What should a driver do when driving downhill with an automatic transmission and the gearshift is in D?

Using the engine brake while driving downhill with an automatic transmission set in Drive (D) is the most effective way to control the vehicle's speed. When the vehicle is in Drive, the transmission automatically shifts between gears based on speed and throttle input, but it can’t provide the same level of braking force as downshifting to a lower gear or engaging the engine brake effectively.

Engine braking occurs when the driver takes their foot off the accelerator, allowing the engine to slow the vehicle down as it resists rotation. This resistance helps reduce the reliance on the foot brake, which can overheat if used continuously, especially on long downhill stretches. Engaging the engine brake aids in maintaining a manageable speed without putting too much strain on the braking system, promoting safer driving conditions.

Coasting without using brakes could lead to uncontrolled speed, especially on steep declines. Shifting to a lower gear position is also a good technique, but if the gearshift is already in Drive, the most immediate way to enhance control and use the braking capabilities of the engine is through engine braking itself. Therefore, while other options might have some merit in different contexts, using the engine brake is the most appropriate and effective method for managing speed on a downhill slope in this scenario.