What is the primary operational benefit of using a retarder (Dauerbremse / hydrodynamic or electromagnetic auxiliary brake) on long downhill gradients?

1. It recharges the vehicle's 24V starter batteries at maximum rate
2. It allows the bus to exceed the legal mountain speed limit safely
3. It provides continuous, wear-free deceleration without overheating or fading the wheel friction brakes
4. It automatically cools the engine radiator fluid