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Originally Posted by northwind
The 90% Rule, as it's known...
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Do you mean the 85% Rule as used by traffic engineers?
For those who don't know, or can't remember :P , it's the "rule-of-thumb" guide which, to papaphrase, says that the speed limit should be set to the 85th %-ile speed were there no speed limit on the road.
For example, if, on a given road link, 85% of drivers were measured at 50mph or below then 50mph will be the set speed limit.