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. 

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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.