Cadillac CTS 2008 Owner's Manual page 38

CAUTION:
{
Never do this.
Never allow two children to wear the same
safety belt. The safety belt can not properly
spread the impact forces. In a crash, the two
children can be crushed together and seriously
injured. A safety belt must be used by only
one person at a time.
1-34
CAUTION:
{
Never do this.
Never allow a child to wear the safety belt with
the shoulder belt behind their back. A child
can be seriously injured by not wearing the
lap-shoulder belt properly. In a crash, the child
would not be restrained by the shoulder belt.
The child could move too far forward
increasing the chance of head and neck injury.
The child might also slide under the lap belt.
The belt force would then be applied right on
the abdomen. That could cause serious or fatal
injuries. The shoulder belt should go over the
shoulder and across the chest.