Citroen XM Forum

There seem to be more and more of these average speed cameras springing up on motorways where there are perpetual roadworks being carried out as well as in a lot of city centres. I believe I understand how they work, your average speed is worked out between the cameras even if you were doing 80mph for a certain distance then dropped to 20mph as long as your average speed works out at 50mph or less you're fine. I just set the cruise control on my XM and relax til I see the national speed limit sign and resume to my usual speed.

So why is it that half the cars on the road are passing me at 60mph or more. I actually set the cruise at about 53/54mph figuring the cameras allow a margin of error and I've been fine. But other vehicles including lorries, some belonging to well known companies go past me at what must be at least 60mph. Do they know something I don't, is there a bit of leeway on what speed you can be doing before they send a ticket?

And then there's the doughnuts who completely miss the point of the average speed cameras....bombing along at 90+ then braking hard before the camera before speeding off again to the next one!

I would say that they either know something that you don't - perhaps that they are not working or they are just stupid. I wouldn't take the chance. In any case 50 mph limits work wonders for your fuel consumption

Peter

Indeed! Around 65mpg according to trip computer when on the level.

yes alot them don't work i do 55.9mph through the ones on the m5 and m6 and nothing through the post also the new managed motorway section m6 j8-j10a - j10a-j8 you can also do over 50mph through the 40 speed restriction without any worries i'm in the 'world's leading records management' lorry

why they don't work i don't know they just don't

Most cameras are set to trip at limit + 10% and maybe 2mph as well, so a 50mph limit can be 57mph before the trigger point.

Don't rely on that as gospel though, it can be as low as speed + 2mph.

As far as i know cameras or any speed measuring device has to be set at speed plus 10%. Vehicle construction and use law states that any speedo in any vehicle is allowed a 10% margin of error unless its tachographed in which case 6%.

I believe these are the manufacturing instructions all vehicle builders adhere to, thus if your caught doing 32 in a 30 and they ticket you argue it quoting construction and use law available on the vosa website .

Note: thats +/- 10% and +/- 6%

nightorb wrote: Note: thats +/- 10% and +/- 6%

No it's not. No speedo is allowed to under-read, but can over-read by a maximum of 10%.

my bad just re-read site and you are right just overread however i think i may have some paperwork that may interest you ill dig it out this weekend and send a pm when i have it. Its from a transport mangers exam theory folder need to check i still have it first.

There is no margin of error for prosecuting a speeding case.
Any excess over the particular limit however small is sufficient to be found guilty.
The speedo reading in your car is no defence and difficult to prove anyway.
The construction and use rules set the speedo to read just a little higher than actual on new tyres.
Worn ones make the speedo read even higher.
It used to be custom and practice not to prosecute for less than 2mph plus 10% over but ACPO rewrote and tightened those rules a few years ago.
That makes relying on using the margin now a bit of a lottery.
One of the major motorway worksite installations was reported (in NCE I think) as generating over 10,000 tickets a day while it was switched on.

John

Hi all, EU law June 1976 decrees that displayed speed must always be greater than actual (true) speed but by no more than 10% + 2mph. Therefore as an example : at 60mph the speedometer could show anything from 60 to 68. Speed cameras are calibrated to true speed, so stay within the 10% + 2mph and all should be well.Bear in mind that any change to manufcturers specified wheel/tyre sizes, gear ratios, speedo gearing, speedo or even tyre wear could adversely affect the displayed speed.

robertm