Speed limit

A speed limit is the maximum speed allowed by law for vehicles on a road. (Also, an axiom of Einstein's relativity theories states that the speed limit of the Universe is the speed of light in a vacuum, i.e., 299,792,458 metres per second.)

85th percentile rule

Since the 1950s, United States traffic engineers have been taught the 85th Percentile Rule. The idea is that the speed limit should be set to the speed below which 85% of vehicles are traveling. The 85th percentile closely corresponds to one standard deviation above the mean of a normal distribution.

Related Topics:
Standard deviation - Normal distribution

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Every state in the United States statutorily or administratively picks a particular speed for a speed limit cap, meaning that no speed limit in that state may be set higher than the cap. A practical effect of this cap is that nearly every rural roadway in the U.S. has a speed limit that is well below the 85th percentile speed.

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