Speed Adjustment Factors for Locked-Wheel Skid Trailer Measurements

Locked-wheel trailers are the predominant friction-testing tool used by state departments of transportation in the United States. These trailers measure friction on a locked wheel on the wetted pavement surface as the wheel slides at a constant speed. The skid resistance is reported as the skid number (SN) at the speed it was measured, with the standard test speed at 40 mph. Speed on most rural interstate and primary roads is rarely lower than 65 mph, whereas in most urban environments, congestion, intersections, and high-traffic volumes can lead to average traffic speeds as low as 20 mph. A methodology was proposed to convert the skid numbers from one speed to any desired speed by using adjustment factors. The data used to develop this methodology came from measurements taken by members of the Pavement Surface Properties Consortium for the three consecutive years from 2007 to 2009. The adjustment factors were developed for the different flexible and continuously reinforced concrete pavement surfaces that are available at the Virginia Smart Road. The friction measurements performed with the skid testers presented different responses to the different pavement surface types. To separate them, principal component analysis was used, and three different groups of surfaces were identified. Principal component analysis was also used to develop a friction conversion method that used correlations between speed and texture. Correlations between speed factors and macrotexture were computed only for smooth tire skid testers, because ribbed tires do not exhibit good relationships, probably because measurements with this tire are not sensitive to macrotexture.