speed limits: do they work? (of course not)

[anthonym]

thanks flemke, interesting. Also vaguely connected to the op in that which was because I have slowly and dimly become aware of the powers that be complaining that their speed limits, enforcement and general measures of suppression are not working; indeed Plato (or similar I forget exactly who) commented on the futility of such behaviour by government - so it seems we never learn and history repeats. Then I started to delve into it all. Really quite surprising and depressing.

[anthonym]

Earlier in this thread evidence was requested about the 85th percentile etc.

Cirillo showed that Interstate speeds of 11 or 12 mph higher than the mean were the bottom of the risk curve. That puts the safest speed (safest by a very small margin) slightly above the 85th percentile - perhaps around the 90th. On surface highways, Solomon found the safest point about 5 mph above the mean, at or very close to the 85th speed.

Anthony

Ps I am not sure that discussion of this should not be a discussion in its own right because it is at the core of road safety and was not the thrust of my op.

[V8 Fettler]

anthonym said:

V8 Fettler said:

I'm not aware of any CDM Co-ordinator who is/was directly employed by a local authority, there would probably be a conflict of interest. CDM Co-ordinators had no powers of enforcement, neither do Principal Designers, but it could be professional suicide and financially disastrous for all parties to allow a high risk design to be developed when a low risk alternative was readily available, e.g http://www.hse.gov.uk/press/2010/coi-sw-739sww09.h...

The risks are generally highest during the construction phase of a project, but there should be consideration of risks applicable to use, maintenance and final demolition, the "use" bit being relevant to Opulent Bob's roundabout project.

as I try to think that through from that link (but in reverse order):

"Notes to editors
Section 3(1) of the Health and Safety at Work etc Act 1974 states: "It shall be the duty of every employer to conduct his undertaking in such a way as to ensure, so far is reasonably practicable, that persons not in his employment who may be affected thereby are not thereby exposed to risks to their health or safety."

are the parties in Opulent Bob's case thus:

employer=the council (i.e. Opulent Bob's employer)
persons not in his employment = road users (I suppose = other persons' employees and/or the public more generally) i.e. everyone regardless unless an employee

That informs me in that while I am aware of public liability, I have not thought of HSE matters in terms of road safety or indeed related to councils in that context. Seems obvious now you point it out - assuming I have understood correctly?

"While it is rare for designers to be charged with breaching health and safety legislation, they must be aware they can be held responsible where bad design is an important contributory factor to a work-place fatality."

The "designer" being "the council"? Though within the council we have those qualified to design, being usurped by a (unqualified) political master?


edit: our (Opulent Bob and me) above two posts posted simultaneously - had to smile at "left the country".

Edited by anthonym on Thursday 20th August 04:31

Although the Health and Safety at Work Act primarily deals with the workplace, it also refers to "others". It's unlikely that the Health and Safety Executive would have an interest in a defective design for a road crossing or similar, the HSE tends to focus on defective designs where failure can lead to catastrophic results on a large scale e.g. nuclear, petrochem. Coroners can indicate that improvements to existing designs should occur, but they're unlikely to delve into the history of a defective design.

Without wishing to annoy Op Bob too much, road design in the UK is like a cottage industry i.e. widely varying standards and concepts; but then this perhaps is linked to incompetent interference from politicians at local and national level.

[flemke]

anthonym said:

Earlier in this thread evidence was requested about the 85th percentile etc.

Cirillo showed that Interstate speeds of 11 or 12 mph higher than the mean were the bottom of the risk curve. That puts the safest speed (safest by a very small margin) slightly above the 85th percentile - perhaps around the 90th. On surface highways, Solomon found the safest point about 5 mph above the mean, at or very close to the 85th speed.

Anthony

Ps I am not sure that discussion of this should not be a discussion in its own right because it is at the core of road safety and was not the thrust of my op.

That is helpful, but please bear in mind that the data are not saying that 85th (or whatever) percentile speed is the "safest speed", per se. They are saying that the drivers who have chosen to drive at the 85th are the safest drivers. It is quite possible, I would say quite probable, that those same drivers would still be the safest if they were driving at the 65th or at the 95th.

Thus the question is probably not, "Why is 85th the safest speed?", but, rather, "Why do the safest drivers choose the 85th?"

