Is there a sound rational basis for the 85th Percentile Speed "rule"?

Iwannahurl

First up, I don't think there is. I believe the 85th Percentile Speed "rule" is flawed at the very least, and based on shaky science.

It may, perhaps, have a limited role in the setting of speed limits in certain situations, but in my view it is a concept that is inherently biased towards motorists, who might prefer higher speed, at the expense of other road users such as pedestrians and cyclists, who might prefer a lower speed.

It is also open to misuse, as by the UK lobby group Safe Speed.

That's for starters.

On the other hand, maybe there is a sound rational basis for the 85th Percentile "rule", one that helps to optimise safety for all road users.

What do others think, and can they point to any evidence (eg from the Behavioural Sciences) that supports the "theory"?

L1011

Have you anything at all to suggest its based on "shaky science"?

Also, as this is usually the basis for deciding if traffic calming or pedestrian protection measures are needed on a road (if the free speed - which is generally the 85th percentile - is higher than the limit, for instance), you might want to tread carefully as I suspect you rather like many of its 'consequences'

schemingbohemia

Why don't you point out the scientifically shaky rationale for the "rule" first?

Iwannahurl

I think Charles Lave was among the first to advocate the 85th Percentile approach based on an analysis of speed and crashes. The supposed U-shaped curve showing that crash risk increases at higher and lower speed partly stems from his work, I believe. Check out the Solomon Curve for more on that.

However, IIRC Gary Davis has argued that Lave's analysis is an example of an ecological fallacy, ie an erroneous assumption about individuals based on aggregate data for a group.

See also: http://rein0398-its.cts-dev.software.umn.edu/Publications/Sensor/2003/Fall/Speed.html

Rune Elvik (who regards the 85th Percentile Rule as "perfect rubbish" by the way) says this supposed u-shaped speed/crash risk relationship is not real, it is simply a product of the way crashes are counted.

The 85th Percentile "rule" is very value-laden, IMHO. As for my values, well I prefer this approach: speed limits should be set to limit casualty risk, not according to driver choices.



EDIT: A crucial point, made by both Elvik and Davis, is that speed level/mean speed is a more important factor in crash risk and severity than speed variance, which is what the 85th Percentile is about.

SeanW

Iwannahurl wrote:

Cars drive to fast ... cars drive too fast ... I want to make motorists slow down just for the sake of it ... I want to make motorists slow down ...

Change the record bro :rolleyes:

For examle, SS focuses on things that realy do have the potential to cause bad accidents, like driver education and a proper response to unsafe driving, by which I assume they (Safe Speed) mean things like cutting off other road users with the right of way, driving the wrong way on a one way system, egregious speeding and things like that, instead of just focusing on speed, speed, speed all the time.

Iwannahurl

Note the title: this thread is about the 85th Percentile "rule" and whether there is solid evidence for it.

If you can't post an informed opinion, insight or comment on the matter here, then perhaps you might feel more at home in the Motors forum. They have some long-playing records that are probably more to your taste.

buffalo

I really don't get this rule at all. So for instance, near my parents' house is Limekiln Road (http://goo.gl/maps/V36tk), a road lined with residential housing on one side, and a park on the other.

The road itself is rather long and straight, and mostly well surfaced. So you can get a car up to 80kmph on it pretty quickly. Most people would probably do close to 60-70kmph down it (before speed ramps were put in).

Does the 85th percentile rule take into account that there are plenty of children playing along the road, that people cross the road to get to the park? Or does it purely say, "this is a decent road with good visibility, let people drive fast down it?"

L1011

buffalo wrote: »

I really don't get this rule at all. So for instance, near my parents' house is Limekiln Road (http://goo.gl/maps/V36tk), a road lined with residential housing on one side, and a park on the other.

The road itself is rather long and straight, and mostly well surfaced. So you can get a car up to 80kmph on it pretty quickly. Most people would probably do close to 60-70kmph down it (before speed ramps were put in).

Does the 85th percentile rule take into account that there are plenty of children playing along the road, that people cross the road to get to the park? Or does it purely say, "this is a decent road with good visibility, let people drive fast down it?"

You appear to be misinterpreting what is meant by "85th percentile rule".

All it is is a method used to derive an 'average' speed of traffic, not by averaging all speeds on the road, but by using the speed which the 85th percentile of traffic is travelling at. This means that 84(.9999)% of traffic is going slower and 14(.9999) is going faster than this.

This figure *can* be used to set speed limits, but in Ireland it generally isn't. It is, however, used to decide if a road needs traffic calming, e.g. if the 85th percentile speed on Limekiln Road was 80km/h even with a 50km/h limit, they'd put traffic calming in.

There is no need for it to "take in to account" anything as all the "rule" is is the mathematics to work out a figure.

I'm not sure what the OP is trying to achieve here - he appears to be calling in to question the merits of working out the 85th percentile speed, but doing so by complaining about what its used for in other countries. He also seems to assume that its used to set speed limits, and nothing else.

