The board of county commissioners shall order the expenditure of all road taxes paid into the county treasury from unorganized territory in the improvement of the highways, paying the district overseer of highways, purchasing implements, and repairing bridges in the road district in which such taxes were levied, under such regulations as it may deem most expedient for the public interests, and for this purpose shall order the payment of such sum by the treasurer to the persons performing such labor upon the certificate of the overseer of highways.