[anonymous-user]

V8 Fettler said:

anthonym said:

V8 Fettler said:

I'm not aware of any CDM Co-ordinator who is/was directly employed by a local authority, there would probably be a conflict of interest. CDM Co-ordinators had no powers of enforcement, neither do Principal Designers, but it could be professional suicide and financially disastrous for all parties to allow a high risk design to be developed when a low risk alternative was readily available, e.g http://www.hse.gov.uk/press/2010/coi-sw-739sww09.h...

The risks are generally highest during the construction phase of a project, but there should be consideration of risks applicable to use, maintenance and final demolition, the "use" bit being relevant to Opulent Bob's roundabout project.

as I try to think that through from that link (but in reverse order):

"Notes to editors
Section 3(1) of the Health and Safety at Work etc Act 1974 states: "It shall be the duty of every employer to conduct his undertaking in such a way as to ensure, so far is reasonably practicable, that persons not in his employment who may be affected thereby are not thereby exposed to risks to their health or safety."

are the parties in Opulent Bob's case thus:

employer=the council (i.e. Opulent Bob's employer)
persons not in his employment = road users (I suppose = other persons' employees and/or the public more generally) i.e. everyone regardless unless an employee

That informs me in that while I am aware of public liability, I have not thought of HSE matters in terms of road safety or indeed related to councils in that context. Seems obvious now you point it out - assuming I have understood correctly?

"While it is rare for designers to be charged with breaching health and safety legislation, they must be aware they can be held responsible where bad design is an important contributory factor to a work-place fatality."

The "designer" being "the council"? Though within the council we have those qualified to design, being usurped by a (unqualified) political master?


edit: our (Opulent Bob and me) above two posts posted simultaneously - had to smile at "left the country".

Edited by anthonym on Thursday 20th August 04:31

Although the Health and Safety at Work Act primarily deals with the workplace, it also refers to "others". It's unlikely that the Health and Safety Executive would have an interest in a defective design for a road crossing or similar, the HSE tends to focus on defective designs where failure can lead to catastrophic results on a large scale e.g. nuclear, petrochem. Coroners can indicate that improvements to existing designs should occur, but they're unlikely to delve into the history of a defective design.

Without wishing to annoy Op Bob too much, road design in the UK is like a cottage industry i.e. widely varying standards and concepts; but then this perhaps is linked to incompetent interference from politicians at local and national level.

Your last paragraph is so true.

Political interference plays a far bigger part, IME, than nearly anything else. I would say 2 out of 3 schemes are scuppered or shelved purely on political decisions, or by the budget holder - who is more often than not politically motivated.

[anthonym]

flemke said:

That is helpful, but please bear in mind that the data are not saying that 85th (or whatever) percentile speed is the "safest speed", per se. They are saying that the drivers who have chosen to drive at the 85th are the safest drivers. It is quite possible, I would say quite probable, that those same drivers would still be the safest if they were driving at the 65th or at the 95th.

Thus the question is probably not, "Why is 85th the safest speed?", but, rather, "Why do the safest drivers choose the 85th?"

Good question, I'll get back to you on that.

[anthonym]

this is just for late night readers in India (plus those not there who can't sleep) and not my considered response, however I found it interesting and moving in the direction of an informative answer:

copy begins

85/67th Percentile Speed

Defined as that speed at or below which 85 percent of the traffic is moving. On urban roadways the 85th percentile speed has been found to be the safest speed, where the 85th exceeds 50 mph the safest speed shifts to the 90th percentile. Speed limits established on the basis of the 85th percentile on urban roadways conform to the consensus of those who drive highways as to what speed is reasonable and prudent, and are not dependent on the judgement of one or a few individuals.

However, in no case should the speed limit be set below the 67th percentile speed of free flowing vehicles.

Determining The Safe For Conditions Speed

The traffic engineer's speed survey is simply a measurement of the "public's consensus" as to what free-flowing speed they have found to be safe. Each motorist drives at a speed they feel comfortable and safe with. Each driver expresses their comfort level by their actions, taking in to account all visual clues that may be present. Engineers have found this to be a better process than basing the speed limits on the arbitrary judgments of a few. Setting all traffic control devices, turn lane lengths etc. based on this measured public consensus has been the most effective in reducing accident rates.

copy ends

from year 2000.

it seems to suggest it is not the drivers who are safe, but the speeds themselves... I'm thinking about that: after all, the search is indeed for "the safest speed"...


and p.s. I share the views above about political interference. I think our engineers should be allowed to engineer.

Edited by anthonym on Saturday 22 August 03:10

[Toltec]

anthonym said:

it seems to suggest it is not the drivers who are safe, but the speeds themselves... I'm thinking about that: after all, the search is indeed for "the safest speed"...

