The commission may cooperate and negotiate with the proper authorities of adjoining states in the construction, purchase, acquisition and maintenance of bridges constructed or to be constructed across streams which constitute boundaries between this State and such adjoining states and may expend for such purposes not exceeding one half of the total cost of such bridges and approaches thereto and bear a proportionate part of the maintenance thereof, such expenditures to be made from the funds available for the construction and maintenance of highways and bridges in the state highway system.