All land within special tax districts shall be subject to the same taxation at the same rate as other property in the highway district for the purpose of meeting the principal and interest requirements of that portion of any bond issue which is not charged against adjoining property but is paid by the highway district as a whole. All bonds issued by a highway district shall, as to a specified portion of the indebtedness created, not exceed fifty per cent (50%), be met and paid by special taxation upon the land adjoining the highways built with the proceeds of the bonds, not exceed a specified sum per acre, and as to the remaining portion shall be met and paid by taxation of all property in the highway district, including the property within special tax districts, and including all property within any incorporated cities included within the limits of the highway districts.