Models of operating speeds for low-volume roads

Recent studies have documented a noticeable disparity between the speeds for which roads are designed and the actual operating speeds. An important reason for driving errors on two-lane rural roads is the misjudgment of the real course of the road by road users. Unfavorable visibility conditions are one of the problems of road infrastructure that can be detected on existing low-volume roads. Therefore an operating speed model was developed to calculate the contribution made by sight distance, and to be a useful tool for planners in the study of the safety of existing roads. Seven low-volume roads in the province of Salerno, Italy, were studied. The study identified operating speeds on 84 curves; the relative sight distance, radius, radius of the previous curve, length of the tangent before the curve, curvature change rate, development of the curve, and number of speed changes made during the study were determined for each curve. Because of the different characteristics of the roads analyzed, once the data were collected it was possible to develop two operative speed models. The first was suitable for roads characterized by consecutive curves alternating with small tangents and the second model was mainly intended for roads characterized by a small number of curves that were preceded and followed by long tangents