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An unsigned highway is a highway that has been assigned a route number, but does not bear conventional road markings that would normally be used to identify the route with that number. Depending on the policy of the agency that maintains the highway, 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.
Routes are often left unsigned when they are extremely short. Roads that serve as a short connector to a major highway are sometimes signed to show the connection to the major road, rather than the road's actual designation.
Some highway maintenance agencies have separate numbering systems for their minor and major routes. To avoid confusion from having multiple highways in the same area with the same number, often the minor routes will be signed differently, or not signed at all. Other agencies use multiple numbering systems for the same set of routes; one number used for public signs and an internal numbering system for inventory purposes. In this case a highway may be unsigned as it has only been assigned an inventory designation, but not assigned a number in the public system.
Some highways are not signed to avoid multiple designations, such as when the entire route runs concurrent with other highways, or the route has been assigned both name and a number. In countries where multiple agencies can create and number highways, a situation can occur where a road was built and signed by one agency, and later adopted or upgraded to become part of another network; yet is only signed with one of the designations. A situation unique to the United States is where some states assign state route inventory numbers to the U.S. Highways and Interstate Highways that enter the state.
 Examples of major unsigned highways
- In several cities in the United States a short connector freeway is assigned a number in the Interstate Highway System; however, the freeway also carries one or more longer highway designations through the downtown area. In some cases, the freeway is only signed as one of the through designations and the short Interstate designation is omitted. Examples include Interstate 444, Interstate 345, Interstate 305 and Interstate 595, all of which are signed only with the U.S. Highway designations.
- The Interstate Highway System includes highways outside the Contiguous United States. While the interstate highways in Hawaii are signed similar to those in the contiguous United states, those in Alaska and Puerto Rico are not signed with Interstate highway shields.
- Interstate 4 is internally designated by the Florida Department of Transportation as State Road 400; however, the state designation continues after I-4 has ended, and SR 400 signs do exist on the portion beyond the termini of I-4.
- Many named toll roads, turnpikes and parkways in the eastern United States are assigned route numbers by the owning agency, but are only publicly signed with a name. Examples include Florida's Turnpike (officially State Road 91), the Garden State Parkway (officially Route 444), the southern portion of the New Jersey Turnpike (officially Route 700), and the Atlantic City–Brigantine Connector, a spur off the Atlantic City Expressway, officially numbered Route 446X and Route 446 respectively.
- The East Los Angeles Interchange is a case where a highway is unsigned with a conflict between the state and federal definition of a highway. The state definition of Interstate 10 has a discontinuity with a stub freeway proceeding west from the northern part of this interchange towards U.S. Route 101, while the federal definition of I-10 is contiguous. The discrepancy is resolved by having the signage at the interchange imply I-10 proceeds south through the main portion of interchange concurrent with I-5. The I-10 stub is signed US 101 while driving west and I-5/10 driving east.
- An unusual case is Washington State Route 41, which is the unsigned designation for a brief portion of Idaho State Highway 41 that crosses the state line. Wyoming Highway 70 also dips into an adjacent state without any signage from the other state; however, in this case the road is entirely maintained by a single department of Transportation and does not have a Colorado highway designation.
- U. S. Route 77 through Dallas County, Texas, is concurrent with Interstate 35E and is not signed there. Also, TxDOT does not sign U. S. Route 67 where it is concurrent with Interstate 30 in Dallas and Rockwall counties.
 Entire unsigned route systems
The C, D, and U road systems in the Great Britain road numbering scheme are systems of routes considered less important than B roads and typically left unsigned.
In New York, state-maintained highways that are not part of a signed state route are assigned a reference route designation. Reference route designations are signed only with reference markers, small green signs posted alongside the highway that typically bear the route number on the first line. The reference route system also includes parkways and other named highways (such as the Inner Loop), which are given a reference route designation for inventory purposes.
Pennsylvania maintains a set of minor routes, called quadrant routes, that are not signed with standard confirming markers. Pennsylvania also maintains relocated signed routes, interchanges, wye connections, rest areas, and truck escape ramps as unsigned state highways.
- "State, Interstate, and Toll Authority highways straight-line diagram" (PDF). New Jersey Department of Transportation. Retrieved 2009-12-29.