It is mounted on a long (usually slender, whiplike) pole that projects high enough from the vehicle to be clearly visible above the dust or sand cloud generated while the vehicle is in motion.
The flag indicates the actual position of the vehicle within the cloud which can be important for safety reasons.
It is similar, although generally larger and taller than, safety flags used by some bicyclists in urban areas, which allow them to be more visible when in front of a tall vehicle, or obscured by an object or vehicle.
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