The effect of combined horizontal curve and longitudinal grade on side friction factors

Abstract

The relationship between side friction factor and longitudinal grade is investigated for horizontal curves considered in three dimensions, and new models for estimating side friction factors for different vehicle types are presented. The study consists of three parts: (1) a series of simulation tests using the CarSim and TruckSim multi-body simulation software packages; (2) multiplex regression analysis; and (3) presentation of recommended formulas for the side friction factor. Two different driving behaviors are considered in the simulation process: in one, the driver negotiates the curve at constant speed; in the other, the driver brakes while passing downgrades. Side friction factors are also studied separately for each axle of a vehicle. Greater side friction factors are found on downgrades for all vehicle types when cornering. From the viewpoint of skidding, the most critical situation is found for the rear axle of a sedan. In addition, braking is found to have a significant effect on side friction factors. The results and models from this study can be used in the geometric design phase by highway engineers to design safer roads.