Cal. Sts. & High. Code § 25205

At any time prior to the approval of the report of the project by the board of supervisors of any county, the matter of the participation of the county in any issuance of bonds proposed in the report may be referred to a vote of the qualified electors of the county. Such reference shall be had upon the petition of 5 per cent of the registered electors of the county, or upon the resolution of the board of supervisors of the county.