Safe Speed Forums

Hi guys, this is my 1st post. Ive been trying to find the answer to this one for a while, this seams to be the best forum ive found so ill give it a go.

its normal for cameras to be set at <Speed Limit> + 10% + 2mph.
So, for 70, the trigger would be 79mph. Although some forces just set it to 80 as a nice round number.

As set by various BS & EN standards, Car speedometer are not permitted to underread at all, by can over read by no more than 10%.
The speed readings on a standard SatNav will be within 1MPH accurate if travelling at steady speed. (could be 0.1 if it desplays a decimal fraction)

The 70mph speed limit says "you can travel no more than 70 miles in 1 hour" it doesnt say "your needle should never go above the number 70"

This means i am not breaking the law (unless there are dangerous driving conidtions) sitting at 76MPH by my speedo, as according to my SatNav i am actually travelling 70 miles every hour. NOT 76 miles every hour.

Now, i could start living on the edge by driving 10% faster, meaning im actaully moving a a rate of 77mph, where my speedo would read 84MPH.

This means, i should be able to drive through a speed camera with 84 on the speedo and 77 on the satnav/actual physical speed, and it not trigger, as i will be 2mph below <speed limit>+10%+2mph

I origionally thought that this was where the Police Patrol Car trigger of 85mph came from. As at 85 on the needle, you will definatly be doing more than <speedo indicated speed whilst physically moving at the speed limit>+10%+2mph

However, ive seen many times on these "police camera" programs that they use the speedo in their patrol car as the initial assessment of speed if they are driving. And ive seen them stop somebody for doing "80mph".

But, is this 80mph by the common everyday speedo, or is it 80 by the police calibrated speedo meaning the person could have been doing around 87 on their speedo?

I guess the culmination of this lot is, if you add up the various allowances and margins of error, then it should be "safe" to go though a speed camera or past a police car with 84 on your needle if your car speedo is over reading by the maximum of 10%.

Working that backwards for clarity:
84 on the needle = 77 actual speed you are moving though space/allong the road
77 is 10% more than the speed limit
this is below the trigger threshold for the speed camera of <speed limit>+10%+2mph

Obviously, in some cars, to actaully move at 77, you may be doing less than 84 if your speedometer is not as much as 10% out. Ive have an 08 Seat Leon and im probably about 7.5% over going by the half mile markers and also satnav readings.

Whats your thoughts on my thoughts? Am i right with my thinking that the set points must be based on physical speed not indicated speedometer speed.

THanks,
Stuart

Interesting interpretation, would love to see somebody try to use that defence in court!

Err - no. If that was true you couldn't be prosecuted for speeding if your journey lasted less than 60 minutes

_________________
When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

or 102.666 feet per second.



Your speedo or satnav are not relevant. If you cover more than 102.666 feet in 1 second, you've broken the law. Few instruments are absolutely correct.

Hi and

You missed out the accuracy of the camera/police speedo, they would only have to overread by 2mph to put you in trouble even if your true speed was 77mph. The 10% + 2mph is also a guideline so if you ended up in court and it turned out you were only doing 74mph you are still technically speeding. I am not sure if you would get a guilty on that or not, there is a magistrate who posts on here and he might be able to answer that one.

The other and much bigger issue is that you will have to spend quite a bit of time staring at your speedo if you want to cruise at 77mph, remember you get pulled at 79mph so giving yourself a 1mph leeway is cutting things a bit fine. Even cruise control could put you over on a down hill section.

Tends to lead panic braking if a cam/car/van is spotted late. Lots of info on these sorts of issues on the main site, but please feel free to keep asking questions, just be aware the answers are opinions of the replier not necessarily fact.

