Puff The Magic Wagon!

iTrader: (2)

Theres some roadworks on the A14 by Cambridge - 40mph limit.

07:00 this morning crusing through. Went through the first at an indiated 45 & the second at 45 too except this time it flashed!!

Anyone able to "advise through experience" how Cambs apply the law?

APCO is 10%+2 so 46 but I doubt my speedo was under-reading. Does Cambs stick to APCO or does it do Limit + 1?

Another 2 week wait...

mart360

ermmmm

limit posted at 40 so i go through at 45

why??? are you exempt, do you have NL status??

if you get a ticket its your own fault here ..


idont agree with cameras, but if you want to push the envelope then its your own fault when you get tagged


M

GC8

James' many scamera partnerships dont observe the ACPO guidelines, Ive heard of people prosecuted for 32 mph!


Simon

Puff The Magic Wagon!

iTrader: (2)

Mart

Fine, I haven't got a problem with it if I get a fp etc. Got a clean license & very low insurance, so its no big deal. Don't need a lecture thanks!

Car speedos are not necessarily accurate which is why APCO guidelines are in place & I would be very surprised if mine was 100% accurate. My tyres are coming up to needing replacement, so are likely to mean that my speedo reads higher than my actual speed etc. I'm just interested in experience of Cambs Camera Partnership at what speed over they prosecute.

New_scooby_04

iTrader: (4)

I'd beg/borrow/buy a Road Angel or other satelite device featuring a speed display on the car and travel at an indicated 45 on the spedo and see what your true speed is. This will indicate if -and by how much- your speedo is out. If it's typicall over-reading you'll have actually been actually doing about 41, which is a bit harsh to get booked for. In which case I'd want them to confirm the camera was calibrated correctly. That said the ACPO recommendations are exactly that, recommendations, and they may want to -and are legally allowed- to prosecute for even 1mph over.

Best of luck mate!

GC8

That isnt nearly as accurate as people think, even since selective availability was turned off. As Im sat here now my PDA GPS (with an antenna outside) cant decide whether Im 500 or 800 feet above sea level and it thinks that the building is moving at between 0 and 3 MPH (and thats with 7 good satellite locks...)!


Simon

OllyK

They are more accurate than a speedo (less so the faster you travel). Never had the moving when not issue you mention with RA or PDA. Your antenna / software may be suspect.

King RA

my PDA and RA used to have differing opinions on how fast I was going so they're not as accurate as people think.

mart360

it wasnt a lecture, it was just asking the question... if the limit is a posted 40 why were you doing an indicated 45??

now if you had a calibrated speedo, and were going to mount a case to set precidence then all well and good, and good on yer etc

Mart

Trucker Ted

Checkout www.pepipoo.com for some good advice!

Puff The Magic Wagon!

iTrader: (2)

The road was pretty empty & I thought that 45 would be OK - more fool me






PLUS 'cos I was chatting to the pretty 20 yr old in the seat next to me

mart360

fairy snuff carnt argue with that then.......



pinkys crossed then!!

M

dsmith

Because so many roadworks are now 40 and not 50 and On a fine sunny morning with light traffic 40 is just ridiculous. I dont know when "someone" decided to drop the 50 to 40 ?

So many roadworks now last 10+ months if you commute though @ non-rush hour times it can be so easy to have mind in neutral and creep a few miles over. as always - speed cameras dont care if people are working, if its foggy, if its raining or if its a bright clear morning with perfect vis.

Puff - recently went to italy and back with a GPS - my WRX reads 10% up on what it shoud be through the entire range. 45 indicated would be 41 tops. Quite startling to see only lows 60s when holding the limit at "70".

Leslie

They decided to drop the speed limits to 40 to ensure the safety of people working on the roads, they say.

PTMW, I should swear blind that you consider you were not speeding and question the accuracy of their speedcam if you get a NIP.

Les

StickyMicky

norty!!

GC8

Actually Olly, this isnt necessarily the case. Since Selective Availablilty was turned off in 2000, 'Civilian' GPS has achieved location accuracy of <25 metres (error) and velocity accuracy of <5 metres per second (error). Five metres per second equates to 18 kilometres per hour, or just over 11 miles per hour. From this Im sure that you can see, that a correctly functioning speedometer may provide a more accurate speed indication at common speeds! (Im not arguing that this is always the case in practice though, bear in mind also that these are the extremes of error, rather than the norm)

Speedometers may not under-read under any circumstances and may over-read by a maximum of 10%. About 3% of the rolling radius of the wheel/tyre will slowly disappear as the tyre wears, which has to be allowed for, as does the deformation of the tyre due to centrifugal force. The speedometer has to be absorb these variables whilst still maintaining -0%/+10% accuracy!


Simon

StickyMicky

thanks, i can use that gps info to bash a few people on a minimoto message board who think there chinese air bikes hit 45mph LOL

back to puffs problems

OllyK

I'm not sure of your sources as you don't quote them however:

Accuracy of 0.5mph - http://gpsinformation.net/main/gpsspeed.htm

Speed Accuracy 0.1 Km/h with signal level higher than -130dBm; speed faster than 40km/h - http://www.pocketgpsworld.com/sanavbt48.php

Dig around in the specs for GPS devices (as above) and accuracy figures of less than 1mph are not uncommon.