Iwannahurl

MYOB wrote: »

Also, as this is usually the basis for deciding if traffic calming or pedestrian protection measures are needed on a road (if the free speed - which is generally the 85th percentile - is higher than the limit, for instance), you might want to tread carefully as I suspect you rather like many of its 'consequences'

buffalo wrote: »

I really don't get this rule at all. So for instance, near my parents' house is Limekiln Road (http://goo.gl/maps/V36tk), a road lined with residential housing on one side, and a park on the other.

The road itself is rather long and straight, and mostly well surfaced. So you can get a car up to 80kmph on it pretty quickly. Most people would probably do close to 60-70kmph down it (before speed ramps were put in).

Does the 85th percentile rule take into account that there are plenty of children playing along the road, that people cross the road to get to the park?

Or does it purely say, "this is a decent road with good visibility, let people drive fast down it?"

Specifically, let motorists travel fast on that road.

These two posts identify some problems I have in trying to accept (or rationally justify) the 85th Percentile "rule".

To save typing time, I'm going to call it 85p.

Firstly, 85p is a very well established concept. For example, this leaflet from the US Institute of Transportation Engineers (undated) says that "according to a Federal Highway Administration study, all states and most local agencies use the 85th percentile speed of free flowing traffic as the basic factor in establishing speed limits."

In fact, I think the use of 85p is a legal requirement in many states of the US.

The ITE leaflet also says:

Speed zoning is based on several fundamental concepts deeply rooted
within the American system of government and law:
A. Driving behavior is an extension of social attitude and the majority
of drivers respond in a safe and reasonable manner as demonstrated
by consistently favorable driving records;
B. The normally careful and competent actions of a reasonable person
should be considered appropriate;
C. Laws are established for the protection of the public and the regulation
of unreasonable behavior on the part of individuals; and
D. Laws cannot be effectively enforced without the consent and voluntary
compliance of the public majority.

The 85th percentile speed is how drivers “vote with their feet.”

So far so socio-political. But where's the scientific basis, and why are motorists the only "reasonable persons" with a vote in this particular poll?

The ITE leaflet includes a graph from a FHWA report which refers to decades-old studies by the likes of Solomon, Cirillo and Hauer, all of which supposedly support this conclusion:

Drivers traveling significantly faster OR slower than this speed are at a greater risk for being in a crash. It is not high speeds alone that relate to crash risk; it is the variation of speed within the traffic stream.

I've already quoted newer analyses by the likes of Rune Elvik and Gary Davis, who suggest that the U-shaped speed vs crash involvement curve is, to put it in simple terms, a statistical artefact.

But even if the Solomon Curve is true and not a statistical illusion, how can the crash risk be calculated solely with reference to motorists? It's as if non-motorists don't exist and therefore their risk is not factored in. Yet we know that lower speed (eg 30 km/h and 20 mph zones) is of proven benefit in reducing casualties among vulnerable road users especially.

Here's a different perspective on that crucial point:

85th Percentile Rule Relaxed

A new California law allows local governments to consider neighborhood quality and pedestrian safety when setting speed limits.

Formerly, the so-called 85th percentile rule was the primary factor on which speed limits were based. The 85th percentile speed is the speed at or below which 85 percent of motorists drive on any given road. The theory behind the 85th percentile guideline is that most drivers will take road conditions into account and choose a reasonably safe speed.

"As traffic engineers themselves freely admit, the flaw in the 85th percentile approach is that drivers are traveling at a speed they feel is safe for themselves," writes Dani Weber in the Spinning Crank, the Silicon Valley Bicycle Coalition's newsletter.

"That speed is not necessarily safe for other road users like pedestrians and bicyclists. High speeds (over 25 mph) are directly correlated with motorists' failure to yield to pedestrians in cross- walks, high injury rates, injury severity, lack of perceived walkability, and high noise levels. "

The bill, sponsored by Assembly member Hannah-Beth Jackson (D-Santa Barbara), authorizes local authorities to "consider residential density [and] pedestrian and bicyclist safety" when conducting engineering and traffic surveys.

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Iwannahurl

MYOB wrote: »

I'm not sure what the OP is trying to achieve here - he appears to be calling in to question the merits of working out the 85th percentile speed, but doing so by complaining about what its used for in other countries. He also seems to assume that its used to set speed limits, and nothing else.

I have to log off now, but here's a quick response.

The RSA's 2008 Survey of Free Speeds contains the following statement:

The 85th percentile speed is the speed at or below which 85% of the motorists drive on a given road unaffected by slower traffic or poor weather. This speed indicates the speed that most motorists on the road consider safe and reasonable under ideal conditions. It is a good guideline for the appropriate speed limit for that road.

The bit in bold is what got me wondering about the scientific basis for the concept.

