Evaluating consistencyof two-lane rural highways: an approach for little twisty alignments

One technique used to improve road safety is to check the consistency of the alignment. Design consistency implies that road geometry and environment does not violate either the expectation or the ability of the motorist to guide and control a vehicle in a safe manner and in predictable ways. One of the most common consistency measure is check speed variation along the alignment. A fundamental question in checking consistency is the speed upon which the road alignment should be based. The design speed concept was used, in the past, and is used again in some standard like Italian one. Recently, in many standards, the use of design speed, to check consistency or to determine individual geometric elements like superelevation rates, was replaced by operating speed, indicated like 85th percentile speed. Operating speed is the real answer to the driver expectancy and is, generally, higher than design speed. In particular all studies have indicated that operating speeds exceed design speeds less than 90 km/h at horizontal curves on rural two lane highways. A recent research in the Department of civil Engineering of University of Trieste on two lane rural highways, showed that, on roads with little twisty alignments, the operating speeds, on horizontal curves and tangents, was not only always higher than design speeds but even higher than maximum permissible speed of road category; for that reason was impossible to use design speed of Italian standards to check consistency on these roads. Moreover it was impossible to use operating speed for checking consistency because these speeds were higher than maximum permissible speed. In these road it should be made provision for a speed control in order to limit the maximum operating speed to the maximum permissible speed. The results of the study indicate that it is possible to use speed consistency on roads with little twisty alignment, according with Italian speed limits, only if there is a control of the maximum operating speed, moreover it is suggested a particular kind of speed control. Speed control was realized limiting inferiorly the curvature change rate (CCR) of the road section according with the environmental speed.