There are ideas along the line that driving too slowly can lead to inattention. From personal observation I see braking gaps being compressed where the speed is lower than most drivers would appear to prefer to travel at.

On the whole I don't think you can remove the drivers from the equation, a particular speed is not innately safe though it may be an optimally safe one to set as a limit.

You could possibly find a maximum safe speed based on physics and visibility, though even that would need to assume no one else made a mistake.

Having rambled a bit here is an idea.

Using the 85th percentile speed creates a range of speeds which optimally allows drivers to compensate for their own and others mistakes.

[Pete317]

I think it's more to do with the 85th percentile being the public consensus of the 'natural' speed for the road in question.
With a large proportion of drivers driving at around that 'natural' speed, there is less stress, less bunching, less need to overtake etc, all of which makes for a much safer dynamic environment.

[anthonym]

think I should have said "the safest speed LIMIT"...

I had an example of that bunching with smaller gaps only yesterday, the peloton of 4 cars with me at the back, all had gaps that were too small, one crash, all crash was my thought. I put the car in 2nd gear and left it there, which widened the gap and saved my brakes down the mountain (an Alp). Everyone else had their speed governed by the lead car which was not local. Once I found the need to brake suddenly or more suddenly that I would have preferred, which ordinarily never happens on that route - my mind was simply elsewhere.

[anthonym]

@flemke (edited to add this)

how about this from Michigan State Police:

copy begins

What is “85th Percentile Speed”?
• The speed that 85 percent of the vehicles are traveling at or below
• Ideal speed to set as the maximum limit:
– Provides the lowest speed variance between vehicles, and thus provides the lowest crash numbers
– Provides optimum enforceability • The SAFEST speed limit

copy ends

So while the 85th percentile (ibid) is not per se the "safest speed", it is the "safest speed limit". So I suppose that lower limits are by definition, not the safest speed limits regardless of driver skill

Does that work?

edit: the above absolutely allows for the "speed limit" and thus the 85th not being the safest speed since the speed limit is a maximum (with flexibility) indicator, not a statement of the speed everyone must travel at to be safe.- since all of us driving at 30mph in towns and villages would be ludicrous.

Edited by anthonym on Saturday 22 August 12:34

Edited by anthonym on Saturday 22 August 13:59

[Toltec]

The answer to your original question would appear to be -

Yes, if the 85th percentile is used - May be an iterative process.

No, if set the way they are now in the UK.

[anthonym]

flemke said:

anthonym said:

Earlier in this thread evidence was requested about the 85th percentile etc.

Cirillo showed that Interstate speeds of 11 or 12 mph higher than the mean were the bottom of the risk curve. That puts the safest speed (safest by a very small margin) slightly above the 85th percentile - perhaps around the 90th. On surface highways, Solomon found the safest point about 5 mph above the mean, at or very close to the 85th speed.

Ps I am not sure that discussion of this should not be a discussion in its own right because it is at the core of road safety and was not the thrust of my op.

That is helpful, but please bear in mind that the data are not saying that 85th (or whatever) percentile speed is the "safest speed", per se. They are saying that the drivers who have chosen to drive at the 85th are the safest drivers. It is quite possible, I would say quite probable, that those same drivers would still be the safest if they were driving at the 65th or at the 95th.

Thus the question is probably not, "Why is 85th the safest speed?", but, rather, "Why do the safest drivers choose the 85th?"

because to do otherwise would make the factors that make him safer disappear i.e. that is where safety resides and it is a moving target (if I can be forgiven for the pun as it seems rather apt)..


Happy to onward discuss that a bit.... more my thinking out loud:

drivers don't choose the 85th as such, they create it with their behaviour, they drive in the safest manner (i.e. least chance of KSI) and it turns out that this tends always to the 85th percentile (or whatever). So driving in some other percentile, isn't where drivers observe safety to be, it is where they see danger (another topic). If all those "safe 85th drivers" are driving in the 65th percentile (as derived in ideal conditions), it won't be the 65th, it will be the 85th. If they are forced into another percentile, then it is not the 85th and no longer safe.

For an attempt at clarity, as I understand it, the 85th (or whatever) (in ideal free flow conditions) is to determine the speed limit for maximum safety in said ideal free flow conditions (another discussion), so in only that instance the safe limit and the 85th (or whatever) are the same. In all other conditions (i.e. less than ideal) the 85th will not be the speed limit, it may even be where the 50th resides in ideal conditions, when conditions are no longer ideal, for example rain etc Drivers (we all) know when conditions are not ideal. It moves around and only in ideal conditions is it the speed limit (with grace), if the speed limit (with grace) was appropriately set.

of course we have multiple speed limits, variable speed limits and so on, but the above is the foundation I see.

