The maximum to be met before starting the car is that all occupants are properly restrained (not doing so carries a fine of 200 euros and the withdrawal of four points from the driving license). But there is an added recommendation (this one without penalty): fasten the rear seat belts even if you are alone in the car.
In Spain, as in most European countries, the use of a seat belt is mandatory in all seats in the vehicle. The mission of this element is to minimize the damage that the occupants may suffer in the event of an accident, whether due to frontal collision, side collision, leaving the road…
Its benefit is double because it protects those it holds from being thrown from the vehicle and/or impacting against the hard parts of the passenger compartment. It also protects the rest of the occupants from the possible impact against the one it retains; this is what the DGT calls the elephant effect and that causes a loose object/person inside the vehicle to weigh tons in the event of an impact.
Why you should fasten the rear seat belts even if you travel alone
This last statement is what explains why road safety experts recommend fastening the seat belts in the rear seats even if no one travels in them.
The advice is especially important when the trunk is full of luggage. And this gives a clue as to why…
In the event of sudden braking or impact, the objects that we transport in the trunk will hit the backrest of the rear bench seat with great force. We remember here the elephant effect that ensures that at a speed of 60 kilometers per hour, the impact of a 75-kilogram person against the front seat can be equivalent to a force of no more and no less than 4.2 tons.
If the rear seat belts are fastened at the moment of impact (even if without occupants) they will help contain the impact and minimize the risk of cargo entering the boot.
For maximum safety, it is recommended that the seat belts be worn even when there is no load in the boot. In this way, in the event of a hit from a distance (from the rear), the behavior of the rear seat backrests will improve.