The **seat belt** is a crucial element for the safety of all vehicle occupants. Its main function is to protect people in the event of an accident, preventing them from being displaced from their seats or ejected from the vehicle. Wearing the seat belt properly fastened **doubles the chances of surviving an accident**.

It is mandatory for **all persons** in a vehicle with installed seat belts to use them, both on **urban roads** (within cities and towns) and on **interurban roads** (highways, expressways, and motorways). Although there are some exceptions for not using the seat belt, these apply only in specific situations and generally within urban areas, such as when a taxi driver is on duty or a delivery person needs to frequently get out of the vehicle.

Therefore, the correct answer is that the seat belt must be used **both on urban and interurban roads**.