L1011

Iwannahurl wrote: »

I have to log off now, but here's a quick response.

The RSA's 2008 Survey of Free Speeds contains the following statement:

The 85th percentile speed is the speed at or below which 85% of the motorists drive on a given road unaffected by slower traffic or poor weather. This speed indicates the speed that most motorists on the road consider safe and reasonable under ideal conditions. It is a good guideline for the appropriate speed limit for that road.

The bit in bold is what got me wondering about the scientific basis for the concept.

The very same report, and the 2011 equivalent, show that limit setting is not done on this basis though.

Iwannahurl

MYOB wrote: »

The very same report, and the 2011 equivalent, show that limit setting is not done on this basis though.

I wasn't aware until just now that a new Free Speed Survey had been published. No (stand-alone) speed report appeared 2009-2010, so it's good to know that there's an update.

I notice that the 2011 report includes the same statement, with some caveats, that the 85p is "a good guideline for the appropriate speed limit" on a road.

The 2008 report also states:

In most cases, a default speed limit applies. This automatically applies to a particular type of road if there is no speed limit sign to show otherwise.

My principal interest is in how it has come to pass that the 85p is widely regarded as being an expression of motorists being reasonable and prudent in their speed choices. To my mind, that is a value-laden minefield, and an area that has more to with Behavioural/Social Sociology and the like, not Civil Engineering.

I suspect it very value-laden, and has a large political rather than scientific component.

The setting of speed limits in Ireland is political, of course, since these decisions are made by, among others, Local Authorities. Here's an excerpt from Cork City Council's Traffic Calming Policy, which is a document I found by chance while searching for info on 85p in Ireland:

There must be a significant majority (two thirds) of residents in favour of the introduction of traffic calming.

If the request [for traffic calming] complies with ... initial requirements a further assessment will be carried out to establish where the request will be placed on a list of priority schemes. This assessment will be carried out by examining a request under the following headings and assigning a score for each heading. The scores for each heading will then be added to establish an overall score which will decide where a scheme will be on a priority list.

The headings are:

Accident History/Data
85th Percentile Speed
Vulnerable Road Users
Traffic Volumes
Local Conditions

Note: a two-thirds majority of residents. There's some politics straight away. Given the scale of car use/dependence in this country, it is highly likely that a majority of those (adult) residents will drive. I have attended meetings where local residents have acknowledged a speeding problem and spoken about their fear for their own kids, but resisted traffic calming. Why? Because they didn't want their own driving to be affected! The speeders were some class of outsider, so apparently what the residents wanted was some intervention which slowed down those other people over there who were the real problem, but left locals unaffected. I've seen the same Nimby attitude to parking controls also.

The belief in 85p as some sort of objective indicator seems to be well-established. Here are a couple of samples:

There are four types of speeding:

1) Unsafe Speed for the conditions

2) Excessive Speed

3) Exceeding the posted limit

4) Speed Variance

By definition, any accident can be considered to have 1) as a causal factor.

Excessive Speed may or may not be hazardous depending on the circumstances, but is certainly, by definition, an extremely risky behavour.

Exceeding the posted limit is the one the Gardai prosecute, but the limit is an artifical number, set by a bureaucrat, and often has no relation to the prevailing circumstances. The Gardai tend to exploit this by locating checkpoints at places where the limit is an anomaly.

International Studies show that speeds on a given section of road tend to be grouped very tightly. The standard way to set a speed limit for a road is to use the 85th percentile as this is the point around which the vast majority of drivers are driving at a reasonably close speed.

Drivers who vary significantly from this tightly packed group tend to introduce higher risks onto the stretch of road because they are either a) roadhogs who create situations where other drivers bunch behind them, and/or have to overtake them, or b) are speeders who are constantly racing up behind ordinary traffic and overtaking them.

http://www.politics.ie/forum/transport/7230-road-safety-june-2006-a-2.html#post257978

Akrasia wrote: »

There is some research out there that suggests that the safest speed to drive your car is at the '85th percentile'

One of the biggest hazards on the roads is where there is a big speed differential between the speed some cars are travelling versus the speed other cars are or want to travel.

If you're driving too fast you're a risk to other drivers, if you are driving too slow, you are also creating a hazardous situation

Type 17 wrote: »

People ignore limits that they disagree with, but they usually disagree with them because they are too low - most people would obey speed limits without having to think about it if those limits were set at a speed that most drivers subconsciously set for themselves anyway. The level of compliance would be higher with more sensibly-set limits.

The 85th percentile rule comes from the fact that when you survey all speeds used by drivers on a given road, the lowest accident rate comes from drivers in the 85th percentile (driving faster than 85% of other users, but slower than 15% of them). This is the safest speed for that road and it is also the speed that most drivers set for themselves (the top of the bell-curve). People who drive at speeds furthest from the 85th percentile are most dangerous (too fast or too slow for the conditions).

