A device for assisting in the driving of a motor vehicle includes a line detector for detecting the boundary line of a traffic lane, a setup module capable of establishing a virtual lane from the line detected, a monitoring module capable of monitoring the risk of the motor vehicle leaving the virtual lane established. The device also includes a module for acquiring an image representing a relevant object. The setup module includes a first map containing values of a position of a virtual-lane boundary according to a lateral shift of the relevant object in the acquired image.