Final answer:
When descending a hill in a commercial vehicle, it is illegal to exceed the posted speed limit. Using the engine brake or service brake is essential for controlling the vehicle's speed by converting gravitational potential energy into heat.
Step-by-step explanation:
When going downhill with a commercial vehicle, it is unlawful to exceed the posted speed limit. Commercial drivers must adhere to all traffic laws to ensure safety on the roads. Observing the speed limit is particularly important when descending hills since the gravitational potential energy of the vehicle can lead to increased speed. Utilizing the engine brake or service brake can aid in maintaining a safe speed by converting this potential energy into increased internal energy (higher temperature) of the brake material, thus controlling the vehicle's kinetic energy. Employing the parking brake for this purpose is not recommended as it is not designed for speed control and can lead to premature wear or failure.