85th percentile speed

The commonly-accepted method of setting speed limits is to unobtrusively measure the 85th percentile speed of free flowing traffic. This is the speed such that 15% of drivers are faster and 85% are slower.

The theory behind this is twofold:

  1. Speed-related safety problems are concentrated in the fastest 5%, so the 85th percentile speed clearly criminalizes those 5% and allows for a small enforcement cushion.
  2. If you plot all measured speeds on a graph, you will get a bell curve. The 85th percentile speed is the break in the upper end of the bell curve.

Interestingly, in actual practice speed limits are often set in the 30th to 50th percentile, meaning that anywhere from 50% to 70% of all drivers on any roadway are criminalized.

Seems goofy, doesn’t it?

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