Passengers of a **quadricycle** must wear a helmet only if the vehicle does not have a **self-protection structure** or **seat belt**. This is because two or three-wheeled vehicles, such as quadricycles, are more fragile and less stable, which increases the risk of injury in the event of an accident. However, if the quadricycle is equipped with a seat belt, it is not necessary to wear a helmet, as the seat belt provides adequate protection.

The helmet is an element of **passive safety** that protects the head in the event of an accident, distributing the force of the impact and preventing serious injuries. However, its use is not mandatory in vehicles that already have safety measures such as the seat belt.