The board of commissioners of any special road district organized and incorporated under the provisions of sections 233.010 to 233.165, for and on behalf of such district, and the county commissions of the several counties under township organization on behalf of any township in their respective counties, are hereby authorized to issue road bonds to an amount, including existing indebtedness, not exceeding five percent of the value of the taxable tangible property of such special road district or township, as the case may be, as shown by the last completed assessment for state and county purposes. Such bonds shall be issued in denominations of one hundred dollars, or some multiple thereof, to bear interest at not exceeding six percent per annum, payable semiannually and to become due and payable at such times as the board of commissioners or county commissions shall determine by order of record, not exceeding twenty years from date of issue.