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What is the 85th percentile and why should I care about it?
The short answer to what the 85th percentile is, is this: 85 percent of the vehicles in this study were going x speed or slower. The x is always
represented as a speed. The reason this value exists is it is used as a measuring stick for how roads should have their speed limits set.
Over the past 100 years of roadways, civil engineers and law enforcement have found that people tend to drive the speed that they feel is safest on a
given road, regardless of the posted limit. This seems to indicate that for the vehicles that are on the road there seems to be a natural safe speed for
individual roadways.
The 85th percentile is the value that has been found to be a good choice for setting speed limits. Research has indicated that the safest group of
drivers on the road are those in the 80th to 90th speed percentiles, above the 90th percentile we tend to find drivers exceeding safe limits and their
accident risk increases as a consequence. It is important to note that research has also found that vehicles going below the 85th percentile, such as
those going at the 50th percentile are ALSO more prone to accidents. The 85th percentile seems to be the safest speed for any given road and this is
why it is used in traffic data collection and reporting.
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