The highway commissioner of each road district within the territory of the Regional Transportation Authority shall have authority, with the approval of the township board of trustees, to contract with the Regional Transportation Authority or a Service Board, as defined in the Regional Transportation Authority Act, for the purchase of public transportation services within the district, upon such terms and conditions as may be mutually agreed upon. The expenditure of road funds, collected under a road district tax, to purchase public transportation services constitutes a road purpose under this Code.