For the purpose of clarifying the federal aid systems provided for in federal highway acts the following classifications are applicable in South Dakota:
(1) "National highway system including the interstate system," the highways designated by the state and federal government that are eligible for improvement with national highway system and interstate federal aid funds;
(2) "Surface transportation program," the urban and rural highways designated as arterial and collector routes not functionally classified as local or rural minor collectors, unless such roads were on a federal-aid highway system on January 1, 1991, including all highways formerly designated federal-aid secondary and federal-aid urban system by the state and federal government, and including the transportation enhancement activities as defined by section 101(a) of Title 23, United States Code as of December 18, 1991, that are eligible for improvement with surface transportation program federal aid funds.