A vehicle lane departure warning system and method are provided having a trailer mode. The system includes an imager capturing images of a roadway forward of a vehicle and an input receiving a towing signal indicative of whether the vehicle is towing a trailer. The system also includes a processor for processing the captured images and determining whether the vehicle or trailer is expected to be departing from a lane on the roadway based on the captured images and a threshold width value. The processor selects the threshold width value based on the input towing signal such that an enhanced threshold width value is employed when the vehicle is towing a trailer. An output is provided for outputting a lane departure warning signal based on the processed images and selected threshold width value.