The state transportation commission shall have full control over all roads designated by the commission as state highways or created as state highways by acts of the legislature. The costs of maintenance thereof shall be paid by the state, provided, that if at any time the money in the state road fund available for improvement or maintenance of all such state highways in the state shall not be sufficient for such purposes, the state transportation commission shall certify to the respective boards of county commissioners the certain sections of the highways in the counties, proportionately to such deficiency, which shall be improved and maintained wholly by the respective counties until sufficient funds are provided in the state road fund to enable the state to take over and maintain them as state highways.