Trunkline

A trunkline is a designation given over a given transportation route, such as over a set of roads, and is identified as such for the purpose of providing ease with respect to guiding transportation along the route over which it covers. An example of this is a state highway such as M-60, which is located in southern Michigan and covers a series of roads such as Spring Arbor Road, Homer Road and so on. The purpose of the designation is to permit travelers the ability to follow the various roads over which it covers without becoming lost or side-tracked from the "main route."

Although not restrictive to state highways, it is certainly the most common usage of the term for the average person. Other uses can be for oil lines, railroads and telecommunications.