The 85th percentile

Alexander Clark made this Freedom of Information request to Department for Transport

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The request was partially successful.

Dear Department for Transport,
In all of the documents I have seen relating to traffic speed control there are references to “the 85th percentile”. I understand this to mean that from any given batch of speed data the highest 15% of measurements is discarded and any subsequent analysis (calculation of mean etc.) is based on just the remaining 85%. There is no corresponding adjustment for the slowest recorded measurements.

Given that the context is about controlling speed, what information do you have that explains the rationale behind excluding the fastest 15%? One would imagine that this is precisely the proportion of the data that would attract the greatest attention.

Yours faithfully,

Alexander Clark

Department for Transport

Dear Sir/Madam,

I am writing to acknowledge receipt of your request for information which
has been allocated reference number P0014218.

A response will be issued to you in due course.


Department for Transport
FOI Advice Team
Information & Security Division
Zone D/04
Ashdown House
Sedlescombe Road North
St Leonards on Sea
East Sussex
TN37 7GA

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Jeff Gilmore, Department for Transport

6 Attachments

Link: [1]File-List
Link: [2]Edit-Time-Data

Dear Mr Clark,


Please find attached my response to your request about the 85^th


I hope this is helpful.






[3][IMG]          Jeff Gilmore 
Policy Advisor, Young Drivers Policy and Casework 

3/29 GMH, Great Minster House
33 Horseferry Road, London, SW1P 4DR  

020 7944 2435       
[4]Follow us on twitter @transportgovuk 


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Dear Jeff Gilmore,
Thank you as it does provide some clarity as to where responsibilities actually lie. My county council is in the habit of blaming its mistakes on DfT rules as if they have no say in the matter.
Yours sincerely,

Alexander Clark