate within the tolerance of the measurement device

If there is an offset to the measurement position , you will no longer be on a constant bearing to the measurement position and it will appear you have travel a greater distance.

As the measurement time will be a constant you be recorded as travelling a greater distance in a given period of time.

Therefore, as the recorded distance appears greater and the time is constant, you will have appeared to increase velocity.

[langers2k]

If there is an offset to the measurement position , you will no longer be on a constant bearing to the measurement position and it will appear you have travel a greater distance.

 

That's not correct, you will have travelled a shorter distance towards the measurement position, giving a lower indicated speed than your actual speed.

[KenONeill]

That's not correct, you will have travelled a shorter distance towards the measurement position, giving a lower indicated speed than your actual speed.

I plotted off adjacent and hypotenuse for Pythagorian triples where hypotenuse <100, and ignoring cases where N+1'adjacent < N'adjacent the result is a straight line.

[langers2k]

I plotted off adjacent and hypotenuse for Pythagorian triples where hypotenuse <100, and ignoring cases where N+1'adjacent < N'adjacent the result is a straight line.

 

I'm not sure if you're agreeing with me or not... There isn't any need for a Pythagorean triple or even a right angled triangle

 

Quick sketch to show the effect, assuming the object covers 20 meters in 1 second:

 

To the first observer they will cover 20m, starting at 100m and now being at 80m giving a speed to the observer of 20m/s.

 

To the second observer they will cover 19.518m, starting at 101.98m and now being at 82.462m giving a speed to the observer of 19.518m/s.

 

This effect is amplified as the angle increases and also if a 3rd dimension is brought into play.

[KenONeill]

I'm not sure if you're agreeing with me or not... There isn't any need for a Pythagorean triple or even a right angled triangle

 

Quick sketch to show the effect, assuming the object covers 20 meters in 1 second:

Untitled.png

 

To the first observer they will cover 20m, starting at 100m and now being at 80m giving a speed to the observer of 20m/s.

 

To the second observer they will cover 19.518m, starting at 101.98m and now being at 82.462m giving a speed to the observer of 19.518m/s.

 

This effect is amplified as the angle increases and also if a 3rd dimension is brought into play.

I don't know what you're drawing, but it's totally irrelevant to a radar, which will derive object speed from the time between propagation and return of a pulse, and hence measure the actual range between radar and object regardless of whether the object course is on boresight or not.

 

The Pythagorean triples are a convenient (if limited) illustration of this.

[thingy]

Hat in ring time for me.

My speedo reads (shows) about 10% faster than my TomTom shows.

Where safe, I drive to the speed limit, as shown on the speedo.

Apologies to all those behind me in the M3 road works when I've got cruise control set at 50.

Same thing on those 'Smart' motorways, if the overhead sign shows a speed limit, that's what I aim for.

So, it's my licence. All those that don't like my speed, overtake. It's your licence.

ps. Thanks for the Maths lesson, bit over my head. Safe driving all.

[33q]

Why do people drive 5-6mph slower than the advised speed limit,

it is so annoying. OK conditions may require it at times but it seems

to be a national disease at the moment and can cause frustration.

This national disease....would that be impatience?, feeling their presence on the road is more important than the one in front?, failling to allow sufficient time for a journey?

 

And what is this 'advised speed limit'?. Is that the same as the legal maximimum speed limit?.....the one where it is illegal to exceed?

 

You learn something everday....well I do.

[langers2k]

I don't know what you're drawing, but it's totally irrelevant to a radar

It was an attempt to show the cosine effect which the OP, probably needed a better explanation...

 

which will derive object speed from the time between propagation and return of a pulse

 

Radar can't derive the object speed from the time it takes a single pulse to return, only distance - at least not without multiple receivers.

 

If you also take into account the frequency of the return pulse, then it's possible to get both distance and the radial component of the objects velocity.

 

As it's missing both the axial and tangential components, it will be lower than the actual velocity when not travelling directly towards or away from the radar.

 

If you take multiple measurements then you can get an average speed but it'll only be the actual object velocity is the angles are measured. Without the angles, you will still be missing the axial and tangential components.

 

Back to the OP statement, would you agree it's not possible to get a higher speed measurement than the actual object speed? At least assuming the radar/lidar is correctly aimed at a consistent part of the car.

[Brijo]

....If you are doing an indicated 50 in a 50 limit you will not be breaking the Law....

A slightly different slant on this if I may, Graham?  A driver doing anything short of the posted speed limit-plus-one, indicated or not, is not breaking the law. 

 

I'ts entirely up to the driver which, if any, device to use as a guide, and to gamble on the presence/absence of authorised officials and approved equipment.  Tyre wear, radiation patterns, Pythagoras, cosines, bridges, bushes, blind bends etc don't count.  If you're over the limit, you're over the limit, period.  If you want to challenge any subsequent legal action, that's your prerogative but AFAIK challenges on other than technical grounds (known/disclosed hardware fault, date/time/identity record errors, the qualifications/sanity of the recording official and so on) stand no more than a snowball's chance. 

 

To paraphrase Clint, 'You have to ask yourself - do I feel lucky?  Well, do you?'  :S

[BJ127]

A speed limit is just that a limit it does not mean you have to drive to that speed.

Drive according to traffic conditions is what I was thought at Ayrshire Constabulary School.

Now 70 and no driving offences

[Ryeman]

What Mercator projections (curving toward the pole), have to do with line of sight over a couple of hundred metres, and rate of closure is a complete mystery.

We have a +\- 2kph error factor which is applied to the 'detected speed' which leads to an 'alleged speed'.

Flat map projections simply don't apply in line-of-sight/rate-of-change measurement .

[fordfan]

There seems to be a lot of discussion about the inaccuracy of pointing a speed measuring device at a moving object.  Post #1 makes a reference to GPS as though it's the last word in accuracy, so how exactly does GPS take into account the extra distance driven in the vertical direction when it is based on a two dimensional system?

[Ryeman]

There seems to be a lot of discussion about the inaccuracy of pointing a speed measuring device at a moving object.  Post #1 makes a reference to GPS as though it's the last word in accuracy, so how exactly does GPS take into account the extra distance driven in the vertical direction when it is based on a two dimensional system?

I would have thought that over a line of sight 200 metre distance it was all irrelevant in any case for practical purposes.

Apparently we now live in a theoretical world justifying a sense of injustice.

Surely we can complicate things even more and end up feeling positively righteous.

[Ryeman]

Stage 5 has no steering wheel and illegal to interfere.

[muddyjim]

so how exactly does GPS take into account the extra distance driven in the vertical direction when it is based on a two dimensional system?

Because it IS based on a 3 dimensional system (it knows your altitude too!)