Setting the speed limits at the 85th percentile would mean that most drivers could drive at the fastest, most efficient, speed possible for the road and could concentrate on actually driving (because their own subconciously-set speed would be close to the set limit), rather than constantly having to watch their speed because the limit is lower than what feels natural for the conditions. The drivers who have poor driving skills (they perceive a higher speed to be appropriate, or just don't care) would be caught by enforcement.

The reason that there is so much discontent about speed enforcement in Ireland is that, whereas speed limits should be set so that about 15% of drivers feel that they are too low (the poor driving skills group), in fact the speed limits on many Irish roads are set so low that the majority of drivers using them sub-consciously set their speed higher than the limit.

The key problem is that too-low limits cause an increase in non-compliance and frustration, but they don't cause a corresponding increase in safety.

cson

I smells me a Thesis in here.

Iwannahurl

cson wrote: »

I smells me a Thesis in here.

I wish. Not a chance, I'm afraid.

I thought someone else might have some leads to suitable research, eg that supports the "85th Percentile speed is the speed that reasonable people (ie you and I) tend to adopt according to the road environment" theory.

It would be interesting and informative to see the specific studies that allegedly support statements like this one, on the US National Motorists Association website:

Federal and state studies have repeatedly shown that the folks most likely to get in an accident are the ones driving at speeds significantly below the average speed of traffic. In fact, the safest motorists, in terms of avoiding accidents, are those who are driving 5 mph to 10 mph above the average speed of traffic.

On the other hand, I have found some authoritative (and more up to date) sources for some very different conclusions, ie that the famous u-shaped Solomon Curve supposedly showing that crash involvement increases at lower speed also is actually an erroneous conclusion, arising out of flawed methodology, substantial bias, and measurement error (eg self-reported speed estimates). With better methodology and improved accuracy in measurement, that u-shaped curve just disappears. The belief persists, however, in my view because it suits a particular narrative.

Here are some useful links to help dispel the myth:

http://casr.adelaide.edu.au/speed/SPEED-V1.PDF

http://casr.adelaide.edu.au/ruralspeed/RURALSPEED.PDF

http://casr.adelaide.edu.au/speed/RESPEED.PDF

http://www.swov.nl/rapport/Factsheets/UK/FS_Speed.pdf

L1011

I still fail to see any connection between any of the links you provide and what you're trying to claim. This is far and away the most confused thread you've ever posted.

All you've done so far is find that the RSA vaguely mention that the rule can be used to set speed limits; but its quite clear that it generally isn't; and some documentation showing that its used to determine the importance or not of traffic calming on a specific road. And quite a lot of completely completely unrelated content has been thrown up too.

Iwannahurl

MYOB wrote: »

I still fail to see any connection between any of the links you provide and what you're trying to claim. This is far and away the most confused thread you've ever posted.

All you've done so far is find that the RSA vaguely mention that the rule can be used to set speed limits; but its quite clear that it generally isn't; and some documentation showing that its used to determine the importance or not of traffic calming on a specific road. And quite a lot of completely completely unrelated content has been thrown up too.

1. What am I trying to claim, in your opinion?

2. Can you identify the specific content that you say is "completely unrelated"?

Sponge Bob

Motorways are designed to be safe at high speeds UNLESS some flute allows bicycles on them....is what it means.

Iwannahurl

I'll bypass that little distraction.

Sponge Bob

Safety is PART of a Level of Service. Part not ALL. It is an abstract measure and bicycles are not really considered as they are incapable of reaching LOS B /C speeds anyway....so why would they be on an LOS B target.

ROADS often fall victim to degredation of service levels owing to an influx of the wrong users...eg unlit cyclists in the evening rush hour. Handing out way more fines would improve safety in that case.

Iwannahurl

There is something in what you say.

Iwannahurl

Sponge Bob wrote: »

influx of the wrong users...eg unlit cyclists in the evening rush hour

A thought occurs: if the proportion of unlit cyclists were to reach 85%, would that suddenly make such dangerous and inconsiderate muppetry "reasonable"?

Sponge Bob

85% of road users on a road breaking the law is Ok with you is it?

Didn't take you long to reach the exact same conclusion you come to in every thread .....'2 wheels good 4 wheels bad' .Forgive me for finding it tedious.

L1011

Iwannahurl wrote: »

1. What am I trying to claim, in your opinion?

2. Can you identify the specific content that you say is "completely unrelated"?

1: I have absolutely no idea. As I said, utterly confused thread.

2: Every single link you've provided has nothing to do with your original claim.

buffalo

Sponge Bob wrote: »

85% of road users on a road breaking the law is Ok with you is it?

This is what I don't understand though - isn't that exactly what the 85% percentile rule says? I admit to being slightly ignorant here, but if a road has a 50kmph speed limit, and 85% percent of people drive down it at 60kmph+, does the rule not say that the limit be set higher at 60kmph? Or if 85% of people drive down it at 40kmph, that the speed limit be lowered?

