2010 Toyota Prius Owners Manual - Page 523

5-2. Steps to take in an emergency *1: Open door warning buzzer: The open door warning buzzer sounds to alert one or more of the doors is not fully closed (with the vehicle having reached a speed of 3 mph [5 km/h]). *2: Driver's and front passenger's seat belt reminder buzzer: The driver's and front passenger's seat belts reminder sounds to alert the driver and front passenger that his or her seat belt is not fastened. The buzzer sounds intermittently for 10 seconds after the vehicle has reached a speed of at lest 12 mph (20 km/h). Then, if the seat belt is still unfastened, the buzzer will sound in a different tone for 20 more seconds.