dcbwhaley wrote:
What is the scientific basis for the 85% rule?
Not scientific but consider the A515 limit reduction (now 18 miles of 50mph) of a couple of years ago, and section 34 in a document I download from the DCC website:
Quote:
Derbyshire County Council
Meeting of Cabinet Member – Environmental Services
13 September 2007
Report of the Strategic Director – Environmental Services
Speed Limit Review – A515 Buxton to Sudbury
Section 34 reads in its entirety:
'The Police, whilst being generally supportive of the changes, have expressed reservations about the proposal to introduce a 50mph limit between Buxton and Sudbury as they feel the road has long straight sections with good forward views and that the route can be driven quite safely. It is mentioned that imposing a lower limit may lead to driver frustration and could actually lead to more, not fewer, collisions. They also consider that speed surveys conducted along the length reveal a good level of compliance with the derestricted limits. It is also a particular concern that wholesale changes to generally reduce speed limits will create a considerable enforcement problem in Derbyshire and it may be difficult to sustain the existing level of enforcement. In view of this, they take the view that additional engineering measures on the long straight sections would be essential to try to ensure any lower limits are more self-enforcing and therefore reduce the need for a regular Police presence. They also feel that the lower limits should not be introduced solely to solve the problems at isolated hazards and pointout that over recent months the County Council has recently done much to improve safety at numerous locations along the A515.'
That's the 85% percentile without recourse to maths or science
As an asides I love the first sentence which, given the document is about the speed limit changes has the police both generally supportive and expressing reservations about the same thing
Chris
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