California Code, Public Utilities Code - PUC § 180201

A retail transactions and use tax ordinance applicable in the incorporated and unincorporated territory of a county may be imposed by the authority in accordance with this chapter and Part 1.6 (commencing with Section 7251) of Division 2 of the Revenue and Taxation Code, if the tax ordinance is adopted by a two-thirds vote of the authority and imposition of the tax is subsequently approved by a majority of the electors voting on the measure, or by any otherwise applicable voter approval requirement, at a special election called for that purpose by the board of supervisors, at the request of the authority, and a county transportation expenditure plan is adopted pursuant to Section 180206.

A retail transactions and use tax approved by the electors shall remain in effect for the period of time specified in the tax ordinance.  The tax may be continued in effect, or reimposed, by a tax ordinance adopted by a two-thirds vote of the authority and the reimposition of the tax is approved by any applicable majority of the electors.