Speedometer tolerance and speed limits

[smeggy]

It would appear that many of us have been under the misconception that speedometers must have a tolerance of 0% to +10%. The tolerance is in fact much greater than that.

www.opsi.gov.uk/si/si1996/Uksi_19963013_en_7.htm
scroll down to paragraph 19

(or www.opsi.gov.uk/si/si2001/20010025.htm#sch3 or www.opsi.gov.uk/si/si2004/20040623.htm)

opsi.gov.uk said:

2. For all true speeds up to the design speed of the vehicle, the true speed shall not exceed the indicated speed. 3. For all true speeds of between 25 mph and 70 mph (or the maximum speed if lower), the difference between the indicated speed and the true speed shall not exceed - V/10 + 6.25 mph where V = the true speed of the vehicle in mph.

Therefore speedos can legally over-read by more than what could be considered to be a significant amount.

It is quite rare for a speedo to be at the limit of this tolerance, but the average speedo will be at the center of this tolerance: Vind = Vtrue(1+1/20) + 3.125mph.
Also, for a driver’s peace of mind, they will usually back off by few mph under the limit when approaching potential speed enforcement sites. A quick spreadsheet based on these figures reveals the average true speed of traffic would be almost 10mph under the indicated limit.

Vlimit / Vmeantruedriverspeed
30 / 23
40 / 32
50 / 42
60 / 51
70 / 61


Consider the active policy of speed limits constantly ‘under review’ such that they will be set to be at the mean speed:

DFT said:

Some Traffic Authorities have therefore adopted the use of Mean speeds in assessing what is an appropriate local speed limit, as they are felt to better reflect what the majority of drivers perceive as an appropriate speed for the road. The Department shares this view and therefore recommends that mean speeds be used in future assessments of appropriate speed limits.

www.dft.gov.uk/stellent/groups/dft_rdsafety/documents/page/dft_rdsafety_032880.hcsp (section 37)

Therefore, it follows that a speed limit is at significant risk of being reduced if everyone were to obey that limit (as indicated by the speedometer), which of course will happen if they feel they are at risk of being targeted for speed enforcement.


Summary of events at such a site:
- Unfortunate (but random) cluster of accidents at site
- Speed camera installed at site
- Drivers slow down, to mean (true) speeds at next lower enforceable increment
- The speed limit on that road is reduced accordingly
- Drivers get caught out exceeding the lowered limit, compounded by the fact that the limit is now set below the natural (and safe) driving speed
- Thanks to the phenomenon of RTTM, the accident rate falls at that site anyway

Hence SCPs score a double whammy: their actions are seen to reduce KSI rate and are seemingly able to justify their inflated level of income.



So much for the phrase ‘if you don’t speed you won’t have anything to worry about’



I’m not advocating that drivers deliberately exceed speed limits, I’m merely highlighting some of the devious tactics used by these SCPs

[Richard C]

Hi smeggy

Funnily enough I read this last night in the context of the apparent hidden agenda of DVLA to enable any modified vehicles to be removed from the roads via the SVA process.

As you say - how can enforcement under the ACPO code ( NEVER guidelines as per CPS lying) be permitted when drivers staring at the speedo be sure they are under the limit plus 'tolerance' ?

[simonprelude]

Does anyone know how accurate GPS based devices are, both at catching up, and displaying the speed when moving along at a constant pace.

[Mr E]

Richard C said:

Funnily enough I read this last night in the context of the apparent hidden agenda of DVLA to enable any modified vehicles to be removed from the roads via the SVA process.

Hello? What?

Over my dead body.

[safespeed]

I don't think it's right to suggest that the speedo design intention is to hit the centrepoint in the legal maximum range. I suggest that the speedo design intention is to hit the centrepoint in the tolerance that can readily be guaranteed by practical instruments. I believe that most manufacturers are shooting at Vtrue-5% with a maximum design tolerance of +/- 5%. Only digital speedometers would need to add +/- 1 digit for quantisation error.

But, and it's a big but, these regulations only apply at time of manufacture. On a ten year old vehicle all bets are definitely off.

smeggy said:

DFT said: Some Traffic Authorities have therefore adopted the use of Mean speeds in assessing what is an appropriate local speed limit, as they are felt to better reflect what the majority of drivers perceive as an appropriate speed for the road. The Department shares this view and therefore recommends that mean speeds be used in future assessments of appropriate speed limits. www.dft.gov.uk/stellent/groups/dft_rdsafety/documents/page/dft_rdsafety_032880.hcsp (section 37)

That horrible document is a consultation proposal and does not represent current advice. It's definitely run into serious problems, but there is a real possibility that it WILL be published as advice shortly.

