GMC 2003 C4C042 Owner's Manual page 110

CAUTION:
It is dangerous to get out of your vehicle if the
shift lever is not fully in PARK (P) with the
parking brake firmly set. Your vehicle can roll.
Don't leave your vehicle when the engine is
running unless you have to. If you have left the
engine running, the vehicle can move suddenly.
You or others could be injured. To be sure your
vehicle won't move, even when you're on fairly
level ground, always set your parking brake and
move the shift lever to PARK (P), if equipped.
CAUTION:
Shifting into a drive gear while your engine is
"racing" (running at high speed) is dangerous.
Unless your foot is firmly on the brake pedal,
your vehicle could move very rapidly. You could
lose control and hit people or objects. Don't shift
into a drive gear while your engine is racing.
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