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Just to put the 85th percentile speed in perspective -- although you may have already seen this stuff -- let me show most data "pre-NMSL".
The "Traffic Speed TrendS reports of the Federal Highway Administration (FHwA) for the years just before the 55-mph National Maximum Speed Limit (NMSL) may be of interest.
Two participating States had no speed limit, eight had 75-mph limits, 22 had 70-mph, two had 65-mph, three had 60-mph.
AVERAGE SPEEDS OF FREE-MOVING VEHICLES AND PERCENTAGES OF PASSENGER CARS TRAVELING IN EXCESS OF VARIOUS SPEEDS:
ON LEVEL, STRAIGHT SECTIONS OF THE COMPLETED PORTIONS OF THE RURAL INTERSTATE SYSTEM FOR 1973
(NOTE: speeds were measured in the summer, before the embargo and oil price crises in the fall.)
|(No Speed Limit)|
|(70-mph) -- slowest, fastest, and Ohio speeds|
I estimate the prevailing (85th percentile) speeds as following:
|SPEED LIMIT||Prevailing speed||Average speeds|
|-NONE-||76- to 77-mph||70-mph|
|75-mph (*)||72- to 76-mph||65- to 70-mph|
|70-mph||71- to 76-mph||65- to 69-mph|
|65-mph||66- to 69-mph||62- to 65-mph|
|60-mph||64- to 70-mph||60- to 66-mph|
I conclude there's little point in a limit lower than 70-mph or higher than 75- mph.
People may feel more comfortable driving faster nowadays -- it was 20 years ago this summer the speeds were taken.
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