Speed limits - keep an eye on this
Discussion
Not read it yet, but it's going to be interesting
www.dft.gov.uk/stellent/groups/dft_rdsafety/documents/divisionhomepage/032869.hcsp
www.dft.gov.uk/stellent/groups/dft_rdsafety/documents/divisionhomepage/032869.hcsp
mybrainhurts said:
Not read it yet, but it's going to be interesting
www.dft.gov.uk/stellent/groups/dft_rdsafety/documents/divisionhomepage/032869.hcsp
It's really bloody dangerous. They have *completely* lost the plot. They are *completely* speed obsessed. I'm working on a press release. Should be fun.
This Safe Speed PR going out right now:
PR149: Backdoor Government plan to ratchet down speed limits
NEWS: for immediate release
A recently published new draft of the Government's primary local speed
limit setting advice document contains some worrying changes.
Astonishingly speed limits will be set such that 50% of motorists will
be exceeding them.
Paul Smith, founder of the Safe Speed road safety campaign explains:
"All around the world and for many decades, speed limits have been set
in accordance with the speed of traffic at such a level that only the
fastest few should be prosecuted. Research evidence shows that this is
the safest and most effective way to set speed limits. There has been
some erosion of this principle over the last decade, but the new
document promises a far more restrictive regime. In future the
Government intends that speed limits should be set at the average
speed of traffic. This immediately means that half of all motorists
will be exceeding the new speed limits."
Paul continues: "The effect of this will be a gradual ratcheting down
of speed limits and far more motorists criminalised despite the fact
that they will be driving at a safe and appropriate speed according to
the conditions."
Paul explains the techincal details. "Speed limits have long been set
at the level not exceeded by 85% of motorists. This is called 'the
85th percentile rule'. The new proposals abandon the 85th percentile
rule and replace it with a 'mean speed rule'. This means that with a
typical speed limit defined under the new scheme, 50% of drivers will
be exceeding the speed limit by definition."
For over ten years now we have speed reducing speed limits and speed
cameras are spreading like a virus. The fact is this policy is
comprehensively failing to reduce road deaths, yet the Government
sticks stubbornly to its guns. They must be saying: 'The medicine
isn't working! Let's increase the dose!'
'Speed kills' road safety policy has become a dogma, encouraged by a
series of vested interests. It isn't working and must be abandoned.
Every rational person knows that 'The competent and careful actions
of a majority of responsible people should obviously be considered
legal.' Yet with speed limit laws, applied over-zealously we are
criminalising the vast majority - and the government proposes to make
it worse.
<ends>
Notes for editors
This is a technical subject, but the effects are enormous and
dangerous. Please try and find a way of explaining the changes to your
readers, listeners or viewers.
The new document:
www.dft.gov.uk/stellent/groups/dft_rdsafety/documents/pdf/dft_rdsafety_pdf_032880.pdf
Contains:
Quote: "37. Circular 1/93 advised the use of 85th percentile speed to
determine local speed limits. This refers to the speed at, or below,
which 85 per cent of the traffic is travelling. Viewed another way it
is the speed only 15 percent of drivers exceed. Practitioners'
thinking has evolved since then and many have expressed concern that
85th percentile speed can be heavily influenced by excessive speeds
travelled by a minority of drivers. Some Traffic Authorities have
therefore adopted the use of Mean speeds in assessing what is an
appropriate local speed limit, as they are felt to better reflect what
the majority of drivers perceive as an appropriate speed for the road.
The Department shares this view and therefore recommends that mean
speeds be used in future assessments of appropriate speed limits."
Understanding speed limits and the 85th percentile rule:
www.safespeed.org.uk/speedlimits.html
PR149: Backdoor Government plan to ratchet down speed limits
NEWS: for immediate release
A recently published new draft of the Government's primary local speed
limit setting advice document contains some worrying changes.
Astonishingly speed limits will be set such that 50% of motorists will
be exceeding them.
Paul Smith, founder of the Safe Speed road safety campaign explains:
"All around the world and for many decades, speed limits have been set
in accordance with the speed of traffic at such a level that only the
fastest few should be prosecuted. Research evidence shows that this is
the safest and most effective way to set speed limits. There has been
some erosion of this principle over the last decade, but the new
document promises a far more restrictive regime. In future the
Government intends that speed limits should be set at the average
speed of traffic. This immediately means that half of all motorists
will be exceeding the new speed limits."
Paul continues: "The effect of this will be a gradual ratcheting down
of speed limits and far more motorists criminalised despite the fact
that they will be driving at a safe and appropriate speed according to
the conditions."