What makes road speeds special, that the same logic can't be applied to bike lights, or parking violations, or any other area of the law?

Sponge Bob

The 85th percentile speed shows whether a road is 'congested' or not relevant to its design spec. It is a key measure of road capacity.

A 2 Lane Motorway becomes severely congested at 120kph motorway limits and 50k+ vehicles a day or 5000 in the peak hour so then reducing the speed limit increases safety as it introduces a platooning effect. In other words any road can become dangerous when the 85th percentile can no longer drive at the design speed and limits should be reviewed or else the road should be widened as they are doing in Dublin where 3 lane and 4 lane motorways are becoming the norm.

However as the M50 with 'space' for 3 lanes at 120kph actually required 4 lanes they made these lanes narrower and then reduced the M50 speed limit to take this narrowness into account.

There are circumstances where it is perfectly safe and reasonable to do 150kph in Ireland but we are not allowed to and large countries like France design for and implement a 130kph speed limit on Interurban Motorways.

Iwannahurl

Sponge Bob wrote: »

85% of road users on a road breaking the law is Ok with you is it?

Didn't take you long to reach the exact same conclusion you come to in every thread .....'2 wheels good 4 wheels bad' .Forgive me for finding it tedious.

How did you read that into it?

One interpretation of 85p is that it represents reasonable and prudent behaviour. The unlit cyclist analogy shows that to be bunkum, since the mere fact that a lot of people do something doesn't necessarily make it right.

Iwannahurl

MYOB wrote: »

1: I have absolutely no idea. As I said, utterly confused thread.

2: Every single link you've provided has nothing to do with your original claim.

So, let me see if I'm reading you right: you've no idea at all what I'm "trying to claim", yet you're certain that the links I provided have nothing to do with it.

Interesting thought process.


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L1011

Iwannahurl wrote: »

So, let me see if I'm reading you right: you've no idea at all what I'm "trying to claim", yet you're certain that the links I provided have nothing to do with it.

Interesting thought process.


.

Your initial post was calling in to question the scientifc theory behind the 85th percentile, while all your links relate to political uses of it.

There's no link there, and I still cannot see what you're trying to do here - although as other posters have noticed, there is likely some attempt to randomly attack private motorists for no reason behind it.

SeanW

Iwannahurl wrote: »

Note the title: this thread is about the 85th Percentile "rule" and whether there is solid evidence for it.

I posted as I did because every post you make is attacking and complaining about motorists. I belive this thread is just another excuse to do so. IIRC you have never posted favourably about motorists or suggested that they be taxed/regulated less in any way.

Iwannahurl wrote: »

... is inherently biased towards motorists, who might prefer higher speed, at the expense of other road users such as pedestrians and cyclists, who might prefer a lower speed.

It is also open to misuse, as by the UK lobby group Safe Speed.

Iwannahurl wrote: »

The belief in 85p as some sort of objective indicator seems to be well-established. Here are a couple of samples:

There are four types of speeding:

1) Unsafe Speed for the conditions

2) Excessive Speed

3) Exceeding the posted limit

4) Speed Variance

By definition, any accident can be considered to have 1) as a causal factor.

So a motorist can be blamed for any accident regardless of its actual cause or what speed they were doing? Is this your position?

Regarding the 85th percentile rule I hadn't heard of it before but it does make sense. I would put my own driving at perhaps the 70th percentile speed wise, but I do belive that motorists in general are capable of selecting a safe, appropriate speed. There will of course be boy racers and assorted a*holes who "take the piss" in how they drive. Speeding, cutting people off, splashing pedestrians with puddles etc.

I make it a point to drive with manners and to treat others, especially pedestrians, children & animals, cyclists, motorists who've had to pull over, with respect. As you went to great lengths to point out in another thread, for reasons you never explained, I got my drivers license about 4 1/2 years ago. In all that time I've never had an accident (thankfully) and not had so much as a parking ticket.

There are times that I exceed the posted speed limit but only if it is CLEARLY and unambiguously safe to do so - examples being urban limits that extend far out of the town, dual carriageways and other case-by-case bases when I have extremely good visibility and can clearly gauge any potential hazards, or usually lack thereof. And even then, usually not by much.

Iwannahurl

MYOB wrote: »

Your initial post was calling in to question the scientifc theory behind the 85th percentile, while all your links relate to political uses of it.

Incorrect: http://www.boards.ie/vbulletin/showpost.php?p=80773633&postcount=9

MYOB wrote: »

There's no link there, and I still cannot see what you're trying to do here

What I said in my first post was: "maybe there is a sound rational basis for the 85th Percentile 'rule'" and asked whether anyone can "point to any evidence (eg from the Behavioural Sciences) that supports the 'theory'?"

I thought someone might be able to provide such sources, seeing as how the "85th Percentile Speed = reasonable and prudent" concept seems to be so well established.

