http://www.motorage.com/motorage/data/articlestandard/motorage/162005/155698/article.pdf
Today’s belts are made from a
military grade (mil spec) nylon or polyester web.
Just as steel is stronger than iron because it can
bend, these materials are stronger because they
can stretch.
In a crash, a driver’s body can exert thousands
of pounds of force on the belts. The trick to
reducing injury is to balance belt stretch with the
need to hold the driver in place. Even a small
amount of stretch helps to reduce what’s called
“the third collision.” The first collision involves
the vehicle, the second is the driver against the
restraining device and the third causes injury to
the driver’s internal organs.
http://www.sfifoundation.com/seatbelt.html