Streets and Highways Code § 1431

Whenever in the interests of commerce, for the benefit of the residents of the county, or for the purpose of expediting travel between points on opposite sides of any navigable body of water, the board of supervisors of any county finds it advisable to construct for the public use any subway under any such navigable body of water, at a location determined upon by the board, the board shall call an election and submit to the electors of the county the question whether bonds of the county will be issued and sold for the purpose of constructing such a subway.