When going downhill with a heavy vehicle, what action is unlawful?

Answer: B — Coast

Coasting while going downhill with a heavy vehicle is considered unlawful because it can significantly reduce the driver's control over the vehicle. When a driver coasts, they take their foot off the accelerator and allow the vehicle to roll freely. This can lead to an increased risk of losing control, especially with a heavy vehicle that needs constant braking and handling to maintain safe speeds. The vehicle can gain excessive speed downhill, making it more challenging to respond to hazards or navigate safely. In contrast, proper actions for managing a heavy vehicle on a downhill slope include using engine braking by shifting gears, maintaining control with steady braking if necessary, and avoiding sudden maneuvers. This enhances safety on steep declines and ensures compliance with regulations designed to prevent accidents caused by loss of control.