I've tried to find good evidence for that "theory" but have so far drawn a blank. The more I look, the more I am convinced that there is no such evidence, and that 85p (in relation to greater crash involvement at lower than mean speed) is merely a pseudoscientific belief that has mysteriously been granted official status, as well as being (for some) a compelling concept "frequently cited and reproduced by opponents of lower speed limits".

For example, this Australian study, which found that the 85p/speed variance/Solomon Curve had no validity, says that "the speed variation idea gained weight, more through successive restatements than through good research."

Here's another official report, this time from New Zealand, which shows that the whole Solomon Curve notion is a result of methodological error in research dating from forty years ago.

And that is what the 85th Percentile is, just a notion:

[M]any jurisdictions in the US use the “85th percentile rule” for setting speed limits, that is, they set the speed limit at a level that is exceeded by no more than 15 percent of the drivers. The justification of this rule is to avoid enforcement problems and the notion, derived from microeconomics, that drivers will consistently choose their speeds to balance their desire to reach their destinations quickly with the risk of a collision. ...[H]owever, ... this microeconomic model breaks down in the case of pedestrian / motor vehicle collisions. In this case, the pedestrian usually incurs nearly all of the personal costs of a collision, with the motorist’s costs limited to minor property damage that is often covered by insurance, while the pedestrian experiences a physical injury resulting in personal pain and suffering and the loss of time and mobility. Consequently, in this “market” the rational motorist will choose a higher travel speed than the pedestrian would like.

So there you go, it seems its origin is in economics, the "dismal science".


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L1011

You don't appear to be able to realise that the use of what is, in effect, a measurement of the average speed on a road to set speed limits is a political use of it.

Just out with it - what specific thing are you trying to oppose that someone is using this to try and defend? This is very, very obviously what you're trying to do, cryptic approach or not.

Iwannahurl

What I said in my first post was "maybe there is a sound rational basis for the 85th Percentile 'rule'" and I asked whether anyone can "point to any evidence (eg from the Behavioural Sciences) that supports the 'theory'?"

I thought someone might be able to provide such authoritative sources, but clearly that's a bit "cryptic" for some.

Iwannahurl

SeanW wrote: »

So a motorist can be blamed for any accident regardless of its actual cause or what speed they were doing?

Take that point up with its author.

SeanW wrote: »

Regarding the 85th percentile rule I hadn't heard of it before but it does make sense.

Obviously you also haven't heard of the reasons why it doesn't make sense.

SeanW wrote: »

I would put my own driving at perhaps the 70th percentile speed wise, but I do belive that motorists in general are capable of selecting a safe, appropriate speed.

Motorists may be capable of selecting a safe appropriate speed, but too often they choose not to, which is why various forms of enforcement are necessary, not to mention perpetual education.

As for your perception that you're at the "70th percentile speed wise", that would mean that on a hypothetical 50 km/h road where the average free speed was, say, 65 km/h and the 85th Percentile 72 km/h, you'd be doing around 68 km/h. Would you call that "reasonable and prudent", "safe and appropriate", or something else?


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An Coilean

Iwannahurl wrote: »

As for your perception that you're at the "70th percentile speed wise", that would mean that on a hypothetical 50 km/h road where the average free speed was, say, 65 km/h and the 85th Percentile 72 km/h, you'd be doing around 68 km/h. Would you call that "reasonable and prudent", "safe and appropriate", or something else?


.

Would the 85th percentile rule not mean that in this case the 50km/h limit is needlessly low?

buffalo

I can't help but notice that nobody has posted any links (apart from one to the Wikipedia article on 'platooning') except IWH, who lists supporting articles and studies. Instead every poster seems to question his/her motives and bias.

Does that mean that the forum admits there is no rational basis behind this concept?

Iwannahurl

An Coilean wrote: »

Would the 85th percentile rule not mean that in this case the 50km/h limit is needlessly low?

I'm glad you asked that question!

Well, the notion that the 85th Percentile speed represents that which is "reasonable and prudent" might invite such a conclusion.

But that takes me back to my original query. What is the evidence to support the contention that the 85th Percentile speed in such a situation is indeed "reasonable and prudent" (and its apparent corollary, that the posted limit is "needlessly low")?

Iwannahurl

buffalo wrote: »

I can't help but notice that nobody has posted any links (apart from one to the Wikipedia article on 'platooning') except IWH, who lists supporting articles and studies. Instead every poster seems to question his/her motives and bias.

Does that mean that the forum admits there is no rational basis behind this concept?

Not necessarily. I don't know if anyone else has searched for the rational basis, but I have, and I can't find it. I would be genuinely interested in seeing good evidence-based arguments for the "reasonable and prudent" 'theory'.




