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85th Percentile Speed (mph) – The 85th percentile speed is the speed at or below which 85 percent of the drivers travel on a road segment. Motorists traveling above the 85th percentile speed are considered to be exceeding the safe and reasonable speed for road and traffic conditions. The 85th percentile speed should be taken from speed data collected during a 24-hour weekday period. Typically the data are collected with commercially available roadside units which sort and present the results in text as well as graphical format.
Speed studies should be conducted using the format and procedures described in your jurisdiction’s publications for establishing speed zones. If your jurisdiction does not have a specific written procedure, additional information is found in ITE Manual of Transportation Engineering Studies, November 2010. In addition, information on using the 85th percentile speed and interpreting the results specifically for this expert system is described in the User Guide.
The road cross section of the speed zone segment being studied should be uniform with similar roadside development. If the number of lanes, road function, or development changes with a study section, the segment should be further subdivided with the measurement of 85th percentile speeds in each segment. Another factor that should be taken into consideration when determining the start and end points of a speed zone is the location of adverse-alignment such as sharp horizontal curves, where the advisory speed is less than the speed limit. The 85th percentile speed used in the analysis for a general maximum speed limit should not be taken from the adverse aligned segment.
50th Percentile Speed (mph) – The 50th percentile speed is the speed at or below which 50 percent of the drivers travel on a road segment. The 50th percentile speed should be taken from speed data collected during a 24-hour weekday period.