49 CFR 172.504(a) states that when required each bulk packaging, freight container, unit load device, transport vehicle, or rail car “must be placarded on each side and each end”. 49 CFR 172.516(a,b) goes on to state that, “Each placard on a motor vehicle and each placard on a rail car must be clearly visible from the direction it faces, except from the direction of another transport vehicle or rail car to which the motor vehicle or rail car is coupled.”
A challenge complying with the relatively simple requirement to placard all four sides and be done with it arises when separate transport vehicles or rail cars are coupled together for a shipment of hazardous materials. It is quite possible that one or more placards will not be “clearly visible” from the direction of the nearest transport vehicle or rail car, what then. The solution for the rail car is simple: 172.516(a) states that a placard’s visibility may be obstructed without a violation of the regulations if the cause of the obstruction is another rail car to which it is coupled. So, when coupled together, the front and rear facing placards on a rail car may be obstructed without a violation. Note however, that though obstructed from view, placards must still be present on each side and each end if required.
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Compliance is trickier on the highway with its myriad of trucks and tractor-trailer combinations. The answer hinges on the definition of a transport vehicle, which is: “a cargo-carrying vehicle such as an automobile, van, tractor, truck, semi-trailer, tank car or rail car used for the transportation of cargo by any mode” (LOI 09-0055 & 08-0098). A tractor-trailer combination fits the definition of two separate transport vehicles, whereas a straight truck is a single transport vehicle. Therefore, a placard on the front of a trailer in a tractor-trailer combination may be obstructed by the presence of the tractor or truck without violating 49 CFR 172.516(a) (LOI 05-0063). Similarly, the front and rear facing placards on a trailer that is part of a multi-trailer load may be obstructed by the presence of additional trailers without a violation. However the front facing placard on a straight truck would be in violation of this part if it was not visible from the front. (LOI 04-0164).
If you wish for the front facing placard on a tractor-trailer combination to be visible to the front, you may mount the placard on the front of the tractor or rig. 49 CFR 172.516(b) allows for the front facing placard of a HazMat shipment to be affixed to the front of the truck-tractor instead of or in addition to the placarding of the front of the semi-trailer. This would also satisfy the requirements of 49 CFR 172.516(a). Note however, that if the tractor and trailer become separated, say if the HazMat shipment is stored incidental to transportation, then a front facing placard would be required for the trailer.
You may also be interested in this article on the General Display Requirements for Placards.
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