While towing a trailer, factors, such as a lane change, inappropriate tongue weight, and crosswind, can cause trailer sway. This may resonate the vehicle to sway also, and put it in a dangerous situation. The Trailer Stability Assist controls the vehicle, using the same system as the VSA system (VSA modulator-control unit and sensors). With signals from each sensor, TSA detects when trailer sway resonates the vehicle, and then decelerates the vehicle speed to its safer zone, stabilizing the vehicle movements. The brake light automatically turns on when TSA activates.