Side friction demanded and margins of safety on horizontal curves

The findings of a research project that was conducted to determine the amount of side friction demanded and provided for a range of roadway curvatures, vehicle speeds and types, and pavement surface conditions are described. Seven horizontal curves located on rural two-lane highways in Alberta as well as curves at the Calgary Police Service's Driver Training Facility were used as test sites. A three-axis accelerometer and a ball bank indicator were installed in seven test vehicles, including passenger cars, a half-ton pick-up, and a tandem-axle gravel truck. Lateral accelerations and ball bank readings were taken as the vehicles traversed test curves at constant speeds. Speeds were increased in increments of 10 km/hr until impending side skid conditions were reached. Ball bank readings are regressed upon lateral accelerations for each vehicle type, and equations predicting the implied value of safe side friction, the safe speed of the curve, and the margin of safety provided by the safe speed are developed. Maximum values of side friction demanded on dry and icy roadways are determined and used to calculate the margin of safety provided at various speeds. It was found that the current design standards are quite conservative and provide a more-than-sufficient margin of safety for motorists.