What is unlawful behavior when driving a commercial vehicle downhill?

Coasting while driving a commercial vehicle downhill is considered unlawful behavior because it can lead to a loss of control over the vehicle. When a driver coasts, they disengage the engine from the wheels, which prevents the engine from helping to control the vehicle's speed. This can result in the vehicle gaining excessive speed and reduces the driver's ability to respond effectively to changes in the road or traffic conditions.

Maintaining a steady speed is important for safety, and using the engine's braking power or transmission to help manage speed is advised when navigating steep grades. Utilizing the parking brake would typically be reserved for stopping the vehicle safely, rather than controlling its speed while in motion. Therefore, coasting does not align with the safe operational practices expected of commercial vehicle drivers, especially in downhill situations that require heightened awareness and control.