Weepej - 70 miles in one hour, that will be SPECS then

_________________
Driving fast is for a particular time and place, I can do it I just only do it occasionally because I am a gentleman.
- James May

or approx 31.3 m/sec for the metrically minded

_________________
Driving fast is for a particular time and place, I can do it I just only do it occasionally because I am a gentleman.
- James May

Yup, you got me there.

or exactly 188.16 furlongs per millfortnight
0r aproximatley 0.18388 smoots per jiffy

_________________
When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

AIUI

Say the speed limit is 70 and the speedo is reading within the allowed tolerances. At a true 70 the speedo will almost always read between 70 and 77 mph, indicating a potentially legal speed but most likely an illegal one (a false positive). At true speeds in excess of the 70 limit i.e. a genuinely illegal speed: the speedo will indicate an illegal speed regardless of its error. Unless the speedo is within calibration, or the error factors are characterised and documented, both done by approved means, one cannot factor in potential errors because those errors cannot be proven beyond doubt.
In short, one cannot assume a speedo is overreading by any amount, let alone the legislated amount.

Building on what Toltec said, if it can be proven by two independent pieces of evidence that a driver was any amount over the limit, the driver can be prosecuted.

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

Thought I recognised the avatar.

Thanks for confirming the point on prosecution, I thought that was the case.

_________________
Driving fast is for a particular time and place, I can do it I just only do it occasionally because I am a gentleman.
- James May

The backup evidence for Gatsos and Truvelos is photos giving average speed over the road markings, i.e. like specs but shorter.

Any idea what the second is on a SPECS system?

_________________
Driving fast is for a particular time and place, I can do it I just only do it occasionally because I am a gentleman.
- James May

The distance/time data for the primary and secondary measurement appears to be generated from the same (pair of) cameras. I have ideas of how that might be done, but no hard facts. I contacted Speedcheck (the SPECS company) many moons ago to ask how it was done, they fobbed me off!
I'm almost tempted to do a patent search. If the search yields nothing then I'll file one with all the possible techniques - just to screw them

_________________
Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

I think the definition of independent would have to be quite flexible.

On the whole though I would prefer to defeat speed cameras on the premise that they are a bad idea rather than accuracy issues.

_________________
Driving fast is for a particular time and place, I can do it I just only do it occasionally because I am a gentleman.
- James May

primary - automatic speed calculation based on matching anpr timestamps in the database ?
secondary - 'manual' speed calculation based on 2 x static images & timestamps ? (if these images are even stored ?)

perhaps.. i dont know.

To be pedantic, 10%+6.25mph. And IIRC that only applies to the vehicle when it rolls out of the factory/dealership.



We had a Mondeo at work which would have to be reading almost 90 on the speedo to put you in the prosecution threshold. Of course anything over 70mph is breaking the law (assuming this is a car or car derived van with GVW less than 2 tonne on a motorway or dual carriageway with the national speed limit in force).

I find this thought very interesting as it challenges the 'miles per hour' phrase. Is it meant to be a precise rate at every foot traveled or an over-all distance related result. Wonderful. The Highway Code states "You must not exceed the max. speed limit for the road and for your vehicle."
So is it stating, that you can average it out and that is OK - is that what you are asking ?

Obviously to maintain a 70 m p h precisely, requires a road where this would be possible for an hour without interruption, (motorway) or delays etc., however if you maintained this speed at the tolerance level then you will do more than 70 miles - perhaps by your example 84 miles, but would you be stopped by the BIB (Boys in Blue) yes, very probably. Could you be convicted of a speeding offence, by any one of the various camera devices, yes. The tolerance levels are not all set identically.
Plus the 'point' is that this 'speed' is maintained to identify the constant forward motion. Could you prove that your 70 miles of journey was all that was accomplished, and therefor you could therefor not have sped ? - well if you ensured this by parking up for a few minutes / seconds, then yes (maybe).



I think that you are thinking of the 85th percentile.
The Police use calibrated equipment in their traffic cars, so that they can take an accurate reading.
In 'normal' cars speedometers can be very inaccurate, when you have different tyre pressures (hot/cold weather & road surface temperatures, / driving style, hard cornering), tyre composition, different wheel diameters, than the manufacturer designed etc.
All cars will read slightly differently too.

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Safe Speed for Intelligent Road Safety through proper research, experience & guidance.

So the law requires you not to exceed a certain speed even though you have no accurate means of measuring that speed. Over to you, Mr Bumble.

Doe the CUR require that the speedometer be driven from the wheels?

_________________
When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown