RFI - 85th percentile
Discussion
I'm confused. Perhaps you can help?
Let's start with what I think I understand:
1. The 85th percentile of speed was originally used as a basis for the introduction of speed limits, reflecting the maximum speed that a large majority of drivers would select free from external constraints.
2. There is evidence to show that drivers travelling at about the 85%ile of speed have fewer accidents
So, questions that I can't answer include:
1. Are the limits, as originally set, no longer valid? If so, then why? Human psychology may be a constant, but some factors would tend to suggest that limits are now too low (eg. better vehicle technology) and other factors might suggest the opposite (eg. increased traffic density)...
2. If the speed limits are no longer appropriate, then how should they be amended? How can the 85%ile experiment be repeated, given that drivers are now conditioned to driving at a prescribed rather than a natural pace (having acclimatised to driving with speed limits)?
3. Furthermore, if the experiment could be repeated, then would we benefit? Do you think the results of a repeated experiment would suggest that 30 limits be raised, and 60 limits lowered, to suit the (seeming) large proportion of motorists who elect to drive at a comfortable 45mph regardless of environment?
I know that the 85%ile has been discussed in passing elsewhere, and apologise if this new thread is poorly regarded. However, I could certainly learn something from a discussion focused specifically on this point, if people are willing to indulge me...
Let's start with what I think I understand:
1. The 85th percentile of speed was originally used as a basis for the introduction of speed limits, reflecting the maximum speed that a large majority of drivers would select free from external constraints.
2. There is evidence to show that drivers travelling at about the 85%ile of speed have fewer accidents
So, questions that I can't answer include:
1. Are the limits, as originally set, no longer valid? If so, then why? Human psychology may be a constant, but some factors would tend to suggest that limits are now too low (eg. better vehicle technology) and other factors might suggest the opposite (eg. increased traffic density)...
2. If the speed limits are no longer appropriate, then how should they be amended? How can the 85%ile experiment be repeated, given that drivers are now conditioned to driving at a prescribed rather than a natural pace (having acclimatised to driving with speed limits)?
3. Furthermore, if the experiment could be repeated, then would we benefit? Do you think the results of a repeated experiment would suggest that 30 limits be raised, and 60 limits lowered, to suit the (seeming) large proportion of motorists who elect to drive at a comfortable 45mph regardless of environment?
I know that the 85%ile has been discussed in passing elsewhere, and apologise if this new thread is poorly regarded. However, I could certainly learn something from a discussion focused specifically on this point, if people are willing to indulge me...
7db said:
Does anyone know what effect the posted limit has on the level of the 85th percentile?
More significantly, does anyone know what effect a visibly enforced limit, coupled with over-reading speedos (must not exceed V/10 + 6.25mph) and speedmeter equipment uncertainty, has on the level of the 85th percentile?
And we’re wondering why many speed limits are being reduced!!
7db said:
Does anyone know what effect the posted limit has on the level of the 85th percentile?
My guess, informed by nothing more than personal observation of free flowing traffic in different environments and reasonable weather, would be that:
In 30 limits, 30mph would lie at about the 40th percentile
In 60 limits, 60mph would lie at about the 80th percentile
In 70 limits, 70mph would lie at about the 60th percentile
Hence my suggestion that a re-evaluation of speed limits based on the 85%ile might have the (undesirable, in my opinion) effect of increasing 30mph speed limits and reducing 60mph speed limits.
I'm sure other contributors to this forum have their own views, and expect many to be more informed than my own. It would be great to hear from them
The other thing that it might be worth taking into account, is that the 70 mph limit on motorways was originally introduced to save fuel during the Suez Crisis. This is no longer a valid reason for it's existance but not only that, at the time the average family car was only just capable of hitting 70 mph and equipped with brakes, steering and suspension to suit. Now even the most basic shopping trolley is capable of way over that, is this not relevant?
Then again, Income tax was introduced to pay the cost of the Crimean War and I think that's been over for some time but the tax is still with us.....
