When is it necessary to use low gears when driving a loaded vehicle downhill?

Using low gears when driving a loaded vehicle downhill is crucial for maintaining speed control. When a vehicle is heavily loaded, the weight significantly increases the momentum as it descends a slope. If the driver relies solely on the brakes, it can lead to overheating or brake failure due to the intense pressure exerted during prolonged braking.

By shifting into a low gear, the driver allows the engine to act as a form of resistance or braking, which helps to naturally control the speed of the vehicle without over-relying on the brakes. This practice not only improves safety but also ensures that the driver maintains better control of the vehicle, especially on steep grades.

The other options do not adequately address the fundamental reason for using low gears downhill. Adverse weather conditions, while a valid concern, are not the only reason for this practice; instead, the act of using low gears serves a more general safety purpose regardless of weather. Approaching a steep hill or being at the hill crest is about the timing of the action rather than the necessity of slowing down, which the correct answer clearly encapsulates.