This paper deals with the design of a lane keeping system in steer-by-wire road four-wheeled vehicles using a nonlinear controller. The controller is designed to take into account the driver's behavior and availability and to control the wheels' angle of the steer system. A high order sliding mode control is chosen as feedback controller, having time-varying upper-bound disturbance strictly depended by the road curvature and by the influence of the wind, which allows an easier tuning process of the parameters based on a physical meaning and a consequent attenuation of the chattering effect. The controller must be robust to minimize the lateral deviation and the heading errors, and it must be sensitive to assist the driver during the shared control mode, i.e. the assist system must give the authority to the driver when the driver is available