Special routes are distinguished from main routes by, in most cases, the addition of an auxiliary plate that describes what type of route it is, while the main highway carries no such sign.

In some locations, a single letter is placed after the route number to denote the special route type in lieu of the auxiliary plate.

Among members of the roadgeek community, these routes are often called auxiliary or bannered U.S.

There are four main types of special routes — alternate, business, bypass, and temporary; though other route types exist.

The following U.S. Highway special routes have existed: