Buick 2010 Lucerne Owner's Manual page 52

Sit up straight and always keep your feet on the floor in
front of you. The lap part of the belt should be worn low
and snug on the hips, just touching the thighs. In a crash,
2-20
this applies force to the strong pelvic bones and you
would be less likely to slide under the lap belt. If you slid
under it, the belt would apply force on your abdomen.
This could cause serious or even fatal injuries. The
shoulder belt should go over the shoulder and across the
chest. These parts of the body are best able to take belt
restraining forces.
The shoulder belt locks if there is a sudden stop or crash.