The initials “HC” on a highway sign denote a route for cars delivering hazardous cargo. A similar sign with the letters “HC” enclosed in a red circle and crossed by a diagonal red bar indicates that hazardous cargo is prohibited.
Signs for hazardous goods vary from state to state. In some states, the symbol for hazardous materials is “HM” rather than “HC.” In most cases, the sign has a square white background with black lettering. When hazardous cargo is permitted, the letters “HC” or “HM” may be enclosed in a green circle. The red circle with a vertical line across it is the traditional symbol for prohibited items.