In severe crashes, when a car collides with an obstacle at extremely high speed, a seat belt can inflict serious damage. As a passenger's inertial speed increases, it takes a greater force to bring the passenger to a stop. In other words, the faster you're going on impact, the harder the seat belt will push on you.
Some seat belt systems use load limiters to minimize belt-inflicted injury. The basic idea of a load limiter is to release a little more excess belt webbing when a great deal of force is applied to the belt. The simplest load limiter is a fold sewn into the belt webbing. The stitches holding the fold in place are designed to break when a certain amount of force is applied to the belt. When the stitches come apart, the webbing unfolds, allowing the belt to extend a little bit more.
More advanced load limiters rely on a torsion bar in the retractor mechanism. A torsion bar is just a length of metal material that will twist when enough force is applied to it. In a load limiter, the torsion bar is secured to the locking mechanism on one end and the rotating spool on the other. In a less severe accident, the torsion bar will hold its shape, and the spool will lock along with the locking mechanism. But when a great deal of force is applied to the webbing (and therefore the spool), the torsion bar will twist slightly. This allows the webbing to extend a little bit farther.
Over the years, seat belts have proven to be far and away the most important safety device in cars and trucks. They are by no means infallible, however, and car safety engineers see a lot of room for improvement in today's design. In the future, cars will be outfitted with better belts, better airbags and, most likely, completely new safety technology. Of course, the government will still have to address the biggest problem with safety devices— getting people to use them.
For more information on seat belts and other safety systems, check out the links below.