Buick 2009 Lucerne Owner's Manual page 72

Your vehicle also has a dual-depth passenger airbag
that adjusts the restraint according to crash severity,
seat location, and safety belt status using electronic
frontal sensor(s) and other special sensors which enable
the sensing system to monitor the position of the front
passenger seat. The passenger airbag inflates to a
reduced depth when the passenger seat is in a forward
position. For more rearward front seating positions,
the passenger airbag may inflate to an increased depth
(a full deployment), based on safety belt status and
the crash severity measured early in the event. (Always
wear your safety belt, even with frontal airbags.)
Your vehicle has seat-mounted side impact and
roof-rail airbags. See Airbag System on page 1-62.
Seat-mounted side impact and roof-rail airbags
are intended to inflate in moderate to severe side
crashes. Seat-mounted side impact and roof-rail airbags
will inflate if the crash severity is above the system's
designed threshold level. The threshold level can
vary with specific vehicle design.
1-68
Seat-mounted side impact and roof-rail airbags are not
intended to inflate in frontal impacts, near-frontal
impacts, rollovers, or rear impacts. A seat-mounted side
impact airbag is intended to deploy on the side of the
vehicle that is struck. Both roof-rail airbags will
deploy when either side of the vehicle is struck.
In any particular crash, no one can say whether an
airbag should have inflated simply because of the
damage to a vehicle or because of what the repair costs
were. For frontal airbags, inflation is determined by
what the vehicle hits, the angle of the impact, and how
quickly the vehicle slows down. For seat-mounted
side impact and roof-rail airbags, deployment is
determined by the location and severity of the side
impact.