Hidden route

An unsigned highway is a highway that has been assigned a route number, but does not bear road markings that would conventionally be used to identify the route with that number. Highways are left unsigned for a variety of reasons, and examples are found throughout the world. Depending on the policy of the agency that maintains the highway, and the reason for not signing the route, the route may instead be signed a different designation from its actual number, with small inventory markers for internal use, or with nothing at all.

There are a variety of cases where roads are officially designated, but have no markings to show that designation. Many highway maintenance agencies assign some form of number to all highways, bridges, and other features they maintain for tracking and inventory purposes. However, policies vary regarding how and when to publicly post these assigned numbers. Several highway maintenance agencies have multiple numbering systems for the different classes of routes they maintain (freeways, expressways, rural roads, etc.) In such cases, one or more class of numbers may be reserved for minor routes and these may or may not be signed. Often roads that serve as a connector to a major highway are signed to show the connection to the major road, rather than the road's actual designation. Some highways are not signed to avoid multiple designations, such as when the entire route runs concurrent with other highways. There are several instances where a route has officially been given a name by government agencies, and is signed with that name, but the route is also assigned a number by the highway maintenance agencies as to fit in their maintenance and inventory systems. Another common reason to not sign a highway is where the highway is government maintained, but is of little value to the general public.

There are numerous cases in the United States where the same physical roadbed has designations in the Interstate Highway System, U.S. Highway system and the state route system. In many cases one or more of the official designations is omitted.

...
Wikipedia