Try reading the research on the subject...
It is a well established axiom that the drivers who centre around the 85th percentile speed have the lowest accident figures - ie they are the safest drivers. Note that they're not the safest *because* they drive at the 85th percentile, they're the safest and *therefore* select the 85th percentile as a safe and appropriate speed.
The "establishment" used to take the pragmatic view that if a particular speed was good enough for the safest drivers, then it's a good speed to set the limit at on the basis that the risks increase quite quickly above that speed. Actually, the 80 - 90 percentile group are statistically the safest, with those in the 90 - 100 percentile having a much higher accident rate matched only by the 0 - 10 percentile group who also have a very high number of accidents.
It wasn't so much a "margin" as just a starting point to set a limit.
Probably not the vast majority, but certainly a significant minority.
-- Jeff. Ironbridge, Shrops, U.K. and don't bother with ralf4, it's a spamtrap and I never go there.. :)
... "There are few hours in life more agreeable than the hour dedicated to the ceremony known as afternoon tea.."
Henry James, (1843 - 1916).
