Any county, city or political subdivision of this state shall have the authority to enter into written agreements with each other or with the secretary of transportation with respect to the planning, designing, financing, constructing, reconstructing, maintaining, acquiring of right-of-way or establishing the controlled access facilities of any existing or proposed highway, road, street or connecting link, including bridges, traffic control devices and other such improvements located thereon. Expenditures made pursuant to such agreements shall be considered proper expenditures of public funds, including state funds, notwithstanding the location of such improvement or facility outside the boundary or jurisdiction of such county, city or political subdivision. This section shall not be construed as amending or repealing any existing law relating to the same subject but shall be construed as supplementary thereto.