85th and 98th Percentile Speed Prediction Models of Car, Light, and Heavy Commercial Vehicles for Four-Lane Divided Rural Highways

In this study, the 85th and 98th percentile speed prediction models of car, light, and heavy commercial vehicles for four-lane divided horizontal curves in rural highways were developed using a stepwise multiple linear regression method. In total 10 sites were considered for developing the models and three for validation. Geometric features of horizontal curve and adjacent tangent sections along with vehicle speed at point of curvature and 50 m prior to point of curvature were considered as independent variables. The 85th and 98th percentile speed of car and sport utility vehicle were found to be comparable, and hence combined for further analysis. The 85th percentile speed of commercial vehicles at midcurve were found to be dependent on its speed at preceding section, which was finally predicted based on geometric features considered. For car, it was dependent on curve length and deflection angle. The 98th percentile speed at midcurve of all vehicle types were dependent on its 85th percentile speed. In validation, the root-mean square errors for the developed models were within 10 km/h. The study revealed that curve length could be an important factor in the geometric design procedure.