I've just spent the better part of an hour trying -- unsuccessfully
-- to find any reference to a US or Washington State law that forbids
carrying acetylene, compressed gas, or any hazardous material, in a
passenger vehicle. The closest I could come was a specific reference
to the Washington State Ferry system, you can't take your acetylene
tank on the ferry, and it seems odd to me that they'd specify that if
you can't legally have it in your car at all. The US DOT regs
discuss hazmat policies, but hazmat is specifically defined by the
DOT as "A substance or material capable of posing an unreasonable
risk to health, safety, or property when transported in commerce."
and I don't think taking one's tank home from the local welding
supply house is considered "transported in commerce", as the regs
discuss passenger aircraft vs. cargo aircraft. But I could be
wrong, stranger laws have been written... could you provide a cite
for the illegality of said transport in a passenger vehicle?
I also can't believe my local welding supply house wouldn't have
mentioned that detail, when they told me to be sure I had it stowed
securely. Like the bartender who lets a drunk patron drive off,
wouldn't they be legally partially liable for any damages, if they
knowingly allowed me to drive off with it, unlawfully?
I'm not suggesting that one should take unnecessary risks with the
stuff, but there's a difference between "illegal" and "risky".