Public improvement bonds.

An insurer may invest in bonds, notes, certificates of indebtedness, warrants, or other evidences of indebtedness which are payable from revenues or earnings specifically pledged therefor of any public toll bridge, structure, or improvement owned by any state, incorporated city, or legally constituted public corporation or commission, all within the United States or Canada, for the payment of the principal and interest of which a lawful sinking fund has been established and is being maintained and if no default on the part of the issuer in payment of principal or interest has occurred on any of its bonds, notes, warrants, or other securities within 5 years prior to the date of investment therein.