First conviction for middle lane hogging

[DanceswithUnix]

Your speedometer needs to be accurate within 10% at 30mph, but an inaccurate speedometer is not a defence, although it might be mitigation. It is also why the discretionary advice for prosecution is 10% +2.

Only for commercial vehicles, which I think are allowed +10%-0%

The consumer speedometer regs are odder than that. It can't lie to you about speeding, so it must not say you are doing 30mph when you are actually doing 31mph. It is however quite OK if when doing 30mph for the speedo to say you are doing 60mph, because it can err on the "safe" side as much as it likes telling you to slow down. Most cars therefore err on the safe side a bit, some quite a lot.

So I think if you claim you are speeding because of a faulty speedometer, then that is admitting an unsafe to drive car which could be another offence

[kalniel]

GPS is handy for checking speeds. Have to say my most recent car (A3) is usually within 1mph of the real speed, which is a bit of a change as last A3 had a little more leeway, and the Astra before it even more (though that's partly because I changed tyre sizes).

[Saracen]

Only for commercial vehicles, which I think are allowed +10%-0%

The consumer speedometer regs are odder than that. It can't lie to you about speeding, so it must not say you are doing 30mph when you are actually doing 31mph. It is however quite OK if when doing 30mph for the speedo to say you are doing 60mph, because it can err on the "safe" side as much as it likes telling you to slow down. Most cars therefore err on the safe side a bit, some quite a lot.

So I think if you claim you are speeding because of a faulty speedometer, then that is admitting an unsafe to drive car which could be another offence

Actually, it wouldn't be OK for the speedo to misreport 30 as 60. Not by a VERY large margin.

The actual speed must NEVER be more than indicated speed, that's true.

But the reported speed mustn't be more than 10% + 4kph of TRUE SPEED.

In other words (well, formula)

0 < V1-V2 < (V2/10) + 4kph

where :-

V1= indicated speed, and
V2 = true speed

And note, where I've used 'less than' symbol <, the actual formula uses "less than or equal to" but I don't have that symbol.

The legislative basis for this is a bit arcane, but UK legislation in speedo requirements refers back to an EEC directive (75/443/EEC) for the tolerance definitions, and the above formula is a direct quote.

But, note also, this is really for defining the specification of speedos to be installed, and the conditions under which those tolerances apply relate to the test environment, which also stipulates tyre requirements, tyre pressure, ambient temperature and even road surface conditions.

So really, our 'on the road' situation is, as someone (PeterB I think) said, speeding is an absolute offence, and if you're over the limit, then you're committing the offence, and speedos are required to comply with those tolerances under test conditions, which aren't necessarily met under real road use, but the speedo is compliant if it complies under those carefully defined test conditions.

[Saracen]

Speeding is an absolute offence, you are either cmpliant or not. The discretion is in whether to prosecute. Your speedometer needs to be accurate within 10% at 30mph, but an inaccurate speedometer is not a defence, although it might be mitigation. It is also why the discretionary advice for prosecution is 10% +2.

Agree with all except the last sentence (and 10%-ish). The real reason is about tolerances in the metrology process.

For instance, how do you measure "true" speed, to compare the speedo's indicated speed? Answer, as with all metrology, you use a known, calibrated reference. But how accurate does that reference need to be? Answer .... +/- 1%.

So there's a chunk of variability right there.

Also, consistency over time. As speedo readings are based on a set of known gearings and the rotational diameter of the wheel/tyre, it will be affected (albeit by a smallish amount) by both tyre wear and tyre pressure, the latter of which can be affected by temperature. More variability.

So there's always going to be a degree of inaccuracy, and indeed variability over time, in how the indicated speed relates to actual vehicle speed, due to the inability of tne measurement system to accurately report vehicle speed, at all times, under all real-world circumstances.

In many ways, GPS-based speed readings are more accurate BUT fail in one essential way, at least. They aren't an indication of current speed. There's always a lag, albeit usually brief. They're also somewhat artificial, as they're based on distance calculated by comparing distance already travelled based on doppler effects between two accurately timed points, but take account only of 2D movement in a 3D environment, and worse, go through a normalisation process in the SatNav to give a usable display of data. That said, generally, it's a more accurate indication of speed, most of the time, but it's dodgy to rely on it.

Personally, I keep aware of the SatNav speed (when it's in the car) but I tend to drive to the car speedo, not the SatNav. That way, I figure to keep speeding convictions to a minimum. So far, that minimum is less than 1.

[Biscuit]

What made me frown was

Surely, that says as much about those 6 drivers as it does the "lane hogger".

Last time I came up behind a slower moving vehicle on the motorway, I slowed down in plenty of time, then over took as soon as I was able.

What you are saying you do, and what the police are saying the other drivers do could easily be the basically the same thing written differently. Without knowing his speed or the speed of the 6 motorists in question you have no idea on the circumstances. Their language is is very blunt, which is typical fo the police, but not indicative of poor driving.

[KeyboardDemon]

What you are saying you do, and what the police are saying the other drivers do could easily be the basically the same thing written differently. Without knowing his speed or the speed of the 6 motorists in question you have no idea on the circumstances. Their language is is very blunt, which is typical fo the police, but not indicative of poor driving.

Good point, well made.

[DanceswithUnix]

I like that he said he was that he was in the middle lane because it was windy.

Is is just me that thinks if it is too windy for your van at 60mph, then you should perhaps be doing 50mph?