The initials “HC” on a road sign indicate a route for cars carrying hazardous cargo. Similar signs indicating that hazardous cargo is prohibited have the “HC” ringed in red and crossed by a diagonal red bar.
State-to-state variations exist for hazardous cargo signs. Some states substitute a “HM,” which stands for hazardous materials, for a “HC.” The sign often has white square background with black square text.
When hazardous cargo is permitted, a green circle may be placed around the “HC” or “HM”. The typical sign designator for something that is forbidden is a red circle with a strike-out bar.