The driver plans a path by preview and controls the lateral deviation of vehicle from the planned path via the steering wheel. However, the traveling vehicle is always affected by unknown lateral disturbances. This paper presents a design method of a lateral driving assistant system based on functional architecture in order to optimize both software and hardware components. Moreover, the control strategy in this paper is not a class of fully automated driving systems but a class of systems for reducing the driver’s corrective steering effort to keep the vehicle at the desired lane in the travel path.