Safe Speed Forums

Would those apply to all vehicles or just cars - if just cars, what limits would you impose on other classes of vehicle and why?

(Incidentally, I have some limited sympathy for the 20MPH in a few urban areas, but it would have to be limited to areas of a particular minimum population density. Saying "residential" is not enough. My house is "residential", but I live on an NSL road, about 100 yards from the houses either side of me)!

Also, I'm amazed to see you want the limit raised to 80 on some motorways - how on earth can you justify that when you think that such a high percentage of accidents have speed as their primary cause?

Simple way of measuring the 85th percentile speed:

1) Measure the speeds of 100 vehicles in free-flowing conditions (and preferably no speed limit)
2) Discard the 15 highest speed readings
3) The highest speed reading you're left with is the 85th percentile.

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Only when ideology, prejudice and dogma are set aside does the truth emerge - Kepler

So if you have 85 of those cars doing 30mph and 15 doing 90mph the 85% is 30mph
but if you have 84 doing 30mph and 16 doing 90mph the 85% is 90mph
Damn queer way to set a speed limit

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When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

Your examples are a wee bit on the ridiculous side.
If you measure enough vehicles to get close to a normal distribution curve, (1000, 10,000?) the 85th percentile will be roughly 1 standard deviation above the average speed.

As an aside, if the distribution curve on a speed-limited road is skewed towards the upper end, it's a good indication that the limit is probably too low.

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Only when ideology, prejudice and dogma are set aside does the truth emerge - Kepler

Deliberately so. To illustrate the danger of using a too small sample size and the danger if assuming normal distribution.


Is there any reason, theoretical or observational, to conclude that, in the absence of speed limits, vehicle speeds will be normally distributed


Too low by what criteria other than the 85 percentile rule? Researching the subject I am beginning to have severe reservations about the soundness of that 85% rule. There seems be no sound theoretical justification and there seems to been very few observational studies. I would be very grateful if you could point me at any papers on this

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When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

I read an article many years ago which stated that the speed distribution on unlimited German autobahns was close to a normal distribution, with the average being 150kph (~90mph)
I see no reason to think that it would be otherwise, as most people tend to drive at a 'comfortable' speed for a road, taking clues (possibly unconsciously) from the road width, surface, geometry, frequency of perceived hazards, etc.
I haven't seen any published research papers on the subject, although the article I mentioned did reference one. (I can't remember the details) I would think that such papers would tend to be few and far between, as, a) there are few unlimited roads left anywhere in the world, and b) state-funded research (it mostly is) has for a long time tended towards the promotion of lower speeds.

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Only when ideology, prejudice and dogma are set aside does the truth emerge - Kepler

Any dafter than letting a local council pick a number out of the hat based on the rantings of a few local busybodies?

Would that lead to a normal distribution? It would if peoples conception of what was comfortable is so distributed.



That is my impression - very little research. But that does make the bold and oft repeated assertion that "the lowest accident rate occurs at the 85th percentile" - an assertion often backed, as on the Safe Speed site, with a pretty graph - somewhat speculative and a poor basis for setting speed limits.

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When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

Surely it has a foundation in basic common sense - if you are seeking to regulate a previously unregulated activity, you ensure that the large majority are within the legal area and only the outliers are excluded. Also it is a fair assumption that in any sphere most people will be acting sensibly and responsibly.

As I have as a sig. on another forum:

"The normally careful and competent actions of a reasonable individual should be considered legal." (Arizona Department of Transportation)

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"Show me someone who says that they have never exceeded a speed limit, and I'll show you a liar, or a menace." (Austin Williams - Director, Transport Research Group)

Any views expressed in this post are personal opinions and may not represent the views of Safe Speed

That rather depends on why you are wanting to regulate the activity. If the activity is proving to be dangerous to others you may prefer to ban it completely or regulate it very tightly. Carrying guns in public places springs to mind. I can see that setting the speed limit at the 85th percentile or, as I understand it better, the one sigma level does appear to be common sense. But the figure is trotted out so often as if it is a well researched result that I wanted to see the research.


Wasn't one of the arguments advanced against travelling by public transport the fact that you meet so many people who are not acting sensibly and responsibly. But where self preservation is involved, as in driving, that is more likely to be true.



That begs a number of questions not least of which is who is to judge reasonableness.

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When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

Safespeed asserts that drivers who travel at the 85th percentile speed are also the "safest". Of course "safest" could mean anything.

It also goes onto assert that slower drivers are at more risk of crashing and then goes onto sort of explain that this is due to people crashing into them for daring to go a bit slower than everybody else.

Could you give some examples of what "safest" might mean in other contexts?

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Malcolm W.
The views expressed in this post are personal opinions and do not represent the views of Safespeed.

I assume you would say the 1st percentile was safest.

And Safespeed doesn't assert it anyway. 85th %ile was the generally accepted principle for setting speed limits for decades until they became politicised.

It most certainly does http://www.safespeed.org.uk/speedlimits.html but it doesn't back it up with reference to original research



Generally accepted consensus of experts is not always correct. If it was you wouldn't be arguing against AGW.

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When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

For instance if you have two knife throwers and one kills 10 of their assistants of an evening and the other kills 20 then the one that kills 10 is the safest right?

I especially love this bit:

The term "safest" does not imply any absolute level of risk. It is a relative term so your example is correct (if a bit extreme). We all strive for the lowest risk commensurate with other factors which make it possible to undertake our day-to-day activities. You said "...safest could mean anything..." but the meaning of "safest" is quite clear.

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Malcolm W.
The views expressed in this post are personal opinions and do not represent the views of Safespeed.

But open to interpretation.

For instance, I don't think it 's acceptable that the "safest" knife thrower kills 10 assistants every night. I.e. "Safest" is a very different concept to "safe".

You are correct about this. However, the meaning of "safest" is not open to interpretation.

It is fine to discuss the acceptability of an absolute level of risk to establish what constitutes the meaning of "safe" and this is a matter of opinion but that's not what you originally said, is it?.

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Malcolm W.
The views expressed in this post are personal opinions and do not represent the views of Safespeed.

OK, if not interpretation, misuse. E.g. it allows people to get away with saying "the safest drivers" and give the impression that they actually mean these drivers are safe.