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.