What is the common name of the marker used for United States Numbered Highways?

The common name of the marker used for United States Numbered Highways is the U.S. Route shield.

It is a distinctive highway marker displaying a route number inside a shield-shaped outline. The shield identifies roads in the U.S. Route system, distinguishing them from Interstate shields, state-route markers, and county-route signs. In everyday speech, people may also call it a U.S. highway shield or route marker, but “U.S. Route shield” is the canonical name used for the sign design.

The Joint Board on Interstate Highways adopted the shield concept in 1925. Its shape was inspired by the Great Seal of the United States, giving the national route system a recognizable symbol. The first U.S. Route signs appeared after the system was established in 1926.

Modern shields generally show black route numerals in a white shield, often placed on a black square background. States manufacture their own signs, so small design differences exist. Three-digit routes may use a wider version to keep the number legible.