1. A warning system for a vehicle, the warning system comprising a back-end system and at least one exceptional road-condition warning device for a vehicle, wherein the back-end system comprises a storage device, storing a traffic information database, wherein the traffic information database is used for storing exceptional road-condition warning event information; and a cooperative self-learning unit, receiving one or more exceptional road-condition events transmitted from the at least one exceptional road-condition warning device, and determining whether to modify the exceptional road-condition warning event information and further update the content of the traffic information database; and each of the at least one exceptional road-condition warning device comprises an advance sensing and warning unit, obtaining vehicle positioning information and obtaining the exceptional road-condition warning event information from the traffic information database, and comparing a plurality of warning locations corresponding to the exceptional road-condition warning event information with the vehicle positioning information, so as to judge whether to generate a warning signal corresponding to the exceptional road-condition warning event information, wherein the determining whether to modify the exceptional road-condition warning event information comprises: judging whether a portion of the exceptional road-condition warning event information corresponding to the received exceptional road-condition events exists, and if not, calculating a confidence count corresponding to the exceptional road-condition events; if another one of the exceptional road-condition events which is the same as the exceptional road-condition events is received again, further adjusting the confidence count corresponding to the exceptional road-condition events; and judging whether the confidence count is higher than a confidence threshold, and if yes, adding the portion of the exceptional road-condition warning event information corresponding to the exceptional road-condition events.