Buick Lucerne 2007 Owner's Manual page 38

The lap part of the belt should be worn low and
snug on the hips, just touching the thighs. In
a crash, this applies force to the pelvic bones. And
you would be less likely to slide under the lap
belt. If you slid under it, the belt would apply force
at your abdomen. This could cause serious or
even fatal injuries.
38
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 safety belt locks if there is a sudden stop or
a crash.
CAUTION:
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You can be seriously hurt if your shoulder
belt is too loose. In a crash, you would
move forward too much, which could
increase injury. The shoulder belt should
fit against your body.