Superelevation, side friction, and roadway consistency

Growing awareness exists of the important role of consistency in highway alignment design in minimizing the frequency and extent of violations of driver expectancy, to improve highway safety. A simple and practical safety margin, based on an established driver behavior model, is used to investigate two sources of inconsistency: (1) The choice of design speed; and (2) the choice of superelevation and side friction when the curve radius exceeds the minimum. This paper describes a method for choosing the design speed for a horizontal curve, taking into account the extent of the curve. The paper also describes alternative methods for choosing the superelevation and side friction. Using a practical numerical example, the design speed selection and superelevation and side friction selection methods are evaluated with respect to consistency of the safety margin. It is concluded that use of the design speed selection method, and having the superelevation and friction varying linearly with the degree of curvature, will lead to greater alignment consistency and improved safety.