When riding a motorcycle, it is **mandatory to wear the safety helmet properly fastened and adjusted** to the head. This is because motorcycles are more fragile and less stable vehicles than other cars, which increases the risk of injury in the event of an accident.

- **Properly fastened helmet:** It is essential to protect the head in case of a fall or collision. An unfastened helmet does not offer the same protection and can be thrown off, leaving the head exposed to serious injuries.

- **Motorcycle safety:** Since motorcycles do not have a bodywork to protect the driver, as cars do, the helmet becomes a fundamental piece of safety. Additionally, it is recommended to wear appropriate clothing, such as tight-fitting leather suits, gloves, and sturdy boots, to increase protection.

Therefore, it is not allowed to wear the helmet unfastened under any circumstances, as it compromises the safety of the driver and the passenger.