Crash safety is about reducing harm when the accident still happens. In a frontal crash at 50 km/h, anything not strapped down continues at 50 km/h until something stops it — belt, airbag, dashboard or the passenger in front. The belt is the most important protection because it spreads the crash across chest and pelvis over the very short time available. The airbag works as a complement, not a replacement. Loose objects in the cabin become projectiles weighing several times their own mass.
Under Trafikförordningen 4 kap 10 § the seat belt must be used by everyone in a seat fitted with one. Children under 135 cm must sit in an approved restraint — child seat, booster seat or booster cushion suited to the child's height. As the driver you are responsible for ensuring everyone in the car is belted, including adult passengers and children. Loose objects in the cabin must be secured or stowed so they cannot fly forward in a sudden brake.
An adult passenger says they can ride without the belt — it is only four blocks. Three options: start driving because the passenger may decide for themselves, drive and accept the consequence yourself if something happens, or stay put until the belt is on. The right decision is the last one. As the driver you carry the responsibility under Trafikförordningen 4 kap 10 §. And the physics does not care how short the trip is — at 50 km/h the impact equals roughly 25 times the passenger's own body weight against the dashboard.
A common slip on the test is believing the airbag saves you even without the belt. The opposite is true — the airbag is designed to catch a person held in place by the belt. Without the belt you are thrown forward and meet the airbag too early or with the wrong part of the body, which can cause severe facial and neck injuries. The airbag assists the belt, it never replaces it.