I also wouldn't want a cylinder of compressed Natural Gas (Methane) in my trunk. Accident = Death.
Fuel cell cars will have a high pressure bottle of hydrogen aboard. High pressure translates into between 10,000 PSI and 15,000 PSI.
CNG bottles will be at a much, much lower pressure, typically in the 3,500 PSI range. Generally, CNG vehicles are considered less of a fire risk than those carrying liquid gasoline.
The drawback is that CNG will occupy a larger volume within the car to obtain the same range. Because CNG bottles cannot be shaped to fill voids like gas tanks can, they waste volume. These bottles are designed to survive extreme impacts.
Most designs have flow check valves on the outlet side of the bottle's pressure regulator to shut off the gas supply should a line downstream be cut and the flow of gas exceed normal rate. Minor leaks in the engine compartment are detected by methane sniffers, which then shut down the gas via a computer controlled solenoid valve.
My regards,
Widewing