Now if you can show me that all these companies have it wrong, I'll be happy to put my hands up and say I'm wrong, but for now I assuming they are lying about the specification.

GC8

This: ".....to reflect the accuracy obtained following the deactivation of Selective Availability. Without SA, SPS accuracy would be of the order of 25m (2 drms, 95%) in the horizontal plane and 43m (95%) in the vertical plane." is from the American government's FAA site here.

There is a direct corelation between the accuracy of positional and velocity figures, to the best of my knowledge.


Simon

hedgehog

I think we need to try and find out what is going on here, I honestly don't know and so am going to tell it as I see it and see if anyone has a better explaination.

My handheld GPS, and any that I have ever encountered, uses doppler shift to determine velocity. This is exactly the reason why it worked when SA was on, otherwise the velocity would have to be calculated based upon distance/time. Now if you use the distance/time calculation today you will still get a significant error just because of the positional error but nothing like it would have been with SA on. However I have never seen a velocity error. How can I make such a statement? Well I am assuming that the error is equally likely to apply at all speeds and I have never seen my GPS register a velocity when it was sitting still. At zero mph the error is an observed zero mph. When walking I have also never seen it give my velocity as, say, 15mph or 0.5mph instead it always shows what I would expect to be a correct and low velocity. So, there is no error at 0mph and if there is an error at 4mph then it is a very tiny error indeed.

From this I extrapolate and conclude that my GPS is extremely accurate when it comes to measuring velocity.

I have also seen maths treatments of the subject which calculate the maximum velocity error with SA off to be in the region of 0.07mph (about 1mph with it on and giving the maximum frequency error of about 1 Hz) which supports my observed findings and, I believe, supports my extrapolation from the 0mph situation.

Of course now i'm up the creek without a paddle it is going to be a tad tricky to explain why two units give a different velocity reading, or why one will give a velocity when stationary. Well, I am tempted to suspect that somewhere in that loop the device has reverted to using the distance/time calculation and so is displaying the velocity of its errors, in effect. I have never seen this with a dedicated handheld GPS device which has DSP dedicated to the calculation of the velocity using the doppler shift and so that would seem to support my conclusion that in embedded applications a software based solution may be applied which may not be using doppler for the calcs.

So that is my take on it but i've no idea if it is correct but if anyone can present the correct answer then it will be most interesting, in the mean time i stick with doppler and maintain that my handheld unit is accurate as makes no difference when it comes to velocity.

GC8

hedgehog

Oh, I had to go out in the middle of writing my last reply and so didn't see this.

I think you are sort of right in what you say but wrong in how you might think it impacts upon velocity.

With SA on the (in)accuracy of the position information bodge was done by some sort of dither error in the clock in the satellite. This had a knock on impact upon the frequency of the signal of up to about 1Hz, as I understand. In turn, because the doppler measurement used to determine velocity is done to an accuracy of about 0.05Hz this could have an impact upon the velocity of about 1mph. So, in this case it is true to say that the factor which makes your position wrong also makes your velocity wrong. However, because the velocity is calculated using doppler the magnitude of the error is not necessarily related to the positional error and it will have a max error of about 1mph. If it had been calculated using the distance/time thing then there would be a direct correlation and the error could be much greater than 1mph.

With SA off the positional accuracy is still not absolute but the measurement of doppler shift should be as accurate as the system will allow which is, i believe, to about 0.05Hz. This will be related to the dilution of precision thing but even so is going to result in a very small error in velocity even if your actual position can bounce about by, say, 25m or so. So, in this case it doesn't matter if your position is accurate to 0.05m or 25m your velocity should be pretty much spot on.

Again all this is based upon how I understand handheld dedicated GPS units to work, embedded units might even do the distance/time calcs (to save on DPS etc.) and so may indeed give inaccurate velocitys. I am keen if anyone has a better explaination as i have no idea if mine is correct.

dinothedude

Hedgehog, you've got to be an Electrical Engineer of some sort.

King RA

do you spend all your evenings searching for dead threads?

speedking

<observation>

Although your speed on the ground might be 0mph, because of the rotation of the earth you are travelling at about 600mph through space. The satellites are travelling at 15-20,000 mph. The GPS has to calculate the difference between the relative positions of you and 4 or 5 satellites. Whether you are travelling at 0, 4, 70, or 150mph would make very little difference to the calculation which result is a small difference between very large numbers, therefore it is not surprising that if the reading is bang on at '0mph' then it will be equally accurate throughout the likely speed range.

Milamber

iTrader: (2)

So Puff, did you get a NIP or not?

Puff The Magic Wagon!

iTrader: (2)

Nope

Leslie

With the inaccuracies of your speedo with worn tyres, and the inaccuracies of radar speed measuring equipment which mean it should never be used as evidence in court anyway, who knows what speed you were actually doing?

Les

gazza-uk

most speedometers are 3mph out, so if your clock was showing 45, you will have been doing 42....

Are you sure it dident flash someone else ?

andy97

iTrader: (1)

might of flashed once indicating that its out of film, but still makes people wary if they flash