National Highway System

On October 1, 2012 the existing National Highway System (NHS) was expanded to include all existing Principal Arterials (i.e. Functional Classifications 1, 2 and 3) to the new Enhanced NHS. Under MAP-21, the Enhanced 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: The Interstate System.

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). Intermodal connectors -- highways that provide motor vehicle access between the NHS and major intermodal transportation facilities. STRAHNET -- the network of highways important to U.S. strategic defense.