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MICHIGAN SPEED LIMITS
Michiganspeedlimits.org is a citizen-run organization focused on how speed limits are set in Michigan, and the effects of car speeds on the safety of drivers, bicyclists, and pedestrians.
One tool for setting speed limits is the 85th percentile method:
The 85th percentile is not the speed the majority of drivers are driving.
It is a speed faster than the majority of others are driving. Faster, in fact, than 85% of the other drivers.
It is the "head of the pack," exceeded by
only 15% of drivers.
Percentile is not the same as percentage.
Think of an achievement test: If you score in the 85th percentile, it does not mean you got 85% correct, or that your score is similar to 85% of the others. It means you have exceeded the scores of 85% of the other students.
Note that speed measurements are taken only of FREE-FLOWING traffic. CARS SLOWING FOR BIKES OR PEDESTRIANS ARE NOT COUNTED.
Procedure for figuring the 85th percentile: Say a hundred car speeds are measured by radar on a given roadway. These speeds are ranked from the smallest number to biggest number, and then are counted until the 85th one is reached--that number is the 85th percentile of all the speed measurements collected.
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