The validity of the 85^th percentile method for determining posted speed limits is a complex and controversial issue. The 85^th percentile speed is the speed at or below which 85% of free-flowing traffic travels. It has been used for decades as the basis for setting speed limits. In recent years, there have been mounting calls to eliminate the 85^th percentile speed as a basis for setting speed limits, especially in urban areas. Recent research and reasoning for moving away from the 85^th percentile in urban areas does not, however, address speed limits on freeways, expressways, and rural highways. Setting safe and reasonable speed limits that encourage compliance without constant enforcement is crucial to reducing crashes and fatalities.

Proponents of the 85^th percentile speed argue it is a good measure of the speed at which drivers feel safe and comfortable on a given road, and that setting speed limits at or near the 85^th percentile can reduce speed variance and improve safety. Critics argue the 85^th percentile speed is not a reliable measure of safe driving. They note the 85^th percentile speed can be influenced by many factors, such as traffic volume, road conditions, and the presence of law enforcement. They also argue that setting speed limits at or near the 85^th percentile speed can encourage drivers to speed.

The objective of this research is to develop guidelines for state departments of transportation (DOTs) and other agencies with authority to set speed limits on freeways, expressways, and rural highways on the measures to be used as a basis for determining posted speed limits, and specifically whether the 85^th percentile speed is still a valid measure for safety and effectiveness of speed limits posted on these facilities. The research should focus on the relationship between the 85^th percentile speed and crash rates on these facilities; driver behavior and psychology; the costs and benefits of reducing speed limits; and  measures to be used by state DOTs and other agencies as the basis for recommending posted speed limits on freeways, expressways, and rural highways.