What must not protrude beyond the sides or top of any vehicle?

The fuel supply cylinder or its appurtenances must not protrude beyond the sides or top of any vehicle because this is crucial for safety and compliance with regulations. When the fuel supply cylinder or any of its components extends beyond the vehicle's dimensions, it increases the risk of accidents, potential damage to the vehicle, and poses hazards to other road users.

Maintaining the cylinder within the vehicle's profile ensures that it is protected from outside impacts and minimizes the risk of leaks or failures that could occur if the components are exposed to external forces. This is particularly important since compressed natural gas is stored under high pressure, and any failure in the cylinder or appurtenances could lead to dangerous situations.

In contrast, elements like the driver compartment, exhaust system, or rear bumper may have different safety and design considerations that allow for some variation in their placement regarding the vehicle's overall dimensions. However, the fuel system's integrity and safety are paramount, which is why strict limits are placed on its visibility beyond vehicle boundaries.