[Einion Yrth]

safespeed said:

smeggy said: DFT said: Some Traffic Authorities have therefore adopted the use of Mean speeds in assessing what is an appropriate local speed limit, as they are felt to better reflect what the majority of drivers perceive as an appropriate speed for the road. The Department shares this view and therefore recommends that mean speeds be used in future assessments of appropriate speed limits. www.dft.gov.uk/stellent/groups/dft_rdsafety/documents/page/dft_rdsafety_032880.hcsp (section 37) That horrible document is a consultation proposal and does not represent current advice. It's definitely run into serious problems, but there is a real possibility that it WILL be published as advice shortly.

How can you possibly use mean speed ( or even 85 percentile speed) as a guide to an appropriate limit if those speeds are likely to be affected by a speed limit that is already in force? Insane.

[smeggy]

safespeed said:

I don't think it's right to suggest that the speedo design intention is to hit the centrepoint in the legal maximum range. I suggest that the speedo design intention is to hit the centrepoint in the tolerance that can readily be guaranteed by practical instruments. I believe that most manufacturers are shooting at Vtrue-5% with a maximum design tolerance of +/- 5%. Only digital speedometers would need to add +/- 1 digit for quantisation error.

I think it would be easier for manufacturers to aim for the center of the band (there’s nothing to stop them). Also, long term drift can go either way, so it’s safer to aim for the center to maximise long term compliance, so I would tend to disagree.

safespeed said:

That horrible document is a consultation proposal and does not represent current advice..

I realised that as I researched into it and I was very careful to check the wording within it:
“Some Traffic Authorities have therefore adopted the use of Mean speeds in assessing what is an appropriate local speed limit” - I took that to mean the policy is already active and the consultation process largely redundant (as ever!)

[ashes]

I have driven sevral new (2002+) cars lately and all overread dramatically - an indicated 85 = 77 GPS!

A sneaky way of slowing everyone down - older cars driven have shown 80 = 77GPS

As if

[safespeed]

smeggy said:

safespeed said: I don't think it's right to suggest that the speedo design intention is to hit the centrepoint in the legal maximum range. I suggest that the speedo design intention is to hit the centrepoint in the tolerance that can readily be guaranteed by practical instruments. I believe that most manufacturers are shooting at Vtrue-5% with a maximum design tolerance of +/- 5%. Only digital speedometers would need to add +/- 1 digit for quantisation error. I think it would be easier for manufacturers to aim for the center of the band (there’s nothing to stop them). Also, long term drift can go either way, so it’s safer to aim for the center to maximise long term compliance, so I would tend to disagree.

I'm pretty sure that average speedos are around 4 or 5mph under at 70mph. That would tend to suggest that they are aiming a little higher than you suggest. I could set up a poll at Safe Speed to get some sample speedo errors and improve the guess from there.

Anyone here involved in motor manufacture who could ask their colleagues?

smeggy said:

safespeed said: That horrible document is a consultation proposal and does not represent current advice.. I realised that as I researched into it and I was very careful to check the wording within it: “Some Traffic Authorities have therefore adopted the use of Mean speeds in assessing what is an appropriate local speed limit” - I took that to mean the policy is already active and the consultation process largely redundant (as ever!)

I expect that was an attempt to spin the consultation, possibly backed by one or two loony traffic authorities. It tends to lend a thin veneer of credibility.

[kevinday]

Here is a list of my observed measured speedo errors:

2000 Skoda Octavia - speedo overreads by 10% at all observed speeds (30, 60, 90, 120 kmh)
2001 Peugeot 206 - speedo overreads by 5% at all observed speeds
2005 Peugeot 206 - speedo overreads by 7% at all observed speeds
2000 Audi A6 - speedo overreads by 8% at all observed speeds
1991 Ferrari 348 - speedo overreads by 2% at all observed speeds (as above plus 180kmh)

All observations made by use of timing device and a measured 2 km stretch of motorway.

[LRdriver II]

simonprelude said:

Does anyone know how accurate GPS based devices are, both at catching up, and displaying the speed when moving along at a constant pace.

In my airplane at 460kts, the update rate is every 1 second. I beleive that be the case also in land-based GPS in cars. So good enough for me..