With the approval of the county court, the commissioners in each district whose roads are not wholly included in the state highway system may annually, as necessary, levy a tax not to exceed one percent (1%) on the assessed benefits in the district for the purpose of constructing, repairing, and maintaining roads within the district which are not included in the state highway system. For that purpose, in order not to delay such necessary work, the road improvement district may issue and sell certificates of indebtedness, bearing interest at a rate not exceeding six percent (6%) per annum and for an amount not exceeding one (1) annual tax. These certificates shall be negotiable, shall mature and be made payable within one (1) year after their issuance, and shall constitute a lien and charge against the funds of the district. This section shall not repeal Acts 1923, No. 180, § 1.