Safe Speed Forums

Hi there,

I regulary travel at 80mph on the motorway whn it is safe to do so. I do this with the knowledge that I wouldnt be prosecuted by a mobile/speed camera as my TRUE speed was nearer to 75/76mph. i,e beloew the prosecution limit of 79mph.

However, I have recently read a reply from forum member 'InGear' and this concerns me a bit...

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"Post by In Gear on Sat Jan 05, 2008 00:37
Now to the OP.... now I cannot guarantee anything but as a general rule of thumb ... professional judgement. discretion and common sense prevail in the human being (Mr Common Sense ain't quite pushing up daisies yet ... though his step-sons per the Mad Doc's post somewhere might be drivin' Miss Daisy around the bend )


If you were registering around 75 mph on speedo.. in a modern car (and the riff raff have played around on track with various gadgets they bought and begged me to test with them as well ) .. but we found cars built 1997 onwards seemed to indicate a 3 - 5 mph average speed above the speed per GPS thing and my various hairdriers .. used correctly and calbrated to a turn.

You would thus not cause them any concerns and they'd just apply normal logic.


If they were ANPR.. then there would be some back up further on as the whole purpose is to swoop down and remove obviously uninsured and illegal vehicles off the road immediately.


Relax and don't worry. Real officers are after morons and not the smidgeon fluctuates within reason.""


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Does this mean that my sat nav isn't truely accurate whilst travelling at 80mph on the speedo, and that I may well be traveling over he prosecution threshold?

Replies ggreatly appreciated.

Karl.

We have this again and again.

If speed cameras were so accurate as to guarantee that 78mph would not set them off but 79mph would, do you think the threshold would be so high, or do you think it would be set to something like 73mph-75mph?

You may of course mention secondary checks. Of course how much error is involved in assessing how far the car has travelled when relying on a picture and wide lines on the road. And how good is the paint job anyway? What if you can only claim to magistrates the minimum speed you were going was 75mph?

CAMS -

The readout of any GPS driven device (SatNav, RoadAngel™) will be spot on, provided -

• You're receiving a good signal from the GPS satellites, and
• Your speed has been steady for at least 1 sec to allow for the time lag your unit may have in processing the data it's receiving from the satellites.

In my current car (Audi) the speedo reads 72-73 at a TRUE 70. It will become more inaccurate as the tyre treads wear down and the circumference of the road wheels decreases.

In light of IG's quote, I can only hope that there are plenty of "real officers" where I drive. Indications are that this is the case.

As said, at a constant speed (I'd give them 2 or 3 seconds to catch up personally, some lag more than others) then they're accurate. I also find that some are only accurate if you're on a hill. My guess is they don't take changes in elevation into account when calculating the speed.

What I'd do is decide what speed you think you can get away with (eg. bang on 70 in a 70mph limit) and on a suitably flat straight empty road, work out what value that translates to on the speedo. Dont forget to recheck every couple of weeks (depending on mileage) to take into account tyre wear. I tend to just check it every time I happen across a suitable road.

And yeah, just because you're accurate don't expect the cameras to be, if you're going to do a constant 78, expect to be done for 79 one day.

Thanks Everyone.

I understand that the fully calibrated cameras have to be accrurate by 2mph, which is why I try and stay below a TRUE 77mph. - So if I get caught doing a TRUE 76, It could be recorded that I was doing 78mph.

I was just a bit unsure about the realibility of the sat-nav. I understand what you mean by having to wait a second or two for the correct speed to register.

Thanks again for your replies.

I run a pocket-pc running tomtom navigator 6 software.
The reading updates every few hundred milliseconds.
It all depends on how may sats are being detected and read.
When the system is running on 6 sats it is faultless and fast.
The speed displayed is accurate to a few hundredths of a mile/hour.

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The world runs on oil, period. No other substance can compete when it comes to energy density, flexibility, ease of handling, ease of transportation. If oil didn’t exist we would have to invent it.”

56 years after it was decided it was needed, the Bedford Bypass is nearing completion. The last single carriageway length of it.We have the most photogenic mayor though, always being photographed doing nothing

Trouble is, if for whatever reason the camera device, or operator, decided that you're speedin' then you will be able to say "No, I wasn't doing 81mph, I was doing 75mph" and they'll still be able to steal your money and points.

That's why I always slow down a lot past cameras, in case the thing goes off accidentally and I have to try and prove my innocence.

For mobile camera vans this usually involves dropping speed from 70-80mph to 40-50mph in a very short period of time.

GPS speed reading made at a constant speed on a straight level road with a good view of the sky and plenty of satellite locks is probably about as accurate as you can get without using a "measured mile".

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Andrew.

The reading on my pda with TomTom only updates every couple of seconds or so and in addition does not display fractions of mph so is only accurate to the nearest 1mph.

If it updated every few hundred milliseconds and displayed to a few hundredths of a mph then it would be a constant blur of numbers and be unreadable.

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Nick

That's utterly ridiculous behaviour IMO.

I agree. Got to protect my license though.

I tend to go into shock whenever I see a mobile camera/van which probably doesn't help.

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Andrew.

But I thought you wanted drivers to slow down weepej.

He'd like them to be doing 40-50 all the time.

My bike is only doing 120mph at an indicated 130

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The views expressed in this post are personal opinions and do not necessarily represent the views of Safe Speed.
You will be branded a threat to society by going over a speed limit where it is safe to do so, and suffer the consequences of your actions in a way criminals do not, more so than someone who is a real threat to our society.