Ten Percent + 2??

[nightdriver]

Hi Chaps

Got a question for any helpful BiB reading this !

I was wondering today if the 10% + 2mph 'rule' is actually regularly used or if its just a bit of a myth?
The reason I ask is that whilst driving along the M4 this afternoon I encountered a camera van on a bridge (nr. jnc 37 in Wales). I was doing 80mph (road angel) and saw the van from quite a distance and broke hard down to 70mph, I'm just slightly concerned that whilst I was quite far from the van in may have still got me???
I think I should be ok it's just I still have the 6 point limit on my licence for another 2 months so I really want to keep it clean!

But anyway, my main quesion is whether there has to be any allowence for speedo inaccuracy for camera vans.

Thanks guys,
ND

[shuvitupya]

www.pistonheads.com/gassing/topic.asp?t=264841&f=10&h=0

[smeggy]

I don't expect anyone to navigate their way through that thread!

A quick answer: Yes the SCPs can prosecute at 10+2; some have their threshold higher, others lower (the latter is quite infrequent but not impossible). Your speedo is very likely to be over-reading but your Road Angel should have been accurate, so I'm afriad you're on dodgy ground.

Only time will tell if you've been done!

[WildCat]

nightdriver said:

Hi Chaps Got a question for any helpful BiB reading this ! I was wondering today if the 10% + 2mph 'rule' is actually regularly used or if its just a bit of a myth? The reason I ask is that whilst driving along the M4 this afternoon I encountered a camera van on a bridge (nr. jnc 37 in Wales). I was doing 80mph (road angel) and saw the van from quite a distance and broke hard down to 70mph, I'm just slightly concerned that whilst I was quite far from the van in may have still got me??? I think I should be ok it's just I still have the 6 point limit on my licence for another 2 months so I really want to keep it clean! But anyway, my main quesion is whether there has to be any allowence for speedo inaccuracy for camera vans. Thanks guys, ND

They can get you for up to a mile or so apparently.

RA ist accurate - und it depend where they target your car as to whether the voice shout "laser laser speed check" in most urgent voice. Mine goes off all the time. Submarinated one's pals torpedoes me each time they see me. But I know where he ist und it confirm my opinion that they do not "form any opinion whatosever but just target willy nilly in hope of raising as much cash as possible"

At short range target - about 200m - they tend to target number plate. This mean that the RA on dash may not pick up as beam at 200m laser can be as little as 200 m across - und they are targetting the plate.

At long range the officer cannot choose which bit of car to target. Hence you may get alert in part of the "c£ossfir£e"

[DamienCBR]

smeggy said:

I don't expect anyone to navigate their way through that thread! A quick answer: Yes the SCPs can prosecute at 10+2; some have their threshold higher, others lower (the latter is quite infrequent but not impossible). Your speedo is very likely to be over-reading but your Road Angel should have been accurate, so I'm afriad you're on dodgy ground. Only time will tell if you've been done!

Is this the same with the speed readout on the TomTom One. ie: More acurate than the car speedo?

Cheers
D
PS: Sorry to high jack

[smeggy]

DamienCBR said:

Is this the same with the speed readout on the TomTom One. ie: More acurate than the car speedo?

All speedos (except for those regularly calibrated [police cars, some trucks]) are set to read between the true speed (v) and V*1.1+6.25mph. That means the speedo can overread (by quite an amount) but not underread (barring mechanical fault or significant change in the mechanics).
All in-car GPS based speed measuring devices should be accurate (some spot speed readings may waver around the true speed but will be precise and accurate when averaged).

[The Saint]

DamienCBR said:

smeggy said: I don't expect anyone to navigate their way through that thread! A quick answer: Yes the SCPs can prosecute at 10+2; some have their threshold higher, others lower (the latter is quite infrequent but not impossible). Your speedo is very likely to be over-reading but your Road Angel should have been accurate, so I'm afriad you're on dodgy ground. Only time will tell if you've been done! Is this the same with the speed readout on the TomTom One. ie: More acurate than the car speedo? Cheers D PS: Sorry to high jack

My Tom Tom One always reads below my speedo - normally between 5 & 10%

[Richard C]

smeggy said:

All speedos (except for those regularly calibrated [police cars, some trucks]) are set to read between the true speed (v) and V*1.1+6.25mph. That means the speedo can overread (by quite an amount) but not underread (barring mechanical fault or significant change in the mechanics).

I think this V*1.1+6.25mph is the SVA band of acceptance for individual vehicles rather that the Cons & Use requirements for new vehicles. The latter is 0 ..- 10%

In the real world calibration is required for any instrument to maintain accuracy. Accuracy is never required to be checked during a vehicles lifetime

[DamienCBR]

The Saint said:

DamienCBR said: smeggy said: I don't expect anyone to navigate their way through that thread! A quick answer: Yes the SCPs can prosecute at 10+2; some have their threshold higher, others lower (the latter is quite infrequent but not impossible). Your speedo is very likely to be over-reading but your Road Angel should have been accurate, so I'm afriad you're on dodgy ground. Only time will tell if you've been done! Is this the same with the speed readout on the TomTom One. ie: More acurate than the car speedo? Cheers D PS: Sorry to high jack My Tom Tom One always reads below my speedo - normally between 5 & 10%

Mine is normally around 5mph lower.

D

[s2art]

I think its a mistake to rely on a GPS speed reading. It may read slower than the speed measured by a laser gun. Just depends on the angle of the slope you are driving on. (lateral movement as measured by Sat versus road speed)

[smeggy]

s2art said:

I think its a mistake to rely on a GPS speed reading. It may read slower than the speed measured by a laser gun. Just depends on the angle of the slope you are driving on. (lateral movement as measured by Sat versus road speed)

That's true but the cosine effect is negligible (1mph error or less at 70) unless the incline exceeds 10 degrees

[s2art]

smeggy said:

s2art said: I think its a mistake to rely on a GPS speed reading. It may read slower than the speed measured by a laser gun. Just depends on the angle of the slope you are driving on. (lateral movement as measured by Sat versus road speed) That's true but the cosine effect is negligible (1mph error or less at 70) unless the incline exceeds 10 degrees

Sure, I wasnt sure how many 10+ degree slopes there are on roads. But I think thats not the only factor. If you are traversing a bend then the Sat will under-read too. Combine both and there could be a big difference.

[shuvitupya]

GPS speed readings are always delayed, so your real speed may be much higher.

If you mantain a steady speed then GPS is quite accurate,
but if you are accelerating, there is a time lag, so you could be 10mph or more over the speed limit for a short time.

[smeggy]

s2art said:

If you are traversing a bend then the Sat will under-read too. Combine both and there could be a big difference.

This depends on the resolution (time wise) of the measurements compared to the amount of 'bend'. I can't give a quantative answer but in principle you are correct (however, I've recently heard that some models work on Doppler Shift of the carrier but I don't know how true that is or if all axes (plural for axis) are considered)

I wouldn't be keen on watching the speedo/GPS when rounding significant bends at speed.