85th percentile speed

The 85th percentile speed or 85th percentile rule is a traffic engineering standard used to set the speed limit for automobiles on a public roadway. It refers to a speed where 85% of vehicles travel at or below.

Critics of the guideline say that it is inappropriate to let drivers set the speed limit for a road via their own recorded speed. Once a speed limit has been set using the 85% rule, motorists tend to drive faster than that new speed limit. A speed limit set using this methodology also does not take into account the safety of pedestrians in the area or bicyclists using the road.

Public safety advocates have advocated for the Federal Highway Administration to change their guidance on the usage of the 85th percentile rule in updates to Manual on Uniform Traffic Control Devices.