A threshold warning system is installed directly on a conventional lift to fully comply with the SAE standard No. J2093. The threshold warning system comprises a sensor device for sensing whether there is an object crossing the boundary of the threshold zone, and a platform position indication device for determining whether the platform is in an unsafe position. The threshold warning system comprises an audible and/or visual warning device for warning a wheelchair user or a passenger when the user or passenger moves into a threshold zone from the vehicle opening while the platform is in an unsafe level for exiting from or boarding into the vehicle.