What do you think?

Anthony



KSI = Killed or Seriously Injured

Edited by anthonym on Saturday 22 August 19:13

[Vipers]

I will just jump in, cant be arsed reading 14 pages. Do speed limits work, of course YES they do, except a military experience, I have never had a speeding ticket in 50 years of driving.

[flemke]

Vipers said:

I will just jump in, cant be arsed reading 14 pages. Do speed limits work, of course YES they do, except a military experience, I have never had a speeding ticket in 50 years of driving.

The question is, for what purpose do they work?

[Vipers]

flemke said:

Vipers said:

I will just jump in, cant be arsed reading 14 pages. Do speed limits work, of course YES they do, except a military experience, I have never had a speeding ticket in 50 years of driving.

The question is, for what purpose do they work?

Think that has been thrashed to death so many times, no point in re-visiting that one.

[anthonym]

flemke said:

anthonym said:

Earlier in this thread evidence was requested about the 85th percentile etc.

Cirillo showed that Interstate speeds of 11 or 12 mph higher than the mean were the bottom of the risk curve. That puts the safest speed (safest by a very small margin) slightly above the 85th percentile - perhaps around the 90th. On surface highways, Solomon found the safest point about 5 mph above the mean, at or very close to the 85th speed.

Anthony

Ps I am not sure that discussion of this should not be a discussion in its own right because it is at the core of road safety and was not the thrust of my op.

That is helpful, but please bear in mind that the data are not saying that 85th (or whatever) percentile speed is the "safest speed", per se. They are saying that the drivers who have chosen to drive at the 85th are the safest drivers. It is quite possible, I would say quite probable, that those same drivers would still be the safest if they were driving at the 65th or at the 95th.

Thus the question is probably not, "Why is 85th the safest speed?", but, rather, "Why do the safest drivers choose the 85th?"

because to do otherwise would make disappear the factors that make him safe(r).

long(er) answer is a few posts above here.

[flemke]

Vipers said:

flemke said:

Vipers said:

I will just jump in, cant be arsed reading 14 pages. Do speed limits work, of course YES they do, except a military experience, I have never had a speeding ticket in 50 years of driving.

The question is, for what purpose do they work?

Think that has been thrashed to death so many times, no point in re-visiting that one.

Has it?

[Vipers]

flemke said:

Vipers said:

flemke said:

Vipers said:

I will just jump in, cant be arsed reading 14 pages. Do speed limits work, of course YES they do, except a military experience, I have never had a speeding ticket in 50 years of driving.

The question is, for what purpose do they work?

Think that has been thrashed to death so many times, no point in re-visiting that one.

Has it?

Someone gives a point of view, somebody comes up with a reason why speed is irrelevant, then we have the debate about 0200 drive on empty motorways, then invariably Germany comes up, Brake v PH's, not worth pursuing it.

[flemke]

anthonym said:

flemke said:

anthonym said:

Earlier in this thread evidence was requested about the 85th percentile etc.

Cirillo showed that Interstate speeds of 11 or 12 mph higher than the mean were the bottom of the risk curve. That puts the safest speed (safest by a very small margin) slightly above the 85th percentile - perhaps around the 90th. On surface highways, Solomon found the safest point about 5 mph above the mean, at or very close to the 85th speed.

Anthony

Ps I am not sure that discussion of this should not be a discussion in its own right because it is at the core of road safety and was not the thrust of my op.

That is helpful, but please bear in mind that the data are not saying that 85th (or whatever) percentile speed is the "safest speed", per se. They are saying that the drivers who have chosen to drive at the 85th are the safest drivers. It is quite possible, I would say quite probable, that those same drivers would still be the safest if they were driving at the 65th or at the 95th.

Thus the question is probably not, "Why is 85th the safest speed?", but, rather, "Why do the safest drivers choose the 85th?"

because to do otherwise would make disappear the factors that make him safe(r).

long(er) answer is a few posts above here.

I see the distinction you are making between "safest" speed and "safest" limit. As alluded to above by Toltec, the question then becomes, how is a limit set in the first place?
Implied in the "safest limit" axiom is that the safest limit for a given road will depend on the sociology of those driving on it.