Then again, Income tax was introduced to pay the cost of the Crimean War and I think that's been over for some time but the tax is still with us.....
gilbertd said:
The other thing that it might be worth taking into account, is that the 70 mph limit on motorways was originally introduced to save fuel during the Suez Crisis. This is no longer a valid reason for it's existance but not only that, at the time the average family car was only just capable of hitting 70 mph and equipped with brakes, steering and suspension to suit. Now even the most basic shopping trolley is capable of way over that, is this not relevant?
Then again, Income tax was introduced to pay the cost of the Crimean War and I think that's been over for some time but the tax is still with us.....
The 70mph limit on motorways was intorduced in 1965 as a four month trial & was never changed back follwoing a series of collisions (some in fog).
gilbertd said:
The other thing that it might be worth taking into account, is that the 70 mph limit on motorways was originally introduced to save fuel during the Suez Crisis. This is no longer a valid reason for it's existance but not only that, at the time the average family car was only just capable of hitting 70 mph and equipped with brakes, steering and suspension to suit. Now even the most basic shopping trolley is capable of way over that, is this not relevant?
Then again, Income tax was introduced to pay the cost of the Crimean War and I think that's been over for some time but the tax is still with us.....
Temporary speed restriction of 70mph imposed on motorways in 1965, and made permanent in 1967. (Suez crisis ten years earlier).
Price of oil now nearly three times that of 1965.
Income Tax was an invention of the Napoleonic Wars. Jolly good thing too - or it wouldn't be as fun to run a train to Paris from Waterloo.
To the best of my knowlege, the 85th percentile is still the accepted way to set speed limits correctly.
Unfortunately many limits are set by councils who haven't the first idea about it and they simply respond to 'those who shout loudest'.
This is, I am sure, at least partly to blame for the failure of road safety strategy; a limit may be changed, but it is simply changed for a lower one that has a similar - or worse - likelihood of there being an accident.
Unfortunately many limits are set by councils who haven't the first idea about it and they simply respond to 'those who shout loudest'.
This is, I am sure, at least partly to blame for the failure of road safety strategy; a limit may be changed, but it is simply changed for a lower one that has a similar - or worse - likelihood of there being an accident.
tlracing said:There are iirc new guidelines, including proposals to make the mean speed the new benchmark.
To the best of my knowlege, the 85th percentile is still the accepted way to set speed limits correctly.
In keeping with the total lack of competence associated with such things, the chit chat that was issued with the proposals said that part of the reason for this - apart from the nonsensical desire to keep on lowering speed limits diue to a fundamental lack of understanding of road safety principles - was that it was wrong for a small percentage of high speed drivers to 'influence' speed limit setting. Take a look at that! Using the 85%ile rule, the fastest 10% of drivers have NO INFLUENCE on the speed limit guideline, if they are already above the 85%ile their actual speed is irrelevant. It's the distribution of speeds that matters not their actual values. However, taking the mean speed for the guideline means that the fastest drivers WILL have an influence on the guideline for the speed limit, as all speeds are added up and included in a calculation of the average (mean) speed.
vonhosen said:Again iirc, the motorway limit was initialy introduced as a temporary measure as a result of AC racing car prototypes being tested on a section of the M1 at well over 100mph, though the official explanation has always been accidents in fog. At the time few family cars could exceed 70mph.
The 70mph limit on motorways was intorduced in 1965 as a four month trial & was never changed back follwoing a series of collisions (some in fog)
willibetz said:
2. There is evidence to show that drivers travelling at about the 85%ile of speed have fewer accidents
It's vital to properly understand the relationship between "safest drivers" and "85%ile speed" - you have stated it back to front. The safest (least crash involved) drivers tend to choose speeds in the 80-90%ile range. That is because the safest drivers are competent and confident but not complacent.
IT DOES NOT FOLLOW that it is safer to drive at the 80-90%ile speed than at any other speed. That range of speeds just happens to be tbe range chosen by the safest drivers. The last thing we want to do is encourage people whose competence/confidence/comfort would naturally lead them to drive at (say) the median speed to believe that they would be safer if they drive faster.
