This article makes it sound like 85th percentile speed is the one and only way speed limits are set. This in not true. Often times, 85th percentile speed is a s

This article makes it sound like 85th percentile speed is the one and only way speed limits are set. This in not true. Often times, 85th percentile speed is a starting point. Other metrics that come into play are pace and mode. The pace of a traffic stream is defined as the 10 mph increment in speed in which the highest percentage of drivers is observed. Mode is the most commonly reported speed. Beyond these statistics based measure, roadside environment is considered. The number of access points per mile, functional classification, geometry, and traffic volumes are relevant when assigning speed limits. There is very little benefit to increasing speed limits beyond the 85th percentile. The capacity of the roadway does not increase because of the increased headway between vehicles. As vehicles speed up, the distance between vehicles increases and thus the total number of vehicles that can occupy a given segment of roadway is decreased. At 30 mph, about 36 vehicles can fit within a one mile stretch (assuming 3 second following distance, 15 ft long vehicles and one travel lane). Under the same assumptions only about 16 vehicles would fit in a one mile section at 70 mph. However, slow speeds everywhere is not reasonable and that is why some measure of central tendency is used to establish speed limits.