speeding - GPS Detector tells me off.

[Funkstar]

i've been trying to find the Department for Transports rules on the accuracy of speedos, but came up blank. I seem to remember they can be up to 11% off up to 70mph and 13% off over that (by off i mean over estimate your speed). under reading of speed is something like 1 or 2%.

You ever noticed those white squares painted in the middle of trunk roads? they are for calibrating police speedos on the fly from what i've been told (by a coppers son).

[Moby-Dick]

Dont forget GPS speed is a horizontal speed and would be most accurate in a straight line on level ground.

[Slamdog]

You ever noticed those white squares painted in the middle of trunk roads? they are for calibrating police speedos on the fly from what i've been told (by a coppers son).

nope, they are measured distances for the vascar type of speed detection equipment.

the distances are not consistent, but they are known measurements. all the copper does is enter the distance then times you between the two points..

full explanation...

http://www.radarfalle.de/technik/ueb...nik/vascar.php

[5lab]

Dont forget GPS speed is a horizontal speed and would be most accurate in a straight line on level ground.

you sure? i thought gps worked on height as well?

[Konan555]

Yes, GPS can determine height. It's down to how they're programmed as to weather they display distance moved in two dimensions or three.

[Moby-Dick]

I'm not convinced it does , but MK is too flat to really test it out !

[mmh]

My dad just bought the road angel 2... hasnt arrived yet, but its supposed to be very good.

[tim_n]

I've heard far too many different comments and opinions on the different speed camera detectors. All I say about my one is that it seems pretty good for the money imho.

[Slamdog]

I'm not convinced it does , but MK is too flat to really test it out !

trust me, it works fine in 2d. although the units i have will give altitude the speed is constant regardless. I have tried all of mine in various lorries and they read the same whether driving in norfolk or if i'm belting up and down the hills on the m62 (the highest motorway in england)

I used the tachograph in the truck to check that the speed hadn't varied because of the altitude.

[Howard]

You needed to be connected to a certain number of satellites (or more) for it to work well in 3D

[5lab]

trust me, it works fine in 2d. although the units i have will give altitude the speed is constant regardless. I have tried all of mine in various lorries and they read the same whether driving in norfolk or if i'm belting up and down the hills on the m62 (the highest motorway in england)

I used the tachograph in the truck to check that the speed hadn't varied because of the altitude.

he's not suggesting that the speed would be inaccurate at a given height, just that it would be inaccurate for measuring a road that travelled uphill, because the road is actually longer than the 2d distance

[divinemadness]

Speedos over read by design to counteract any mishaps that may cause them to under read slightly when they are built.

You'd be mighty annoyed if you thought you were doing bang on 60 and it turns out you were actually doing 70mph through a camera

[Slamdog]

he's not suggesting that the speed would be inaccurate at a given height, just that it would be inaccurate for measuring a road that travelled uphill, because the road is actually longer than the 2d distance

it was just me not explaining properly...

just a thick lorry driver me!

[Rave]

Well....more to the point, if you're done for speeding by a patrol car and you're not 100% sure it's a fair cop (I dunno, say doing 80 in a 70 zone) you should always ask to see the patrol car's speedo calibration certificate. As I understand it, if they ain't got a recent one, you're in the clear*.

*but google the precise legalities first

[TheAnimus]

An engineers perspective:

First off tire tread. How much does tire tread change by? Think how many times that has to rotate once for a 70mph reading? As such that changes the accuracy imediately.

So spedo's over rate, to be on the safe side.

Then GPS, speed isn't velocity as such, remeber that.

Then remeber that GPS will try and mesure it in 3dimensional space and time how long it takes for you to travel alone a single vector in it. However, there are approximation errors, (as the speed is rounded). I really should draw a diagram. But the idea is quite straight forward its a rough approximation, most GPS units that size have about 50cm accuracy, but, they stay quite accurately in-acurate (ie, say it thinks your 30cm to the right, it doesn't matter because your looking at the relative speed). As such the accuracy of the reciver dosen't matter. Its more a case of how many readings its taking a second. As dips in a road will effect it. a nice straight level dual carrageway/motorway is a better test, when there are no things that will effect radio reception. It will be far more acurate than the car spedo.