Final answer:
An adult passenger over 18 without a seat belt is often held responsible for this violation, as laws mandate seat belt use for personal and societal protection.
Step-by-step explanation:
When a passenger over 18 is found without a seat belt, typically, the passenger themselves is held responsible for this violation. Laws regarding the use of seat belts vary by jurisdiction but broadly share the intent of protecting individuals and society from the negative consequences of vehicle accidents. In many places, adults are expected to take responsibility for their safety, and government enacts laws that require seat belt use to minimize both potential injuries to the individual and the subsequent societal costs that can arise from such injuries.