There shall be a system of state highways which shall include (1) state primary highways, which are highways serving the predominant flow of traffic between the principal towns of this state and between such towns and similar towns of the surrounding states; (2) state secondary highways, which are connecting and feeder highways which supplement the state primary system by serving the predominant flow of traffic between the smaller towns of the state and between such towns and other towns in this state and in surrounding states; and (3) state special service highways, which are highways which provide access from the primary and secondary systems of state highways to federal and state facilities.