Paul explains the techincal details. "Speed limits have long been set
at the level not exceeded by 85% of motorists. This is called 'the
85th percentile rule'. The new proposals abandon the 85th percentile
rule and replace it with a 'mean speed rule'. This means that with a
typical speed limit defined under the new scheme, 50% of drivers will
be exceeding the speed limit by definition."
For over ten years now we have speed reducing speed limits and speed
cameras are spreading like a virus. The fact is this policy is
comprehensively failing to reduce road deaths, yet the Government
sticks stubbornly to its guns. They must be saying: 'The medicine
isn't working! Let's increase the dose!'
'Speed kills' road safety policy has become a dogma, encouraged by a
series of vested interests. It isn't working and must be abandoned.
Every rational person knows that 'The competent and careful actions
of a majority of responsible people should obviously be considered
legal.' Yet with speed limit laws, applied over-zealously we are
criminalising the vast majority - and the government proposes to make
it worse.
<ends>
Notes for editors
This is a technical subject, but the effects are enormous and
dangerous. Please try and find a way of explaining the changes to your
readers, listeners or viewers.
The new document:
www.dft.gov.uk/stellent/groups/dft_rdsafety/documents/pdf/dft_rdsafety_pdf_032880.pdf
Contains:
Quote: "37. Circular 1/93 advised the use of 85th percentile speed to
determine local speed limits. This refers to the speed at, or below,
which 85 per cent of the traffic is travelling. Viewed another way it
is the speed only 15 percent of drivers exceed. Practitioners'
thinking has evolved since then and many have expressed concern that
85th percentile speed can be heavily influenced by excessive speeds
travelled by a minority of drivers. Some Traffic Authorities have
therefore adopted the use of Mean speeds in assessing what is an
appropriate local speed limit, as they are felt to better reflect what
the majority of drivers perceive as an appropriate speed for the road.
The Department shares this view and therefore recommends that mean
speeds be used in future assessments of appropriate speed limits."
Understanding speed limits and the 85th percentile rule:
www.safespeed.org.uk/speedlimits.html
85%ile my arse.
Locally there are no longer any NSLs...most have gone to a straight 40. Some are 50's........which means that LGVs are now legally going faster and the 'safe' speed that I adhered to in my primitive vehicles of old is now deemed unsafe and prosecutable in more modern vehicles.
There's progress for you
Locally there are no longer any NSLs...most have gone to a straight 40. Some are 50's........which means that LGVs are now legally going faster and the 'safe' speed that I adhered to in my primitive vehicles of old is now deemed unsafe and prosecutable in more modern vehicles.
There's progress for you
safespeed said:
Practitioners'
thinking has evolved since then and many have expressed concern that 85th percentile speed can be heavily influenced by excessive speeds
travelled by a minority of drivers.
Have I missed something. 85th percentile speed is not a measure of speed but a measure of distribution of speeds. So how can it "be heavily influenced by excessive speeds?"
Observer said:
Practitioners'
thinking has evolved since then and many have expressed concern that 85th percentile speed can be heavily influenced by excessive speeds
travelled by a minority of drivers.
Have I missed something. 85th percentile speed is not a measure of speed but a measure of distribution of speeds. So how can it "be heavily influenced by excessive speeds?"
If you define the top 20% of drivers as driving at excessive speeds, then hey presto, you can say excessive speeds of a minority are dragging the 85th percentile up. I guess what they might be alluding to, albeit in an incompetent fashion, is that the distribution has fat tails? Given the repetition of the naieve ...
prune said:... this circular isn't written by a statician.
with each 1mph reduction in average speed reducing
accident frequency by 5% (TRL, 1993 & 2000).
One thing I'll say in this document's defence is that it clearly gives the idea that if you have to resort to the police to enforce a speed limit, then you have failed to engineer the road and its signage properly. It also states that although the dept has a target of reducing accidents (who knows if the target is realistic?) a cost benefit analysis of, amongst other factors, journey time versus safety (not that the two are necessarily in opposition) must be conducted.
I guess the nub of the argument is whether paragraph 100 is reasonable or not. Anyone have any idea how this stacks up against current policy, or, for example, what a typical B-road's injury accident rate per 100 million vehicle km is?
If Paragraph 100 is reasonable, then paragraph 101 might be worth rubbing under the noses of some local authorities.
I guess the nub of the argument is whether paragraph 100 is reasonable or not. Anyone have any idea how this stacks up against current policy, or, for example, what a typical B-road's injury accident rate per 100 million vehicle km is?
