Subject to the provisions of this article the boards of county commissioners of the several counties in this state are hereby authorized to issue bonds for the construction and repair of roads and bridges within the limits of their respective counties to an amount, which including the existing indebtedness of said counties, shall not exceed four per centum on the value of the taxable property within any such county, as shown by the last preceding assessment for state and county taxes.
NMS § 67-6-1