§AASHTO
¥ A 1969 ÒResolution of the
annual meeting of the American Association of State Highway OfficialsÓ
§ÒThe review of existing practices revealed that most of the
member departments use, primarily, the 85th
percentile speed. Some agencies use the 90th percentile speed, and of secondary consideration are
such factors as design speed, geometric characteristics,
accident experience, test run speed, pace, traffic
volumes, development along the roadway, frequency of intersections, etc.Ó
¥ ÒOn the basis of the forgoing review, the Subcommittee on
Speed Zoning recommends to the AASHTO Operating
Committee on Traffic for consideration as an AASHTO Policy on Speed Zoning that:
¥ The 85th percentile speed is to be
given primary consideration in speed zones below 50 miles per hour, and the 90th percentile speed is to
be given primary consideration in establishing speed zones
of 50 miles per hour or above. To achieve the optimum
in safety, it is desirable to secure a speed distribution with a skewness index approaching unityÓ