The Importance Of Shoulder Belts - Ford 2010 Explorer Sport Trac Owner's Manual

Seating and Safety Restraints
Children and booster seats vary in size and shape. Choose a booster that
keeps the lap belt low and snug across the hips, never up across the
stomach, and lets you adjust the shoulder belt to cross the chest and
rest snugly near the center of the shoulder. The drawings below compare
the ideal fit (center) to a shoulder belt uncomfortably close to the neck
and a shoulder belt that could slip off the shoulder. The drawings below
also show how the lap belt should be low and snug across the child's
hips.
If the booster seat slides on the vehicle seat, placing a rubberized mesh
sold as shelf or carpet liner under the booster seat may improve this
condition. Do not introduce any item thicker than this under the booster
seat. Check with the booster seat manufacturer's instructions.

The importance of shoulder belts

Using a booster without a shoulder belt increases the risk of a child's
head hitting a hard surface in a collision. For this reason, you should
never use a booster seat with a lap belt only. It is generally best to use a
booster seat with lap/shoulder belts in the back seat.
Move a child to a different seating location if the shoulder belt does not
stay positioned on the shoulder during use.
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2010 07+ Explorer Sport Trac (esp)
Owners Guide (own2002), 1st Printing
USA (fus)