A lane departure warning system has a trailer detector for detecting that a trailer is attached to a vehicle, a lane boundary detector for detecting a position of a lane boundary on a roadway over which the vehicle is travelling, and an information processor including a calibrator for generating calibration parameters used in determining an unintended lane departure event with respect to the lane boundary. The information processor initiates a process for allowing a vehicle operator to provide trailer width information when the trailer detector indicates that the vehicle has the trailer attached thereto. The trailer width information is used by the information processor for generating the calibration parameters as a function of a width of the trailer.