Proceedings for the authorization and issuance of bonds for the acquisition, construction or completion of any public utility or utilities may be initiated by electors in the following manner: Whenever a petition, signed by qualified electors of the City and County equal in number to at least fifteen percent of the votes cast for all candidates for Mayor at the last proceeding general municipal election for Mayor, requesting the Board of Supervisors to submit to the voters of the City and County a proposition or propositions for incurring bonded indebtedness for the acquisition, construction or completion of any public utility or utilities shall be filed with the director of elections, the Board of Supervisors shall submit to the voters the proposition or propositions for incurring bonded indebtedness of the City and County for purpose or purposes set forth in that petition at the next general municipal, statewide or special municipal election.