225 ILCS 440/3.02

"Interstate highway" means any highway designated by the Department and approved by the United States Department of Transportation as a part of the National System of Interstate and Defense Highways on the effective date of this Act or thereafter. A highway constructed after the effective date of this Act shall become a part of the National System of Interstate and Defense Highways upon the date of approval of the Route Location Decision and the approval of the addition of the highway to the National System of Interstate and Defense Highways by the Governor and the United States Department of Transportation.