I've done a search on this. Lots of responses, most of them of the wise-guy "Buckle your seatbelt, dummy" variety, and the rest with broken links. Can someone please post how to disable the damnable thing? No, I don't drive without a seatbelt, or carry a passenger without one. A couple books or a gallon of milk can just take their chances, though.
Lol! Yes, I've heard about Prius owners with dogs or 60 pound bags of dog food or groceries in the passenger seat had this problem! Good luck.