In the United States, there are currently eight routes in the Interstate Highway System that are signed with letter suffixes to the route number.

Other suffixed Interstates include I-69C, I-69E and I-69W in South Texas, and I-480N in Ohio, which is designated as such on mile markers but is otherwise unsigned.

The state of Maryland has four unsigned suffixed Interstate designations that are designated by the Maryland State Highway Administration, rather than by the Federal Highway Administration (FHWA).

Most of these were spurs; the suffixed route did not return to its parent.

In 1980, the American Association of State Highway and Transportation Officials (AASHTO) abolished the majority of suffixes due to confusion, renumbering them as three-digit Interstates.