A common feature of curve design standards of the United States, Germany, and the United Kingdom is designers' freedom in applying above-minimum values for curve radii. On the basis of research findings suggesting a strong dependence of operating speed on degree of curve, it appears that, if consistency rules are not applied, this freedom may result in unrealistic assumptions for operating speed, with potential implications for safety. In addition, a comparison of the assumed values of the side friction coefficient (f) with empirical data for f-values "acceptable" by drivers confirms that, even for minimum-radius curves, actual operating speeds are underestimated for design speeds below 90 kph (56 mph). Innovative approaches to curve design suggest that, in addition to the incorporation of explicit consistency rules in design guidelines, the link between curvature and speed should be taken into account. A design procedure reflecting the association between curvature and speed, as well as enabling simplification of the superelevation - curvature relationship, is suggested in this paper and compared to existing design guidelines. It is suggested that the advantages and disadvantages of reconsidering design values for the f coefficient could be discussed at a future stage.
Abstract