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PostPosted: Sat Dec 12, 2009 06:31 
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USA - understanding the 85th percentile ! :)
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hereCounty leaders agree to 35 mph maximum on new section of road
By JON DUVAL
Express Staff Writer
The Blaine County Commission approved an increase to the speed limit along a one-mile stretch of East Fork Road at a special meeting on Tuesday. The speed limit has been increased to 35 miles per hour, about six months after the commission had voted to lower it to 25 mph.

Less than half a year after the speed limit was reduced to 25 miles per hour for a stretch of East Fork Road, drivers will once again be able to travel 35 mph without fear of receiving a ticket.

At a special meeting Tuesday night, the Blaine County Commission approved a speed limit change for the second time in less than a year for a mile-long section of the road running from the bridge over the Big Wood River to the intersection with Ranch Road, between Hailey and Ketchum.
This part of the road was improved and widened over the summer, and residents with driveways abutting the heavily used East Fork Road voiced concern in May that the project would encourage drivers to step even harder on the gas pedal.
The commission members reversed course this week in part because traffic statistics showed that the average speed was well over the current limit.
According to data recorded by the county, 68 percent of the cars along that portion of the road were travelling over 30 mph and that 74 percent were going over 25 mph.

"From a road engineering standpoint, 85 percent of people will drive a safe speed," Commissioner Tom Bowman said in an interview. "And that part of the road was engineered for 40 mph, with its curves and restrictions."

About 60 members of the public attended the meeting, with the majority of those who spoke arguing in favor of returning to the higher speed limit.

"There were some good arguments that it's difficult to drive 25 mph there, especially heading downhill toward [state Highway 75]," Bowman said. "We're just going to trust that people will be good neighbors and stick to the new speed limit."

Given that the road is seeing increasing use for accessing recreational areas in the Pioneer Mountains, Bowman said it was not surprising that people were passionate about the issue.
At the meeting, Blaine County Sheriff's Capt. Ed Fuller reported that in the last six months, the police issued 10 citations for speeding on that stretch of East Fork Road.
Jon Duval: jduval@mtexpress.com

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PostPosted: Sun Dec 13, 2009 08:48 
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How was that 85th percentile figure established. Not arguing it, just seeking knowledge.

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PostPosted: Sun Dec 13, 2009 14:22 
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This is a good place to start.

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PostPosted: Sun Dec 13, 2009 15:38 
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See here : http://www.safespeed.org.uk/speeding.html
That's a good place to start. :) If anything is still unclear just ask.

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PostPosted: Sun Dec 13, 2009 21:16 
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Steve wrote:
This is a good place to start.


Interesting. But that demonstrates that "At the 85th to 90th percentiles we tend to find drivers with above average skill " rather than that an average motorist chosen at random would be safer travelling at the 85th rather than the 50th percentile. And surely speed limits should be set for the average motorist rather than for those with above average skills.

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PostPosted: Sun Dec 13, 2009 21:49 
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dcbwhaley wrote:
Interesting. But that demonstrates that "At the 85th to 90th percentiles we tend to find drivers with above average skill " rather than that an average motorist chosen at random would be safer travelling at the 85th rather than the 50th percentile. And surely speed limits should be set for the average motorist rather than for those with above average skills.

If those with 'average skill' are indeed voluntarily going slower than those with 'above average skill', then it is right to set the limit in line with those with 'above average skill'; those of 'average skill' won't suddenly won't start going faster.
No UK motorist is being forced to travel faster than they feel comfortable (within reason). No one is calling for those at the 50th% will be forced to travel at 85th% speeds; those who are at the 50% can remain where they are.


The issue here is the needless criminalization of safe drivers.
The normally careful and competent actions of a reasonable individual should be considered legal.

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PostPosted: Sun Dec 13, 2009 22:08 
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Steve wrote:
The normally careful and competent actions of a reasonable individual should be considered legal.

Which indeed, attributed to the Arizona Department of Transportation, is my sig. on another forum.

And underlines the point why speed limits should not be set below the 85th percentile speed.

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PostPosted: Sun Dec 13, 2009 23:11 
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dcbwhaley wrote:
Steve wrote:
This is a good place to start.


Interesting. But that demonstrates that "At the 85th to 90th percentiles we tend to find drivers with above average skill " rather than that an average motorist chosen at random would be safer travelling at the 85th rather than the 50th percentile. And surely speed limits should be set for the average motorist rather than for those with above average skills.

If you 'chose' any driver at random, then you are only observing an unknown single example - useless for overall knowledge and facts. You need to look at the overall picture. Observe what is going on where accidents are least and that can guide you as to the 'best practice'. The you need to understand good behaviours and where and how do people 'obtain' a good attitude ... etc.
The 50th are safer than the 20th%ile but are less safe (more accidents) than the 85th%ile that have the least accidents.
Speed limits need to be set for each road :
1) against the type, location,
2) dry road conditions
3) a typical average of hazard expectations

Hence then why distance between vehicles is recommend to be 2 sec at standard and then increase this gap when conditions are worse (etc).

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PostPosted: Mon Dec 14, 2009 00:17 
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Quote:
Interesting. But that demonstrates that "At the 85th to 90th percentiles we tend to find drivers with above average skill " rather than that an average motorist chosen at random would be safer travelling at the 85th rather than the 50th percentile. And surely speed limits should be set for the average motorist rather than for those with above average skills.


which is why mean (or average) speeds which we are using now to supposedly set speed limits are always lower than 85percentile speeds, however even mean (average) speeds are still being ignored by many authorities.

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My views do not represent Safespeed but those of a driver who has driven for 39 yrs, in all conditions, at all times of the day & night on every type of road and covered well over a million miles, so knows a bit about what makes for safety on the road,what is really dangerous and needs to be observed when driving and quite frankly, the speedo is way down on my list of things to observe to negotiate Britain's roads safely, but I don't expect some fool who sits behind a desk all day to appreciate that.


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PostPosted: Mon Dec 14, 2009 00:59 
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I think there is a conflict here.
One the one hand we have the 85th percentile way of calculating what a reasonable speed limit should be for a particular road.
On the other hand we have councils, who generally would like to see the back of cars/trucks/vans/motorcycles etc.
There would seem to be no common ground.

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