The director shall, with the approval of the commission, prorate the costs of the department which will be expended for highways and such costs shall be paid from money appropriated from the road use tax fund. Prorated costs payable from the road use tax fund shall be based upon that portion of the department's duties related to the construction, maintenance, and supervision of the public highways within the state or for the payment of bonds issued for the construction of public highways and the payment of interest on such bonds. The general assembly shall appropriate from the general fund of the state the remaining necessary departmental costs.
Iowa Code § 307.28