2011 Toyota Sienna Owners Manual - Page 776

5-2. Steps to take in an emergency *1:Open door warning buzzer: A buzzer will sound if the vehicle reaches a speed of 3 mph (5 km/h) with any door open. *2:Driver's and front passenger's seat belt warning buzzers: The driver's and front passenger's seat belt warning buzzers sound 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 12 mph (20 km/h). Then, if the seat belt is still unfastened, the buzzer will sound at a different tone for 20 more seconds.