The state highway commissioner, subject to the approval of the state administrative board and highway advisory board, is hereby authorized and directed to lay out and establish trunk line highways and other highways as are necessary to, upon or through state parks or other state property, and to build connections between state trunk lines and highways, running to, upon or through state parks and other state property: Provided, That such highways, if and when laid out and established, shall be a part of the total mileage authorized for such year.