Significance of Directional Skid Resistance of Pavements in Road Safety Evaluation

Recent research studies have revealed that all road pavements exhibit directionality in their skid resistance. Field measurements confirmed that at any point along a pavement wheel path, the skid resistance in the forward and backward directions were significantly different. Such differences are caused by the directional polishing effect of vehicular traffic flow. Along a straight tangent road section, the relative magnitudes of directional skid resistance at any given point vary with the degree of traffic polishing. The higher the degree of polishing, the large is the difference between the forward and backward skid resistance. Such differences in the directional skid resistance is also dependent on the mode of vehicle movements. In a case where traffic flow is dominated by acceleration mode of vehicle movements, the skid resistance would be higher than the backward skid resistance. On the other hand, in a case where traffic flow is dominated by deceleration mode of vehicle movements, the skid resistance would be lower than the backward skid resistance. It is illustrated in our analysis that the differences in directional skid resistance could have major impacts on the results of road safety analysis, especially for the calculation of braking distances and geometric design of stopping sight distance. In particular, it has significant implications on safety analysis of the safe sight distances for the design of road intersections, and safe design of weaving sections along expressways.