Under what conditions is it obligatory to operate a “speed retarder” while driving downhill?

1. When a “warning sliding” sign is placed on the road.
2. When the slope rate of the decent exceeds 6% and the length of the decent is more than one kilometer.
3. Correct answer: When the vehicle is equipped with a speed retarder and a sign warning about a dangerous descent is placed on the road.
4. If a vehicle is equipped with a speed retarder, it should always be operated, also while driving on plane surfaces.

WhyIf the vehicle has a speed retarder, use it when a sign warns of a dangerous descent. It is not needed on a flat road.