An official alternate route is a special route in the United States that provides an alternate alignment for a highway. They are loop roads and found in many road systems in the United States including the U.S. Route system and various state route systems. Alternate routes were created as a means of connecting a town (or towns) desired to be on a route which had been routed differently to put another important town or city on the route.
Originally, the term for these routes was "optional"; but in 1959, the American Association of State Highway and Transportation Officials (AASHTO) changed the designation to "alternate". In some cases where needed, an additional business route exists as a third alignment (this was the case with former Alternate US 71 which bypassed Joplin, Missouri).
AASHTO defines and specifies that alternate routes of the U.S. Route system should have the following behavior:
An "Alternate Route" shall be considered a route which starts at a point where it branches off from the main numbered route, may pass through certain cities and towns, and then connect back with the regular route some miles distant. Since it is the purpose of the U.S. numbered system to mark the best and shortest route available, an alternate route should be designated only where both routes are needed to accommodate the traffic demand, and when the alternate route has substantially the same geometric and structural design standards of the main marked routing. It is recommended that in case an alternate route is marked, that the shorter and better constructed route be given the regular number and the other section designated as the "Alternate Route". It is further recommended that the Highway Department erect signs at the junction points of the regular and alternate routes giving the distance between the cities or points concerned... In no instance should an alternate routing be used for the purpose of keeping an obsolete section on the U.S. numbered system after a new routing has been constructed and available to traffic.
In at least one case, the banner "Optional Route" was retained when a second alternate route existed. One example of this occurred in Kansas City, Missouri with U.S. Route 40 which had an alternate and an optional route simultaneously.
In some US states, an alternate route will be designated by adding an "A" after the number instead of a sign marked "Alternate" above it. Example: "US 69A" means "Alternate US 69".