US Dot
Us Dot requires any pressure vessal that transports on the highway any flammable liquids to be hydrotested and recertified every 12 years, my propane guy even has a go-nogo gage for dent sizes.... the tank in your yard is sort of exempt, until it leaves and is then retransported to a new customer. I have seen thirty year old 1000 gal. tanks, but they get Eyeballed now and then by the service company. Not sure about vehicle LP or LNG tanks, but I gotta believe they get tested too! The LNG tanks are about 3600 psi, going down the road just waiting for an accident, and they are fibre glas, Scary thought.
I just paid $1.948 a gallon here in MI. for Lp. It was $2.37 a couple of years ago.