The National Highway System consists of a network of roads important to the economy, defense and mobility. It is made up of the Interstates, Intermodal Connectors, the STRAHNET and its connectors, MAP 21 Principal Arterials and other routes designated by the FHWA. Starting October 1, 2012 the existing NHS was expanded to include all existing Principal Arterials (i.e. Functional Classifications 1, 2 and 3) to the NHS. Under MAP-21, the NHS is composed of rural and urban roads nationwide serving major population centers, international border crossings, intermodal transportation facilities, and major travel destinations. The NHS includes:

1) The Interstate System.

2) Other Principal arterials and border crossings on those routes (including other urban and rural principal arterial routes, and border crossings on those routes, that were not included on the NHS before the date of enactment of the MAP-21).

3) Intermodal connectors -- highways that provide motor vehicle access between the NHS and major intermodal transportation facilities.

4) STRAHNET -- the network of highways important to U.S. strategic defense.

5) STRAHNET connectors to major military installations.