Final answer:
The question relates to a traffic safety law which prohibits passengers from hanging onto or riding on the outside of a moving motor vehicle to ensure their safety.
Step-by-step explanation:
The question being asked pertains to traffic regulations which is an element of law. Specifically, it is concerning the safety rules that govern how passengers should behave in or on moving vehicles. The blank should be filled in with the appropriate part of the vehicle which passengers are prohibited from hanging onto or riding on when the vehicle is in motion.
In most jurisdictions, the legal phrasing of the rule is that no person shall hang onto or ride on the outside of any moving motor vehicle. This rule is in place to prevent injuries or fatalities that could occur from falling off a vehicle or being struck by another object while the vehicle is moving.
This rule is typically taught in driver's education programs and is important for both the driver and passenger to understand for their safety.