Safety comparison of superelevation changes in broken back curves on the side friction of vehicles on rural highways

Horizontal curves as one of the most dangerous places on roads affect the vehicle's balance, which depends on various factors such as vehicle type and superelevation. Side friction, one of the most critical parameters in designing curves, ensures driver safety and comfort. Previous studies have not adequately addressed the specific safety characteristics and challenges of broken back curve as a type of horizontal curve, especially for different types of vehicles and superelevation modes. Therefore, in this research, the impact of superelevation changes on the side friction of broken back curves was investigated on rural highways. To examine the effects of various factors on vehicle dynamics, CarSim and TruckSim software were applied to simulate 378 scenarios. These scenarios included various road geometries, vehicles, curves, and speeds. The simulations were performed on three types of vehicles (E-class Sedan, E-class SUV, and two-axle conventional loaded truck). Finally, the multiple regression analysis was carried out to assess the impact of various parameters on the side friction factor. According to the results, the longitudinal grade, vehicle speed and curve radius significantly affected the side friction factor, so that -8% grade and lower amounts of speed and curve radius increased the side friction of the broken back curves. Sedans and SUVs in continuous and trucks in discontinuous superelevation change conditions experienced more side friction, which allowed drivers more control over the vehicle. The multiple regression analysis results of the side friction factor showed that the truck regression models had the highest R² values.