Development of Skid Testing in Indiana

Many studies have been made in recent years with various types of skid equipment to evaluate skidding characteristics of pavement surfaces. This paper briefly summarizes the equipment used and the results found in these studies and presents a detailed description of a semi-automatic braking device used on a conventional automobile in Indiana. The device is electrically operated and when activated applies the brakes and initiates measurement of stopping distance simultaneously. The speed at which the brakes were activated is also recorded. The method used eliminates much of the human variable from the measurement of stopping distance and makes it possible for the good reproduction of stopping distance. The skid testing program in Indiana is also outlined and preliminary results are presented. A number of experimental surfaces were tested along with four major surface types used in Indiana. These four were rock asphalt, Portland cement concrete, bituminous concrete, and other bituminous surfaces. A total of 233 different roads were tested, each road being tested at three locations with two skids being performed at each location. The skidding properties of the various roads were compared in terms of mean skid distances at 30 mph. Variability of the skid distances was determined along with the means. The tests showed that rock asphalt had the best skidding properties of all the surfaces tested with respect to both average distance and variability. Its mean skid distance changed little between the wet and dry condition. Portland cement surfaces provided relatively good skid characteristics but were subject to some polishing by traffic during the first few years of their life. The bituminous concrete surfaces tested had poorer skid characteristics than any other major type considered. The bituminous surfaces tested, other than rock asphalt and bituminous concrete, had a relatively low mean but were extremely variable. This variability was almost invariably associated with bleeding. Those roads with no bleeding yielded a mean 18 feet less than those that displayed some bleeding. The bituminous roads constructed with limestone aggregate had a lower mean than those containing gravel, although the limestone in some cases polished extensively under prolonged heavy traffic.