What do red and blue signs in the shape of a shield indicate?

Red and blue signs in the shape of a shield specifically indicate interstate highways. These shields are a standardized design in the United States and are used to mark routes that are part of the Interstate Highway System, which is crucial for facilitating long-distance travel and connecting cities across the nation. The colors help the signs stand out and convey information quickly to drivers.

While other shapes and colors represent different types of signs, the distinct shield shape combined with the red and blue coloring is unique to interstate routes. City limit signs, school zone signs, and parks and recreation signs each have their own designs and color schemes that do not resemble the shield format used for interstates.