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What does this mean in the context of a traffic speed survey?
"The results show that the mean speed is 29mph and [b]the 85 percentile is 35.5."[/b]
Sounds like standard local government obfuscation to me but what does it mean in terms of the actual speed of the vehicles racing down our street?
Mean figures can be a bit skewed by the extremes. In this case 15% of cars are going over 35.5mph. If that is a bit fast, which I'd say it was for a 30mph residential street, then it is a bit of ammo for something to be done. The mean shows as under the limit but the 85th percentile shows that around 1 in 7 are going way significantly over the limit.
With some actual real life context
http://en.wikipedia.org/wiki/Speed_limit#Maximum_speed_limits
The speed limit is commonly set at or below the 85th percentile operating speed (being the speed which no more than 15% of traffic is exceeding)
So from your data it means that 85% of people are under 35.5mpg, 15% exceed it.
The mean is the sum of all the values divided by the number
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Nope it's a method of expressing the facts that appears to be fairly universal in this context. Just because you don't understand it doesn't mean they are trying to obscure it.Sounds like standard local government obfuscation to me
If you want to see what it all means you need to see the full plot which shows all of the percentile results (what 90, 95 look like)
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IAMA_statisticians
Mean is the average. 85th percentile basically means 85% vehicles are doing less than 35.5mph and 15% doing more. If you think of putting all the speeds in ascending order and going 85% of the way along the list the speed you would get to is 35.5
Brilliant, thanks, I knew the forum wouldn't let me down!
id statistically say there are some statisticians on here !
There are two types of statisticians: Those who can extrapolate from incomplete data
There are two types of statisticians: Those who can extrapolate from incomplete data
I'm the other kind.
The 85th percentile is used to determine speed limits.
For example, if a council want to make a road into a 20 limit, the police will only support it, without traffic calming, if it can be shown that 85% of people already drive down that road at less than 20.
It's the exact opposite - it's unvarnished facts.Sounds like standard local government obfuscation
That being the case, speeding is actually a necessity! If we all obeyed the speed limit it would keep dropping until it reached 0... 😕The speed limit is commonly set at or below the 85th percentile operating speed (being the speed which no more than 15% of traffic is exceeding)
Mean is [b]an[/b] average. 🙂Mean is the average
For example, if a council want to make a road into a 20 limit, the police will only support it, without traffic calming, if it can be shown that 85% of people already drive down that road at less than 20.
And it's that kind of backwards thinking that leads to things like the pedestrian crossing near my old shop that only worked when there were no cars coming.
Mean is the average. 85th percentile basically means 85% vehicles are doing less than 35.5mph and 15% doing more. If you think of putting all the speeds in ascending order and going 85% of the way along the list the speed you would get to is 35.5
Should also point out that there's no upper limit on the speed the top 15% could be doing - the original data could well include a couple of boy racers doing 60+ down your road on a daily basis.
There are two types of statisticians: Those who can extrapolate from incomplete data
There three types of lies;
1) Lies
2) Damned Lies
3) Statistics
@nickjb - nearly spat my coffee out when I read that :lol:. Exactly my kind of joke!
@globalti - I asked on here a while ago about some maths resources as I wanted to improve my understanding a bit. Now I don't know if you are or not but if you are people recommended the Khan Academy - online and free and very very good...
Sounds like standard local government obfuscation to me
[i][b]Nope it's a method of expressing the facts that appears to be fairly universal in this context. Just because you don't understand it doesn't mean they are trying to obscure it[/b][/i].
Depends entirely on who the intended audience is.
I'd bet a pound to a penny that most of the parents at my kids school would not understand what was trying to be communicated.
Insane isn't it?
Why don't they use the 85% rule for other laws? I'm sure more than 15% of people would rob banks if it was entirely legal.