EDIT: Here's a US Federal Highway Administration manual that elevates the 85th Percentile criterion to the status of official policy, without providing a scrap of independent evidence to support it. See Chapter 3, Section 2. What I see in it is a complete lack of references, an absence of definitions for key terms, and (I would contend) circular arguments. I'm amazed that this stuff is official dogma.


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Sponge Bob

Where is it dogma in Ireland? YOU keep posting US refs...they have an Interstate network you know.

Iwannahurl

'Lay' belief in the pseudoscientific 85th Percentile dogma seems to be fairly strong in this general neck of the woods.

At the official level, my impression is that the RSA accepts to some degree the FHWA "reasonable and prudent" assumption, albeit as just one criterion among several in relation to the setting of speed limits.

As an aside, here's an RSA presentation which mentions the 'Solomon Curve' as well as some other research.

As another aside, and as a curiosity if nothing else, here's an excerpt from a 2005 report on the use of safety cameras. Make of their phraseology what you will:

The private operator will carry out a speed survey over a specific time period at the site. If the 85th percentile speed is above the actual speed limit (ie. 15% of drivers exceed the speed limit) the site will be considered for enforcement. If the 85th percentile speed is below the actual speed limit, the Gardaí will propose to the local authority that it consider reviewing the actual speed limit.

L1011

Iwannahurl wrote: »

'Lay' belief in the pseudoscientific 85th Percentile dogma seems to be fairly strong in this general neck of the woods.

Yes, its very heavily used to determine if traffic calming is needed... not speed limits.

So, effectively, you're calling in to question the validity of decisions on traffic calming measures. Fantastic.

Iwannahurl

MYOB wrote: »

effectively, you're calling in to question the validity of decisions on traffic calming measures. Fantastic.

By using the word "effectively", you're effectively making a Straw Man argument. No surprise there.

The use of the 85th percentile as one indicator of the speed of traffic on a given road compared to the posted speed limit is not what I have been referring to. To repeat, yet again: what I queried in my first post was whether there is a sound rational basis for the 85th Percentile 'rule' and I asked whether anyone can point to any evidence (eg from the Behavioural Sciences) that supports the 'theory' that it represents reasonable and prudent behaviour on the part of motorists.

Such information is not forthcoming (and couldn't be, because apparently such evidence doesn't exist, as far as I can ascertain at least). Whether or not the 85PS is used in Ireland the same way as it is used in the US, there does seem to be a belief in its validity, as evidenced by threads and posts that crop up on Boards and other forums from time to time. As mentioned already, the 85PS "rule" is often cited and reproduced by opponents of lower speed limits.

http://www.boards.ie/vbulletin/showthread.php?p=69032141

http://www.boards.ie/vbulletin/showthread.php?p=56789059

http://www.boards.ie/vbulletin/showthread.php?p=58638759

http://www.boards.ie/vbulletin/showthread.php?p=64440086

http://www.boards.ie/vbulletin/showthread.php?p=59993950

http://www.boards.ie/vbulletin/showthread.php?p=55698071

http://www.octane.ie/forum/archive/index.php/t-44267.html

See also.



EDIT: Just found this post from July 2012 (emphasis added):

Theanswers wrote: »

This graph shows it all.



Driving slower than an average speed of all other cars is actually more dangerous than driving faster.

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Sponge Bob

There is no straw man argument involved in saying that the 85th percentile speed is used as a measure of road congestion vis a vis design and vis a vis desired outcomes. Try not to introduce straw man arguments or accuse others of same please.

If traffic speeds up beyond the speed LIMIT there are ways of dealing with that...where it is an issue.

Iwannahurl

Sponge Bob wrote: »

There is no straw man argument involved in saying that the 85th percentile speed is used as a measure of road congestion vis a vis design and vis a vis desired outcomes.

Did I say there was?

Sponge Bob wrote: »

Try not to introduce straw man arguments or accuse others of same please.

Let's see if I can work this out.

You seem to be accusing me of making straw man arguments, apparently on the basis of something I didn't actually suggest (see above, and guess what that rhetorical trick is called!), while at the same time insisting that I shouldn't accuse others of straw man arguments. Have I got that right?

That's so multi-faceted it probably deserves to be called the Combine Harvester Fallacy or some such.

This is getting too silly altogether...

Sponge Bob

There was a bloke called Darwin. Darwin had a theory that ( to paraphrase) nature will deal with the 86th percentile and the rest of the lads who drive too fast all the time because the 85th percentile make them drive too fast.

It went something like that anyway and Nature was deffo involved.

As far as I know Mr Dawin never owned or drove a car though so I might be wrong. MYOB will set me straight next time they are around.

SeanW

Iwannahurl wrote: »

Take that point up with its author.

You posted it: I want to know if its your position that a motorist can always be blamed for an accident regardless of actual cause?

Obviously you also haven't heard of the reasons why it doesn't make sense.

No, I just don't care because I look at your posts and all they are is about bashing motorists and "Waaaaaah ... motorists drive too fast" obviously anything you write is going to support that, regardless of whether its true or not."

