Major changes in vehicular design have occurred in the United States during the last 25 years that have affected both the computation and measurement of stopping sight distances (SSD). First, a brief review of the changes in the design parameters of symmetrical-crest vertical curves, as reflected in the 1984 AASHTO manual, is presented. Second, an analytical approach computing the length of unsymmetrical curves to provide SSD requirements, for which no design guidelines are currently available, is presented. Unsymmetrical curves may be warranted in special situations with constrained geometrics, e.g., freeway ramps, grade separation structures, multiple control points, etc. A parameter (gamma) is introduced as an indicator of nonsymmetry in the computation of length of unsymmetrical-crest curves. For values of gamma less than unity, the procedure presented results in a longer curve length than that used for a symmetrical curve, with the maximum length occurring at gamma equals 0.38 for driver's eye and object heights of 3.5 and 0.5 ft, respectively. Overall, unsymmetrical curves required longer lengths than those currently used for symmetrical curves. A complete procedure for setting up unsymmetrical curves to meet SSD requirements is presented along with a technique to locate the highest point. Finally, recommendations are made for further research for formalising additional design guidelines for unsymmetrical curves.
Samenvatting