PC*MILER State + National Network routing favors using the U.S. Federally designated National Network (primary Interstates with reasonable entry/egress points up to 1 mile off the Interstate), combined with a preference to favor using the state-designated extensions to the Federal National Network (additional highways and supporting roads that can be any distance off the Interstate, as determined by the individual states).
State + National Network routing can be selected using the Road Network route option under Routing Preferences, either as part of a route profile or in the Route Options dialog for an individual route