Limited Response of Curve Safety Level to Friction Factor and Superelevation Variation under Repeated Traffic Loads

Abstract

Although road horizontal curves are high-risk sections for accidents, current road safety assessments often neglect the dynamic evolution of superelevation and the friction factor. The connotation for road safety level was clarified by examining the significance of road factors in traffic safety through the systemic characteristics of roads. Among these characteristics, curve safety level is determined by the ratio of the supply and demand of the lateral friction factor. On the basis of international standards and specifications, this study clarified the design supply and demand of friction factors for curve by considering the distribution of tangential and lateral friction factors. Expanding on the steady-state bicycle model while accounting for road geometric parameters and vehicle operation characteristics, the lateral friction factor demanded for vehicles was quantified. Meanwhile, the characteristics of the friction factor supplied and the superelevation variation were analyzed by using the road service life as a variable, along with their influence on the actual supply of the friction factor and the curve safety level. The results of the analysis indicate a rapid decrease in curve safety level during the first two years of road utilization, followed by a slower declining trend, with a significant 27% reduction in curve safety level by the end of the second year. Furthermore, the decline in the curve safety level is mainly attributed to variations in the road surface friction factor, whereas the influence of superelevation variation on the curve safety level is restricted. In the absence of maintenance interventions, the curve safety level will decrease by over 30% after three years of operation. Controlling operational speed is one of the effective measures for ensuring traffic safety. Meanwhile, the impact of the friction factor and the superelevation variation on the curve safety level accumulates over time, thus causing drivers to have difficulty perceiving these alterations. Therefore, dynamic safety evaluations that account for the fluctuation in the friction factor and superelevation induced by repetitive vehicle loading must be undertaken.