Influence of Combined Highway Grade and Horizontal Alignment on Skidding

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Influence of Combined Highway Grade and Horizontal Alignment on Skidding

D. F. Dunlap / P. S. Fancher / R. E. Scott / C. C. MacAdam / L. Segel

As the result of greater-than-normal highway skidding accident experience at several downgrade curve sites, this project was initiated to develop guidelines for highway geometrics and pavement surface characteristics to ensure adequate vehicle control during anticipated maneuvers at such highway sites. Based on the combined findings from (1) an extensive analysis of accident data, (2) computer simulation studies, and (3) an in-depth field investigation of two high-accident sites, the researchers have concluded that drivers are not likely to lose control of their vehicles on properly designed and built curve-grade sites unless they are attempting severe maneuvers on slippery road surfaces with fair to poor tires. AASHTO design procedures provide practical methods for arriving at reasonable geometric designs for sites with combined horizontal curvature and vertical grade, provided (1) the selected values of superelevation are large enough to result in adequate pavement surface drainage and (2) the pavement skid resistance is sufficient for anticipated vehicle maneuvers. However, misinterpretation of the AASHTO design procedures has resulted in the design and construction of some long-radius curves with inadequate superelevation for surface drainage, thus contributing to an extraordinary wet-weather accident rate at this type of site.