2012 Toyota Prius Owners Manual - Page 744

5-2. Steps to take in an emergency *: Driver's and front passenger's seat belt buzzer: The driver's and front passenger's seat belt buzzer sounds to alert the driver and front passenger that his or her seat belt is not fastened. Once the engine switch is turned to the "ON" or "START" position (vehicles without a smart key system) or the "POWER" switch is turned to ON mode (vehicles with a smart key system), the buzzer sounds for 6 seconds. If the vehicle reaches a speed of 12 mph (20 km/h), the buzzer sounds once. If the seat belt is still unfastened after 30 seconds, the buzzer will sound intermittently for 10 seconds. Then, if the seat belt is still unfastened, the buzzer will sound in a different tone for 20 more seconds.