The major hazard of not being in the right gear while descending a mountain road is that:

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The major hazard of not being in the proper low gear before starting a descent on a mountain road is that the driver may not be able to shift into a lower gear once the vehicle picks up speed. It is critical that drivers be in the proper gear before they begin their descent. There have been times when drivers have attempted to downshift during the descent and were not able to get into any gear at all, thus travelling downhill, in effect, in neutral. This can lead to serious, even fatal crashes.

The Major Hazard of Not Being in the Right Gear