What is unlawful to do when going downhill with a heavy vehicle?

Coasting while going downhill with a heavy vehicle is considered unlawful because it can lead to dangerous situations. When a driver allows the vehicle to coast, they are not fully in control of the vehicle’s speed. This can cause the vehicle to gain momentum and speed up unintentionally, increasing the risk of losing control.

Additionally, heavy vehicles require sufficient braking force to maintain safe speeds, especially on downhill grades. By coasting, the driver may not be using the vehicle's braking system effectively, leaving them unable to respond adequality if rapid speed reduction is necessary.

The use of low gear is an appropriate technique for controlling speed when descending a hill, as it helps to utilize the engine’s braking power. Speeding up, on the other hand, is unsafe because it can quickly lead to an uncontrollable situation on a slope. Continuously applying the brakes can also be problematic since it may lead to brake overheating. Thus, coasting is specifically unlawful in this context because it compromises the control and safety of the vehicle when navigating downhill.