§ 503-b. Supplemental tax. 1. In addition to the taxes imposed by sections five hundred three and five hundred three-a of this article, and in addition to any other tax or fee imposed by law, there is hereby imposed on and after July first, nineteen hundred ninety a supplemental tax, for the privilege of operating any vehicular unit upon the public highways of this state and the purpose of recompensing the state for the public expenditures incurred by reason of the operations of such vehicular units on the public highways of this state, equal to the tax imposed by section five hundred three of this article.

2. The provisions concerning returns under section five hundred five of this article shall be applicable to this section. The supplemental tax imposed by this section shall be payable to the commissioner of taxation and finance in full at the time the return is required to be filed.

3. Except as otherwise provided in this section, all of the provisions of this article applicable to the tax imposed by section five hundred three of this article shall apply with respect to the supplemental tax imposed by this section to the same extent as if it were imposed by such section five hundred three, insofar as such provisions can be made applicable to the supplemental tax imposed by this section, with such modification as may be necessary to adapt such provisions to the supplemental tax imposed by this section.

4. The provisions of this section shall not apply to (i) any vehicular unit used almost exclusively to transport boltwood, logs, pulpwood or woodchips, and (ii) any vehicular unit used almost exclusively to transport raw, unprocessed milk in bulk, provided, however, the provisions of clauses (i) and (ii) of this subdivision shall not apply to any carrier or owner which operates more than three such vehicular units.

5. Notwithstanding any other provision of this article, commencing January first, nineteen hundred ninety-nine and ending March thirty-first, two thousand one, the supplemental tax imposed on the privilege of operating any motor vehicle unit shall be fifty percent of the tax imposed by section five hundred three of this article and commencing April first, two thousand one, the supplemental tax imposed on the privilege of operating any motor vehicle unit shall be forty percent of the tax imposed by section five hundred three of this article.