Observer2 said:Quite right too, which is why I pointed out in an earlier post concerning the 85%ile that road user education and training is the best way to recover our stalled road safety policy, moving more road users into that type of driver category. Pedestrians could be helped by public information clips to understand the need to estimate speed and distance, while cyclists could have eye tests to help diagnose the seemingly large number with red-green colour blindness that emerges suddenly in front of traffic lights
2. There is evidence to show that drivers travelling at about the 85%ile of speed have fewer accidents
It's vital to properly understand the relationship between "safest drivers" and "85%ile speed" - you have stated it back to front. The safest (least crash involved) drivers tend to choose speeds in the 80-90%ile range. That is because the safest drivers are competent and confident but not complacent.
IT DOES NOT FOLLOW that it is safer to drive at the 80-90%ile speed than at any other speed. That range of speeds just happens to be tbe range chosen by the safest drivers. The last thing we want to do is encourage people whose competence/confidence/comfort would naturally lead them to drive at (say) the median speed to believe that they would be safer if they drive faster.
Fining motorists using GATSOs and Scamvans for minor technical offences will do diddly squat for road safety. Indeed, it IS doing diddly squat.
Observer2 said:
It's vital to properly understand the relationship between "safest drivers" and "85%ile speed" - you have stated it back to front. The safest (least crash involved) drivers tend to choose speeds in the 80-90%ile range. That is because the safest drivers are competent and confident but not complacent.
IT DOES NOT FOLLOW that it is safer to drive at the 80-90%ile speed than at any other speed. That range of speeds just happens to be tbe range chosen by the safest drivers. The last thing we want to do is encourage people whose competence/confidence/comfort would naturally lead them to drive at (say) the median speed to believe that they would be safer if they drive faster.
No we defintely don't want to encourage people to drive beyond their ability level.
willibetz said:
Does anyone know what effect the posted limit has on the level of the 85th percentile?
My guess, informed by nothing more than personal observation of free flowing traffic in different environments and reasonable weather, would be that:
In 30 limits, 30mph would lie at about the 40th percentile
In 60 limits, 60mph would lie at about the 80th percentile
In 70 limits, 70mph would lie at about the 60th percentile
Hence my suggestion that a re-evaluation of speed limits based on the 85%ile might have the (undesirable, in my opinion) effect of increasing 30mph speed limits and reducing 60mph speed limits.
I'm sure other contributors to this forum have their own views, and expect many to be more informed than my own. It would be great to hear from them
No need to worry. In all normal circumstances 90% of drivers do not drive 'too fast'.
Observer2 said:
2. There is evidence to show that drivers travelling at about the 85%ile of speed have fewer accidents
It's vital to properly understand the relationship between "safest drivers" and "85%ile speed" - you have stated it back to front. The safest (least crash involved) drivers tend to choose speeds in the 80-90%ile range. That is because the safest drivers are competent and confident but not complacent.
IT DOES NOT FOLLOW that it is safer to drive at the 80-90%ile speed than at any other speed. That range of speeds just happens to be tbe range chosen by the safest drivers. The last thing we want to do is encourage people whose competence/confidence/comfort would naturally lead them to drive at (say) the median speed to believe that they would be safer if they drive faster.
Many thanks - that's a clear and reasoned explanation that I understand and accept.
Is it fair to presume that a safe driver (competent, confident, not complacent, tends to choose speeds in the 80-90%ile range) would not become less safe if compelled (by traffic or legislation) to travel at a lower speed? Please note, I'm not suggesting that safe drivers should be compelled to drive more slowly - I just wonder what the effect on safety would be.
>> Edited by willibetz on Thursday 16th March 09:33
Einion Yrth said:
I think we all want pretty much the same thing, safer roads; we just disagree as to how that may be achieved.
