The board, subject to the approval of the voters within the portion of the county to which the tax would apply, may impose a maximum tax rate of one-half of 1 percent under this article and the Transactions and Use Tax Law (Part 1.6 (commencing with Section 7251) of Division 2 of the Revenue and Taxation Code). The board shall not levy the tax at a rate other than one-half or one-fourth of 1 percent unless specifically authorized by the Legislature.