Abstract: A fasten-on supplemental restraint system comprising a detecting circuitry and an air bag assembly is provided. The supplemental restraint system is independently located in the vehicle and works with the existing safety belt system. The supplemental restraint system can be readily attached by means of buckle so that the air bag assembly can be readily placed in front of the driver/passenger. Once the air bag body of the air bag assembly is inflated, the direction of expansion is directed forward and outward from the driver/passenger. With this arrangement, the potential risk of being injured by the deployment of the air bag body is therefore eliminated.