What does the sign with the letters "HC" on a highway mean?

A highway sign with the letters "HC" on it designates a route for vehicles transporting hazardous cargo. A similar sign with the "HC" circled in red and crossed with a diagonal red bar means that hazardous cargo is not allowed.

Hazardous cargo signs vary from state to state. In some states, an "HM," standing for hazardous materials, is used instead of an "HC." In most cases, the sign features black letters on a white square background. When hazardous cargo is allowed, the "HC" or "HM" may be surrounded by a green circle. The red circle with a strike-out bar is the standard sign designation for something that is not allowed.