Development and Validation of Vehicle Safety Margin for Design of Highway Horizontal Curve

The basic equation for designing highway horizontal curves in standards of China and Western countries is the result of dynamic analysis of the rigid-body vehicle model. The critical situation for a vehicle running in a curve is derived by balancing the desired lateral acceleration and the available lateral friction coefficient of pavement. However, due to the complex lateral nonlinear characteristics of tires, a vehicle loses track holding capability before a total sideslip occurs, especially on pavement with low friction. Besides, maximum lateral acceleration cannot reflect the steering capability of a vehicle. In this work, new algorithms determining the safety margins of yaw rate and sideslip angle of vehicle mass center were proposed to characterize the critical scenarios of vehicles running in curved road sections. The sideslip angle margin was determined by considering the controllable lateral tire forces on roads with various adhesion conditions, while the yaw rate margin was derived based on stability analysis of a 7 DOF (degree-of-freedom) vehicle model which included magic formula tire models. The comparison between the proposed safety margin and the upper bound of lateral acceleration employed in standards in terms of accuracy were undertaken through simulations in curves with various radius, speeds, and friction coefficients using Carsim. It was indicated that the proposed safety margin was more accurate and capable of indicating total sideslip and steering failure comprehensively.