Observer wrote:
IanH wrote:
I'd like to see more which incontrovertibly stated that 85th to 90th percentile was the safest speed as
this link tends to suggest that such research is in short supply.
Wikipedia wrote:
85th Percentile Rule
Traffic engineers are taught the 85th Percentile Rule, which claims that maximum speed limits should be set to the speed at which 85% of vehicles are traveling. (Thus 15% of vehicles are speeding.) The rule has a mathematical basis; the 85th percentile almost exactly corresponds to one standard deviation above the mean of a normal distribution. This rule has been used for many years, yet no scientific evidence has been produced that this particular rule is safer than any other.
Surely the point is
not that the 85th percentile speed is the safest speed but that the speed which the 'safest' (not sure how that's defined so let's assume it is "least crash involved") group of drivers tends to select in free flowing conditions is the 85th-90th percentile speed. (I'm sure you realise this, Ian, but thought it worth clarifying for others.)
We don't want drivers whose 'comfort' speed is at (say) the median speed thinking they must drive more quickly to be safe(r).
Good point well clarified, Observer
Your concern also highlights the problem of over-reliance on speed limits as a guide for safety. The two distinct and equally important factors of individual ability, and hazard density mean that the safest speeds necessarily vary routinely while driving.
Clinging to the speed limit as the guide to safety is not only wrong, it carries added risk