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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?

matt_black
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Thomas O
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    What's the maximum speed limit for the motorway in the UK? – Jason Plank May 14 '11 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 '11 at 20:13
  • @Jason, 70 mph (for cars.) For other vehicles, like trucks, it's different. – Thomas O May 14 '11 at 20:29
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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 '11 at 20:33
  • Okay, I'll add this. Yes it is *possible* to drive at 70 mph. But if everyone else drives at 80 mph, then you will be met by angry drivers. – Thomas O May 14 '11 at 20:35
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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 '11 at 20:36
  • @Thomas: maybe, maybe not, but is that what is claimed here? No. – F'x May 14 '11 at 20:37
  • @F'x I've adjusted the question to clarify "nobody" – Thomas O May 14 '11 at 20:41
  • Why the downvote...? – Thomas O May 14 '11 at 20:42
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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. – Martin Scharrer May 14 '11 at 22:07
  • Just as a comment from a boomer in the US - as we age (and you will) our reactions slow down, so we drive slower to compensate. We slow down to the speed limit :) Another point - I worked with a guy with a fast bike. He thought nothing of commuting at over 100mph and complained of the slow traffic at 70. He's aged 15y now (hopefully), so I suspect he's slowed down (to 80). – Mike Dunlavey May 18 '11 at 13:10
  • I reference my own experience when I say that I used to commute a 100 mile a day round trip on a busy section of the M6 motorway. During rush hour it was difficult to get over 70 mph due to the amount of traffic present. So I would say that it depends on the time of day. Most articulated lorries are limited to 56mph max so this would remove a large proportion of vehicles from the total. – Matt Wilko May 17 '11 at 20:07
  • Try driving in the Netherlands, where no-one surpasses speed limit, and many people drive well below. It's absolutely terrible to have a guy driving 98km/h (61mph) overtake guy driving 97km/h (60mph) in 120km/h zone with only 2 lanes. – vartec Mar 19 '12 at 13:47

2 Answers2

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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.

Sklivvz
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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 '11 at 00:03
  • 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 '11 at 00:47
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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 '11 at 07:06
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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 '11 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.

ʍǝɥʇɐɯ
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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. –  May 15 '11 at 02:44
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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](http://en.wikipedia.org/wiki/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. – user unknown May 15 '11 at 03:04
  • Only the tolerance of police measurements can be accepted. – user unknown May 15 '11 at 03:06
  • The question is regarding the UK not the USA where you don't even have the right sized gallon. In the USA the maximum that the speedometer can show is 5 mph over actual. The speedometer cannot show less than the speed you are going over there (like here, the UK). Hence the manufacturer in the USA sets the speedometer to be within this range, over estimating speed by a small percentage to be in the 5 mph safe zone. – ʍǝɥʇɐɯ May 15 '11 at 04:57
  • @mat: please put your references in the question, not in the comments :-) – Sklivvz May 15 '11 at 06:43
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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 '11 at 06:51
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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. – Konrad Rudolph May 15 '11 at 11:15
  • @Konrad: it's not common knowledge, especially in the US where (forgot the reference) speedometers are required to be more precise than in Europe. – jwenting May 18 '11 at 12:02
  • @jwenting Jup, that’s the great thing about this site, it challenges my assumptions at every step. – Konrad Rudolph May 18 '11 at 12:17
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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 http://en.wikipedia.org/wiki/Speedometer. – Suma Jul 22 '11 at 01:28
  • Comparing GPS-reported speed to speedo can be interesting. While GPC is not hugely accurate on a single point, over any reasonable distance (say, distance travelled in 1 minute, more than 1 mile at motorway speeds) it should be pretty accurate. – Marc Gravell Mar 19 '12 at 09:51
  • Indeed, in my experience in numerous cars the speedometer reading was up to 12% more than that of GPS (eg. 140km/h on meter, 125km/h on GPS). But also in some cars the difference was much less (130km/h meter, 125km/h GPS). – vartec Mar 19 '12 at 13:53