2005 Commercial Driver’s License Manual four directions. They must beat least 10 3/4 inches square, turned upright on a point, in a diamond shape. Cargo tanks and other bulk packaging display the identification number of their contents on placards or orange panels. Identification Numbers area four digit code used by first responders to identify hazardous materials. An identification number maybe used to identify more than one chemical on shipping papers. The identification number will be preceded by the letters NA or UN. The US DOT Emergency Response Guidebook (ERG) lists the chemicals and the identification numbers assigned to them. Not all vehicles carrying hazardous materials need to have placards. The rules about placards are given in Section 9 of this manual. You can drive a vehicle that carries hazardous materials if it does not require placards. If it requires placards, you cannot drive it unless your driver license has the hazardous materials endorsement. See Figure 2.25.