It depends on who you mean by we! It would appear that those tasked with making the roads safer want a regular revenue stream more than they want safer roads. If this were not the case then they would be more open to informed debate regarding alternatives and would be accepting of the limitations and risks associated with any and all (especially the current) approaches to improving safety on the roads.
If, by we you mean all those without a financial vested interest, then I totally agree.
Observer2 said:
It's vital to properly understand the relationship between "safest drivers" and "85%ile speed" - you have stated it back to front. The safest (least crash involved) drivers tend to choose speeds in the 80-90%ile range. That is because the safest drivers are competent and confident but not complacent.
IT DOES NOT FOLLOW that it is safer to drive at the 80-90%ile speed than at any other speed. That range of speeds just happens to be tbe range chosen by the safest drivers. The last thing we want to do is encourage people whose competence/confidence/comfort would naturally lead them to drive at (say) the median speed to believe that they would be safer if they drive faster.
I totally agree. Why set the limits at the 85%ile and then hammer home the message "it's 30 for a reason" almost in isolation. At the very least it should be "it's NO MORE THAN 30 for a reason". The message has beed "dumbed down" in search of a soundbite, and becomes an incorrect message in the process. The thing with soundbites is they are meant to reach even the dumbest - who will get the soundbite and nothing else.
safespeed said:
On every single road journey speed must be moderated far below the speed limit for serious and immediate safety considerations. This is an essential and unavoidable skill that all drivers must use if they are not going to crash almost immediately. It is fundamental to safe driving and cannot be replaced with speed limits and speed limit compliance. We call it safe speed behaviour. It is usually about being able to stop safely within the distance that you know to be clear. In real driving this clear distance ahead changes frequently (especially in town where most accidents happen).
It's wholly unrealistic to expect speed limits to replace proper speed setting decisions from drivers. That has never been the purpose of speed limits.
But modern oversimplified thinking keeps returning to the idea of a speed limit as a means to ensure that drivers use speed safely. This is the speed limit trap. It's the unfulfilled and unfulfillable promise that speed limits will make drivers choose safe speeds.
7db said:
No need to worry. In all normal circumstances 90% of drivers do not drive 'too fast'.
That's quite an assertion, Paul. But I know you'll have the facts to back that up...?
I can back it up four ways.
By observation: I see nutters and I see normal drivers. The proportion of nutters is small. Way under 5%. The next question of course is 'do normal drivers on occasion drive too fast?' and of course they do - But it is rare indeed to observe a normal driver driving so much to fast that there is immediate danger. This is a judgement based on degree. Someone asked me once: If I was a passenger in a car giving instruction, how often would I say 'a little bit too fast just there'; and the answer is of course 'frequently'. But this isn't about fundamental danger, this is about sub-optimal safety - or if you prefer insufficient margin for error.
By crash stats: Despite well over half of drivers at free flowing sample sites exceeding the speed limit on most road types, less than 5% of all crashes involve exceeding a speed limit. Speeding is underrepresented in crash stats by a factor of 10. Contrast this with drunk driving. Less than 2.5% of drivers are drunk, yet 25% of crashes involve drunks. As we expect, drunks are over-represented in the crash stats, but 'speeders' are massively under-represented.
By logic: I know that if you are driving 'too fast' for the condition you can't stop in time and you crash. Yet with 32 million licenced drivers doing 500 billion vehicle kilometres per annum we only have 214,000 injury crashes. And most of those crashes are caused by carelessness or inattention. You might get away with driving too fast for a year or five - but you can't get away with it for a lifetime - yet with 12% of crashes involving 'excessive speed' and 32 million drivers we get 32,000,000 / (12.5% * 214,000) = one excessive speed injury crash in 1,200 driver years.
By science: Solomon et al as discussed.
The whole thing fits a pattern. Routine speeding isn't dangerous. And don't forget that in the illustrations above we have NOT excluded the nutters and the speeding drunks and the speeding stolen cars from the analysis. If you take them out then the risks affecting the typical motorist halve again (probably).
>> Edited by safespeed on Thursday 16th March 11:37
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