GMC 1999 Envoy Owner's Manual page 30

CAUTION:
If something is between an occupant and an air
bag, the bag might not inflate properly or it
might force the object into that person. The path
of an inflating air bag must be kept clear. Don't
put anything between an occupant and an air
bag, and don't attach or put anything on the
steering wheel hub or on or near any other air
bag covering.
When should an air bag inflate?
An air bag is designed to inflate in a moderate to severe
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frontal or near frontal crash. The air bag will inflate
only if the impact speed is above the system's designed
"threshold level." If your vehicle goes straight into a
wall that doesn't move or deform, the threshold level is
about 14 to 18 mph (23 to 29 km/h). The threshold level
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can vary, however, with specific vehicle design, so that
it can be somewhat above or below this range. If your
vehicle strikes something that will move or deform, such
as a parked car, the threshold level will be higher. The
air bag is not designed to inflate in rollovers, side
impacts or rear impacts, because inflation would not
help the occupant.
In any particular crash, no one can say whether an air
bag should have inflated simply because of the damage
to a vehicle or because of what the repair costs were.
Inflation is determined by the angle of the impact and
how quickly the vehicle slows down in frontal or
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near frontal impacts.
The air bag system is designed to work properly under a
wide range of conditions, including off
Observe safe driving speeds, especially on rough terrain.
As always, wear your safety belt. See "Off
Driving" in the Index for more tips on off
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road usage.
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Road
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road driving.