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I live in the UK, and my dad has told me "It is not possible to drive at 70 mph on the motorway. Everyone goes at 80 mph and [the police] don't care."

When he says nobody he means the majority of people drive at 80 mph.

The speed limit for motorways in the UK for cars is 70 mph.

This seems to be a fairly common belief, but is it true? And does it apply for other countries?

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    What's the maximum speed limit for the motorway in the UK? May 14, 2011 at 20:13
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    I'm from the US, and many people go 10-15, even 20 mph over. However, in some areas the police do care, and I see people stopped every day. In other areas, you could go 30 over and nobody would do anything. I have heard especially in Chicago that it is expected to drive the same speed as everyone else to be safe - which is sometimes even 45 mph over - and you'll get stopped for going the speed limit if it's too slow!
    – mellamokb
    May 14, 2011 at 20:13
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    I've deleted my answer, but my point stands: as stated, the claim is trivial to disprove. Yes, it is possible to drive at 70 mph.
    – F'x
    May 14, 2011 at 20:33
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    Regarding anecdotal evidence, the scientific method is clear: one counter-example is enough to disprove a generic affirmation. Anecdotal evidence would be “if observed X a few times, so probably X is true”. A counter-example is “I observed not-X once, so X is not true”, and it's a valid reasoning.
    – F'x
    May 14, 2011 at 20:36
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    Could you please rephrase the question. Maybe to "Is it true that the majority of people drive faster than allowed on motorways". Than please add the fact that 70mph is the max. speed to the question, not only as a comment. This is not clear to people on an international site like this. May 14, 2011 at 22:07

2 Answers 2

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No it's not true, although a lot of people drive faster than they should.

On motorways in 2009, 52 per cent of cars exceeded the 70 mph speed limit. In addition, 16 per cent of cars were recorded as travelling at 80 mph or faster.
source

In response to your edit: the majority of people drive below 80mph. only 1 in 6 drives at 80mph or over.

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  • And in addition, in some areas even fewer people speed ( I'm thinking specifically of that 30m section of road immediately prior to a speed camera :-)
    – Rory Alsop
    May 15, 2011 at 0:03
  • I'm sure the stats don't lie.. but my recent experience is that in all but the left hand, the average speed easily exceeds 70mph on motorways - and on highways with more than 2 lanes, you should not travel at less than 80 mph in any other lane.
    – Dexter
    May 15, 2011 at 0:47
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    For what it's worth - I live in the UK and generally do respect speed limits. Sure, there are people going much faster than the limit, as everywhere, but there is no problem traveling at 70 on motorways, even in the fast lane.
    – Sklivvz
    May 15, 2011 at 7:06
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    @Sklivvz only if you're overtaking something in lane 2 - there's technically no such thing as "the fast lane". It's just a lane. Lane hogging is evil.
    – bcmcfc
    Oct 18, 2011 at 22:51
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Most people are under the illusion that the speed they are travelling at is the speed shown on the speedometer. This is a fair assumption but not a correct one.

The speedometer - whether electronic or analogue - makes some assumptions about tyre size. Because the tyres wear/get changed and with other inaccuracies in the measurement can be introduced between the tyres and the speedometer, the speedometer is calibrated to be typically 10% or so 'out', over estimating the speed. In this way, no matter what happens to the tyres, the speedometer is not underestimating the speed, which would be illegal (UNECE 39).

Hence, the motorist travelling at '80 mph' is more likely be travelling nearer 70 m.p.h. and, with the police allowing 'a few m.p.h.' over 70, what that means is that, when travelling at a speedometer-measured 80 m.p.h. there is no real reason to slow down to speedometer measured 70 m.p.h. just because there is a police car in the mirrors. However, in practice, 85 rather than 80 mph might be the speed of the traffic and all the drivers around might slow down to 70 instinctively on sight of a police car, forcing you to do likewise.

What would be nice is if there was scope to tune one's speedometer to show actual speed rather than '110% + up to 6.5 m.p.h.'. Theoretically this could result in a more accurate car mileage that, come resale time, could be ~10% less than it would be if set to manufacturer defaults. This could result in higher resale value for the car. However, any effort at 'adjustment' may be deemed 'tampering', get it wrong and it could cost around £700 for a proper, garage installed replacement.

How Stuff Works have more information on how the car speedometer works including the history of the invention and some maths about how tyre size can affect measured speed.

What amuses me about motoring programs is how the likes of Richard Clarkson-May will have a cutaway shot of the speedometer hitting some exciting top speed. Now, if you were the manufacturer, what would you want that speed to show, 180 mph or 200 mph? (where the 200 mph would be ~ 180 mph x 110% + 6.5 mph) Certainly the exaggerated speed would be more impressive and unlikely to be achieved in real life by any customers outside of Germany.

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    References to any of this, please? These are extremely bold claims that contradict my personal experience and my knowledge of, at least, typical United States laws with regards to odometer tampering and accuracy. Although it's quite possible that car manufacturers only tamper with UK odometers.
    – user1770
    May 15, 2011 at 2:44
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    I don't buy your math. From 70 to 80 mph, the difference is much closer to 15% than to 10%. And to have a speed which is 10% faster, your wheels circumference has to be 10% longer, which means, the diameter of your wheel must be 10% bigger see the definition of Pi. A typical tire size seems to be 14"-17", 40 to 45 cm in diameter. A 14% bigger tyre would need to have a 5 to 6 cm bigger diameter, 2.5 to 3 cm on each side. You don't tell us, that your profile is about one inch (2.54 cm), that you lose 2.5 cm of grit. You should control your tachometer with GPS. May 15, 2011 at 3:04
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    As a side note, speedometers are calibrated to show a bit higher speed than actual, as anybody with a GPS can tell, especially over 100Km/h. Reason: it's safer and protects car manufacturers. Also, car speedometers are not precision instruments - they do have an inherent accuracy of around +/-5Km/h (besides calibration), or ~10%. Police tend not to stop people going below 110% of the limit for this reason, because one could be in perfectly good faith and complain, and there are plenty of worse offenders to choose from.
    – Sklivvz
    May 15, 2011 at 6:51
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    I’m surprised that this answer is controversial – I thought the impreciseness of speedometers was common knowledge. Either way, the reference should clear this up. May 15, 2011 at 11:15
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    @jweting: Interesting. Do you have some reference? What I see is EU requires error max. +10 % + 4 km/h (same as UNECE), while US requires +- 10 % - both from sources linked in en.wikipedia.org/wiki/Speedometer.
    – Suma
    Jul 22, 2011 at 1:28

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