ROAD CURVE SUPERELEVATION DESIGN: CURRENT PRACTICES AND PROPOSED APPROACH

Horizontal alignment design standards in the United States and Australia have two basic common features: firstly, the absence of a single nationwide maximum superelevation rate and, secondly, designers' freedom in applying above-minimum values for curve radii. Taking into account the proven dependence of operating speed on curve radius, Australian standards introduce the concept of speed environment (characterising highway sections as a whole) to be used alongside the traditional design speed concept (corresponding to individual curves) and incorporate consistency checks as a feedback loop in the design process. In this paper a proposal for simplifying the relationship between radius and superelevation is applied to Australian guidelines for speed environments ranging between 60 km/h and 120 km/h. Consistent application of this proposal, for which specification of nationwide maximum superelevation rates is a precondition, would result in curve radius serving the driver both as a guide for selecting speed and as a signal for the centrifugal acceleration to be expected, thus enhancing horizontal alignment consistency. (a)