Good question to ask about..

One thing I have been told by the folks at Tractor Supply that sells welding gases is that the cylinders MUST be securely transported standing upright versus laying down.. There are huge dollar amount fines if you are caught violating..

Guess this is so the valve is not down in liquid contents which would make a huge difference in escape rate if the valve were to leak... or be damaged in an accident..

One thing to keep in mind with CO2 (as in CO2/argon shielding gases or the CO2 bottles people carry to air up tires with) is while is is not flammable, you could easily die from asphyxiation if there was much of a leak in a confined area.. From what I've read, you loose consciousness w/o warning as with carbon monoxide.

Propane for example expands from a liquid to a gas at a volume rate of 270:1.. So a small liquid leak would present a huge gas volume..

I'm familiar with NFPA 58 which is the fire code for propane.. very stringent regulations when dealing with compressed gases in liquid form vs gaseous form..

btw the little propane torch/oxygen bottles (i.e. Bernz-o-matic) have specific exemptions, at least in NFPA58, regarding transport, storing in your house, etc due to their small size.. Any larger cylinder has to meet increased requirements/prohibitions.

Maybe some one else has more specifics on the transportation laws...

Just happens that I'm going to a welding supply store this morning.. I'll see what they have to say...