This section does not apply to a bond that: matures within 1 year after the date of issue and is issued: in anticipation of tax receipts; to meet current expenses; or to meet an emergency; is sold to the United States or a unit or instrumentality of the United States; is issued under a plan of composition approved in a proceeding under Chapter IX of the United States Bankruptcy Act; or is issued under any other plan to refund or refinance in exchange, bond for bond, an outstanding maturing debt, other than: a current or floating debt; or a bond under item (i) of this paragraph. This section does not apply to bond or grant anticipation notes issued under Part III of this subtitle. This section does not apply to bonds issued under an enabling act that specifically states that this section does not apply or that provides a different method for establishing the maturity of the bonds. A State unit shall issue bonds on a serial maturity plan. The State unit may: vary the amounts of the series; and provide for the maturity of a series in consecutive annual installments or at longer intervals. The maturity date of the final series shall be based on the purpose for which the bonds are issued: within the time limit that applies under the schedule in paragraph (2) of this subsection; or if more than 1 time limit applies, within the shortest applicable time limit. The schedule of maturity dates is as follows: Limit on Maturity Purpose of Issue                                of Issue Paving existing highways or streets …………………      10 years Airports and buildings constructed or to be constructed on airports …………………………………      15 years Highway construction ……………………………………     20 years Electric light and power systems ………………………     25 years Gas systems ………………………………………………     25 years Grade crossing eliminations ……………………………     25 years Harbor improvements ……………………………………     25 years School construction ………………………………………     25 years All unscheduled permanent structures of durable Materials ……………………………………………………     25 years Bridges ………………………………………………………     30 years Land acquired for permanent improvements …………     40 years Sewerage installation ………………………………………     40 years Water Systems ………………………………………………     40 years All other unscheduled proprietary or public purposes     40 years