Final answer:
When driving downhill in a commercial vehicle, it's illegal to use brakes excessively, exceed speed limits, carry excess weight, and operate without headlights, due to the risks of losing control.
Step-by-step explanation:
When going downhill in a commercial vehicle, it is unlawful to engage in several practices that may endanger the driver or others on the road.
These include excessive use of brakes, exceeding the speed limit, carrying excessive weight, and operating without proper lighting such as headlights. When a truck is going downhill, the gravitational potential energy converts into increased kinetic energy.
The brakes are used to control the truck's speed by converting that kinetic energy into heat. However, a large mass such as that of a commercial truck can overwhelm the brake system, leading to extremely high temperatures that prevent the brakes from functioning efficiently.
It's important to manage speed by using engine braking or a lower gear, and by adhering to proper vehicle maintenance and operational practices for safe driving conditions.