Hate to tell you, but placarding is federal, it is NOT state by state. 49 CFR 172.504 (Title 49 of the Code of Federal Regulations section 172.504) If they have not bothered you it is because you have not been pulled over by a DOT trained officer yet. Diesel tanks are usually no more then 110 gallon. Most often people are not caught until something happens. The fines get into 5 figures quickly, at my old job a competitor got a 10,000 fine for shipping 1 gallon of a flammable liquid as non-hazmat. The liquid got damaged and they got popped.
NY had a good summary on their DOT website, but this part applies everywhere I believe.
Cargo fuel tanks shall not exceed 150 gallon capacity per vehicle. Vehicle transporting fuel in cargo tanks of 115 gallons or more shall display either a hazardous materials placard indicating identification number NA 1993 for diesel fuel or a placard indicating 'Fuel Oil'. When empty, the ......
I'm not being pompous, I'm just listing it point by point so I don't leave anything out.
1) Semi's routinely have (1) 150 gallon tank on each side of the truck. Some of these trucks even run auxillary tanks and carry even more fuel. Diesel tanks on semi's are almost always more than 110 gallons and do not have placards on them.
2) The summary that you quoted was from the NY DOT website, but it was in the section dealing with "NYDOT Employee Safety and Health", meaning that those are the regulations for their employees when operating NY State vehicles, not the rules for the general public.
3) I would be interested in seeing your competitors ticket for $10,000 over a gallon. Was it a dangerous corrosive liquid? I'm not disputing that it happened, I'd just like some additional facts.
4) I have never been stopped by the DOT but I have stopped and spoken with the DOT. All have indicated that unless it is a random fuel check, they do not engage private vehicles UNLESS the vehicle is blatantly unsafe. Even then, they'll usually call in the local or state police to finish the check. I have been stopped with my 105 gallon tank in the bed of my truck along with a half full tote (about 135 gallons) and haven't had any problems. I've crossed from MI to Canada with ~175 gallons of WMO in a tote on my trailer and haven't had any problems. The DOT (Department of Transportation) and Small Business Commercial Transportation Federal and State laws pertain to commercial motor vehicles, not private, and definitely not to RV's (technically they do apply to RV's but not really).
5) I don't know everything but I do know that in NY I am violating NY law when I transport WMO in greated capacities of 60 gals since they have a limit of 500 lbs. That being said, I've talked to several LEO's who all indicated that unless I'm pulled over, have oil leaking all over my truck and on the ground, and am using containers that clearly aren't safe, they could care less about my 60+ gals of oil since there's trucks flying by with 350+ gallons of much more volatile diesel, not to mention all the equipment trucks that have 100 gal auxillary bed tanks.
If I were to go with Federal Regulations, they don't seemed concerned in the least as long as I'm a private guy carrying less than 3500 gallons and/or the WMO is going to be used to power the vehicle to which it's attached.
http://www.gpo.gov/fdsys/pkg/CFR-20...011-title49-vol2-subtitleB-chapI-subchapB.pdf
True, my experiences are anecdotal but it's also a reality that a private guy carrying a tote (or two) full of diesel in a covered trailer, with a blanket on the tote, isn't going to attract any attention.