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  What are DOT placards?
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Each state has its own requirements for how to label dangerous chemicals in factories or in labs, for example (and by and large, these rules are adopted from the NFPA or other non-governmental sources). The open roads are a special case, though – what if a tanker carrying per chlorate crashes en route to a factory? Emergency personnel need to be able to identify the substance in a hurry, and they shouldn't have to figure out what state the tanker's from to do so. Clearly, the states need a way to label toxic substances as they're in transit.

That's why Congress passed the Hazardous Materials Transportation Act in 1975, which put the regulation of hazardous cargo squarely on the shoulders of the Department of Transportation. In order to keep the public as safe as possible, the Code of Federal Regulations now requires shippers to label the goods they're transporting, to keep paper documentation on dangerous chemicals, and to maintain a placard on the outside of any vehicle that's transporting toxic cargo. Labels on chemical drums serve to help inspectors and anyone loading or unloading the substances, but placards are meant to help law enforcement and first responders, but the pictograms are also easily deciphered enough as to let the public know what's onboard in general terms.

Small amounts don't need to be placarded – anything over 1,000 lbs. does need a placard, but small amounts don't (which is why you don't need a placard on your car for that bottle of peroxide from the pharmacy). Placards are 273 mm by 273 mm (10.75" x 10.75") diamonds, and they have to be visible from the front, back, left and right of the vehicle. Besides a placard designating the general type of hazard, vehicles carrying chemicals also need UN/NA numbers, which are four digits that designate the chemical or family of chemicals being carried. NA are for North America, and are mostly identical to UN (United Nations) numbers, with a few more specific entries at the top end of the range (8,000-9,000). These are often located next to the placard, although some placard designs include space for the UN/NA number in the middle of the diamond.
Flammable Placards This tanker is carrying several flammable petroleum substances, sodium hydroxide solution (a corrosive), and two general classes of chemicals: environmentally hazardous liquids and combustible liquids.
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