Enhancing the Superelevation Runoff Method in Circular Arcs for Mountainous Terrain Alignments

Despite the recognized importance of spiral curve implementation in highway design, several design manuals permit spiral omission depending on the geometric layout and the performance characteristics of road users. A critical safety issue associated with these methodologies arises from the potential exceedance of the maximum allowable side friction coefficient and the design utilization factor on a circular arc, particularly under wet pavement conditions. The present study aims to address a gap in geometric design manuals and the international literature by optimizing the superelevation design of the runoff section in the tangent-to-curve transition for circular arcs in mountainous terrain with a maximum design superelevation rate of up to 5%. The proposed methodology is supported by an analysis based on fundamental vehicle dynamics equilibrium equations, aiming to resolve concerns among practitioners regarding the elimination of superelevation rate transitions within the circular arc itself, with particular focus on the evaluation of the utilization factor and the applicability in icy conditions. The comparative evaluation of the demanded utilization factors resulting from this method and those defined by existing guidelines, along with the safety levels it maintains under icy conditions (i.e., compound slope up to 10%), encourages its immediate implementation in circular arcs with a design superelevation rate up to 5%, as well as further investigation into the potential application of this method in circular arcs with a design superelevation rate greater than 5% in mountainous and rolling terrains.