If Paragraph 100 is reasonable, then paragraph 101 might be worth rubbing under the noses of some local authorities.
Actually, speed limit being set at the average speed doesn't mean that half of the drivers will exceed it.
However it is a slightly self fulfilling prophecy.
Suppose someone decides to measure the speed limit on road.
Even if people actually drive faster than the current limit, the sight of men in bright coats with measuring gadgets will cause them to obey the speed limit.
On most non-30 limits, there will be some people who will not drive at near to the limit.
So what they will measure is a spread of data with an upper bound of the current speed limit.
Thus, any measurement to determine the "average speed limit" will be lower than the current speed limit - (unless it's a 30mph where virtually everyone does 30mph)
However it is a slightly self fulfilling prophecy.
Suppose someone decides to measure the speed limit on road.
Even if people actually drive faster than the current limit, the sight of men in bright coats with measuring gadgets will cause them to obey the speed limit.
On most non-30 limits, there will be some people who will not drive at near to the limit.
So what they will measure is a spread of data with an upper bound of the current speed limit.
Thus, any measurement to determine the "average speed limit" will be lower than the current speed limit - (unless it's a 30mph where virtually everyone does 30mph)
There's a simple solution to this; everyone should speed. This would bring the average speed above the speed limit, hence the speed limit would have to be increased.
I'm doing my bit already
Seriously though, as autismuk says, everyone would slow down when the measurement is taken. What if the powers that be decided to repeat the process again with the new limits enforced, for 'optimisation' purposes? - The speed limit would be reduced again!
I'm doing my bit already
Seriously though, as autismuk says, everyone would slow down when the measurement is taken. What if the powers that be decided to repeat the process again with the new limits enforced, for 'optimisation' purposes? - The speed limit would be reduced again!
autismuk said:
Actually, speed limit being set at the average speed doesn't mean that half of the drivers will exceed it.
I think it does mean exactly that.
By definition, half the drivers will be going faster than the average speed. If the speed limit is set at the average speed, this means that half the drivers will be exceeding the speed limit.
GreenV8S said:
Actually, speed limit being set at the average speed doesn't mean that half of the drivers will exceed it.
I think it does mean exactly that.
By definition, half the drivers will be going faster than the average speed. If the speed limit is set at the average speed, this means that half the drivers will be exceeding the speed limit.
[pedantic] I'm sure you actually mean (no pun intended) the median. If you have 1 driver travelling at 1 million MPH and another 9 at 30MPH, then only the one nutter will be above the average [/pedantic]
GreenV8S said:
Actually, speed limit being set at the average speed doesn't mean that half of the drivers will exceed it.
I think it does mean exactly that.
only if it is perfect normal distribution would the arithmetic mean suggest half the drivers will exceed.
to get the situation where 50% are above and below independant of distribution you would take the median value.
FunkyGibbon said:Ideally correct, but as antismuk said
only if it is perfect normal distribution would the arithmetic mean suggest half the drivers will exceed.
to get the situation where 50% are above and below independant of distribution you would take the median value.
autismuk said:......resulting with a 'skewed distribution'.
So what they will measure is a spread of data with an upper bound of the current speed limit.
I will stop now
Either way, I think we can all agree with piccy mate
Before we get too wound up about whether the mean, median, 85th percentile or whatever should be used, it would be interesting to know what the distributions of speeds actually looks like on, say, a typical B-road. If the distribution has a low dispersion, then it doesn't matter which percentile is taken. I've no idea what the distributions actually look like. Does anyone know if this data is available?
ATG said:
pr149 said:
Practitioners'
thinking has evolved since then and many have expressed concern that 85th percentile speed can be heavily influenced by excessive speeds
travelled by a minority of drivers.
Have I missed something. 85th percentile speed is not a measure of speed but a measure of distribution of speeds. So how can it "be heavily influenced by excessive speeds?"
If you define the top 20% of drivers as driving at excessive speeds, then hey presto, you can say excessive speeds of a minority are dragging the 85th percentile up. I guess what they might be alluding to, albeit in an incompetent fashion, is that the distribution has fat tails? Given the repetition of the naieve ...prune said:
with each 1mph reduction in average speed reducing
accident frequency by 5% (TRL, 1993 & 2000).
... this circular isn't written by a statician.
"Excessive speeds of a minority" cannot 'drag up' the 85th percentile. If a speed is above the 85th percentile, it makes no difference to the 85th percentile speed whether it's 1mph above or 100mph above.
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