You are driving a heavy truck down a steep and long descent. When should you shift to an appropriate low gear?

1. The gears are insignificant for slowing down, but you must use the speed retarders intelligently.
2. There is no need to shift to low gear. You should shift to high gear and regulate your speed with the speed retarder, to increase safety.
3. You should shift to low gear towards the end of the descent.
4. Correct answer: You should shift to low gear at the beginning of the descent.

WhyOn a long, steep descent, shift to a low gear at the very beginning, not once you have already picked up speed.