The short version
A seat belt and an airbag are designed as a pair. The belt holds you in place so that your body is where the airbag expects it to be, and the airbag cushions the part of you that the belt cannot, mainly the head and chest. An airbag is formally described by many manufacturers as a supplemental restraint, which is a polite way of saying it is meant to supplement the belt, not replace it.
What the seat belt does first
In a collision, the car stops very quickly, but anything unrestrained keeps moving at the car’s earlier speed until something stops it. The belt ties you to the car so you slow down with the vehicle, rather than moving into the steering wheel, dashboard or windshield. It also keeps you inside the car, which matters in rollovers and side impacts.
In many cars, a pretensioner fires within the first instant of a crash to pull the belt snug, and a load limiter then lets a small amount of belt out to reduce the peak force on your chest. These parts are why a properly worn belt does more than just hold you in.
What the airbag adds
Sensors detect a rapid deceleration or impact, and a control unit decides whether to inflate the bags. If it does, a gas generator fills the bag within a few thousandths of a second, far faster than you can react. The bag then begins to deflate through vents right away, so it acts as a cushion that you sink into, not a hard balloon.
Airbags are placed for different kinds of impacts:
- Front airbags in the steering wheel and dashboard.
- Side airbags in the seat or door, protecting the torso.
- Curtain airbags along the roof edge, protecting the head in side impacts and often in rollovers.
- Knee airbags, in some cars, which reduce leg injuries and help keep the body positioned.
Airbags do not inflate in every crash. A minor impact, a gentle rear hit or a collision from the side may not trigger the front bags, and that is normal. The belt, however, is working in every crash.
Why they need each other
If you are not wearing a belt, you may move toward the bag before it is fully inflated, or at an angle it was not designed for. An inflating bag is a fast-moving object, so contact in the wrong position can cause injury instead of preventing it. Without a belt, the bag also does little in side impacts, rollovers or a second impact, since it has already deflated.
With a belt, the body is positioned so that the bag meets the chest and head in the intended place, and the two share the forces between them. This pairing is the reason that seat belts are legally required in most places and that regulators generally treat airbags as an addition.
Why seating position matters
Because the bag inflates with force, how close you sit to it matters.
- Distance from the steering wheel. Safety organizations often advise leaving about 10 inches (about 25 cm) between your breastbone and the center of the steering wheel, if you can still reach the pedals and see the road. Tilting the wheel so it points toward your chest instead of your face can help. If you find you must sit much closer than this to drive, a qualified specialist can advise on options.
- Sit upright, back against the seat. Leaning forward, slouching or reclining far back changes where the belt and bag meet your body. A reclined seat can let the lap belt slide up, which is a particular concern.
- Belt fit. The lap belt across your hips, the shoulder belt across the middle of your collarbone and chest. Never put the shoulder belt behind your back or under your arm.
- Hands and arms. Holding the steering wheel at the sides, roughly at the nine and three positions, keeps your arms out of the bag’s path better than holding at the top.
- Nothing between you and the bag. Do not put feet on the dashboard, hold objects on your lap in front of a bag or place pets or items on the dashboard. Do not attach accessories over airbag covers.
You can see how setting up your driving position also improves your view in the article on mirror adjustment.
Children and airbags
Children are lighter and smaller, and a deploying front airbag can be dangerous for them. They are generally safest in the rear seat in a restraint suited to their size. A rear-facing child seat must not be used in front of an active front airbag. The guide to child seats covers this in more depth, and your vehicle manual explains whether a passenger airbag can be turned off.
Warning lights and repairs
An airbag or restraint warning light usually indicates that the system has found a fault and may not deploy properly. Do not ignore it. After a crash in which airbags deployed, the bags, and often the seat belt pretensioners, need to be replaced by a qualified repairer. Be cautious about unusually cheap repairs, and check that a used car has had any airbag recall work completed. A history report may show whether airbags previously deployed.
What to take away
Buckle up on every trip, adjust the seat so you sit upright with some distance from the wheel, and treat any restraint warning light as a reason to get the car checked. Airbags are well designed, but they are most effective when they are working with a correctly worn seat belt.