Motorists may be capable of selecting a safe appropriate speed, but too often they choose not to, which is why various forms of enforcement are necessary, not to mention perpetual education.

Agreed. People who take the piss with speed, or text while driving, or drive drunk need to be dealt with.

As for your perception that you're at the "70th percentile speed wise", that would mean that on a hypothetical 50 km/h road where the average free speed was, say, 65 km/h and the 85th Percentile 72 km/h, you'd be doing around 68 km/h. Would you call that "reasonable and prudent", "safe and appropriate", or something else?

It would depend on the road: if everyone was speeding on towns main street or through housing estates, that would indeed be proof that the 85th percentile was a bad rule, at least in that area.

However, starting about 4 years ago, I've seen a proliferation of ridiculous speed limits like urban 50kph limits going for a mile into the countryside, 30kph limits that should be time-dependent rather than fixed, not to mention speed limits that bear no relation to reality like 30kph on the N3 as it crosses the M50 - the couple of times I used it I did 90kph in absolute safety and was overtaken by just about everyone else.

The private operator will carry out a speed survey over a specific time period at the site. If the 85th percentile speed is above the actual speed limit (ie. 15% of drivers exceed the speed limit) the site will be considered for enforcement. If the 85th percentile speed is below the actual speed limit, the Gardaí will propose to the local authority that it consider reviewing the actual speed limit.

Comes as no surprise. At that time, the RSA was stuffed with people like you - extremists whose sole purpose in life was to make life hell for motorists, demonising motorists, especially men out of all proportion to reality.

The report basically says - and you would agree - that there is no such thing as a speed limit that's too low: if motorists are breaking ANY limit, it's because they're lawbreaking scum. But if they're not driving at the posted limit, the posted limit is too high and should be revised downwards.

I am happy to say however that the RSA has changed its tune a little bit since and has taken to actually doing something helpful: running TV ad campaigns to educate people to use roundabouts properly, overtake safely, and has recently called for the Garda to check the mobile phone records of drivers involved in fatal accidents, to see if they were texting etc at the time.

All of which I agree with 100%.

Iwannahurl

SeanW wrote: »

You posted it: I want to know if its your position that a motorist can always be blamed for an accident regardless of actual cause?

No, I just don't care because I look at your posts and all they are is about bashing motorists and "Waaaaaah ... motorists drive too fast" obviously anything you write is going to support that, regardless of whether its true or not."

Agreed. People who take the piss with speed, or text while driving, or drive drunk need to be dealt with.

It would depend on the road: if everyone was speeding on towns main street or through housing estates, that would indeed be proof that the 85th percentile was a bad rule, at least in that area.

However, starting about 4 years ago, I've seen a proliferation of ridiculous speed limits like urban 50kph limits going for a mile into the countryside, 30kph limits that should be time-dependent rather than fixed, not to mention speed limits that bear no relation to reality like 30kph on the N3 as it crosses the M50 - the couple of times I used it I did 90kph in absolute safety and was overtaken by just about everyone else.

Comes as no surprise. At that time, the RSA was stuffed with people like you - extremists whose sole purpose in life was to make life hell for motorists, demonising motorists, especially men out of all proportion to reality.

The report basically says - and you would agree - that there is no such thing as a speed limit that's too low: if motorists are breaking ANY limit, it's because they're lawbreaking scum. But if they're not driving at the posted limit, the posted limit is too high and should be revised downwards.

I am happy to say however that the RSA has changed its tune a little bit since and has taken to actually doing something helpful: running TV ad campaigns to educate people to use roundabouts properly, overtake safely, and has recently called for the Garda to check the mobile phone records of drivers involved in fatal accidents, to see if they were texting etc at the time.

All of which I agree with 100%.

Quite.


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SeanW

Iwannahurl

SeanW wrote: »

Quite. :rolleyes:

L1011

Iwannahurl wrote: »

Quite. :rolleyes:

It appears you're now even confused by the completely inexplicable mess of a thought process you're having in public on this thread.

Iwannahurl

buffalo wrote: »

I can't help but notice that nobody has posted any links (apart from one to the Wikipedia article on 'platooning') except IWH, who lists supporting articles and studies. Instead every poster seems to question his/her motives and bias.

Does that mean that the forum admits there is no rational basis behind this concept?

You can lead the horse to the water, but you can't make him think.


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monument

Iwannahurl wrote: »

buffalo wrote: »

I can't help but notice that nobody has posted any links (apart from one to the Wikipedia article on 'platooning') except IWH, who lists supporting articles and studies. Instead every poster seems to question his/her motives and bias.

Does that mean that the forum admits there is no rational basis behind this concept?

You can lead the horse to the water, but you can't make him think.


[COLOR="White"].[/COLOR]

You're an evil motorist hater and anything you say is about hating motorists right to drive!