Final answer:
1) Coast. It is unlawful to coast downhill with a commercial vehicle because it reduces the driver's control. Drivers should use gears to regulate speed based on road conditions and safety considerations.
Step-by-step explanation:
When going downhill with a commercial vehicle, it is unlawful to coast. This unsafe practice involves putting the vehicle in neutral or pressing the clutch pedal, which diminishes the driver's full control over the vehicle. Instead, one should remain in gear, and it may often be necessary to shift to a lower gear to increase engine braking and control speed. It's incorrect to say that it's unlawful to remain in the same gear or to travel more than 25 mph downhill, as the appropriate action depends on the specifics of the road, vehicle capabilities, and speed limits. For example, a truck of mass 1.2 tons would need a controlled descent from a 150 m high hill for safety.