Friction measurements were made with a skid trailer at 70 mph on 820 miles of rural, four-lane, controlled-access routes on the interstate and parkway systems in Kentucky. These facilities were subdivided into test sections and half-mile sites. Accident experience, friction measurements and traffic volumes were obtained for each subdivision.

The expression of accident occurrence which correlated best with skid resistance was wet-surface accidents per 100 million vehicle miles. There was a definite trend exhibiting a rapid decrease of accidents with increasing Skid Number (70 mph) to 26 ± 1; thereafter, with increasing Skid Numbers, the rate of decrease was considerably lessened. This trend was developed using test-section data and verified using half-mile sites. Analysis of Peak Slip Numbers and accident occurrences indicated similar trends to those developed with Skid Numbers.

Report Date


Report Number

No. 339

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The contents of this report reflect the views of the author who is responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Department of Highways or the Federal Highway Administration. This report does not constitute a standard, specification, or regulation.