Sharp, horizontal curves on steep downgrades represent a potential safety concern for vehicles, especially heavy vehicles. Examples where this combination may occur are interchange ramp movements, curves on mountainous roads, or high-speed downgrade curves on controlled-access roadways. At these locations, the complicating factors of grade, pavement cross slope, and pavement friction fully tax the driver’s ability to provide correct vehicle positioning without compromising control of the vehicle. Superelevation criteria, horizontal curvature, and other associated geometric criteria needed to be developed for situations where steep grades are located on sharp horizontal curves. The objective of NCHRP Project 15-39 was to develop superelevation criteria for horizontal curves on steep grades. Other criteria associated with design of horizontal curves (e.g., tangent-to-curve transitions, spiral transitions, lateral shift of vehicles traversing the curve, need for pavement widening, and determination of curve radii) were also considered. The research was performed by MRIGlobal and the Pennsylvania State University. Design criteria were developed based on a series of field studies and vehicle dynamic simulations. Field studies were conducted to collect vehicle speed and lane-changing manoeuvre data from locations across the United States, as well as representative samples of tire—pavement friction data for various pavement surface conditions. Vehicle dynamic simulations used AASHTO design criteria in combination with field-measured data. Three classes of passenger vehicles and three classes of trucks were considered for safety analysis. The report provides design guidance based on the analyses for sharp horizontal curves on steep grades. (Author/publisher)
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