Buick 2001 Century Owner's Manual page 16

But don't have a seatback reclined if your vehicle
is moving.
1-4
CAUTION:
Sitting in a reclined position when your vehicle is
in motion can be dangerous. Even if you buckle
up, your safety belts can't do their job when
you're reclined like this.
The shoulder belt can't do its job because it
won't be against your body. Instead, it will be
in front of you. In a crash you could go into it,
receiving neck or other injuries.
The lap belt can't do its job either. In a crash the
belt could go up over your abdomen. The belt
forces would be there, not at your pelvic bones.
This could cause serious internal injuries.
For proper protection when the vehicle is in
motion, have the seatback upright. Then sit
well back in the seat and wear your safety
belt properly.