California Code, Streets and Highways Code - SHC § 1434

Whenever any such subway is proposed to be constructed under any navigable body of water forming the dividing line between counties, the boards of supervisors of each of the counties into which any such subway will reach shall first agree as to what portions of the expense of such subway will be paid by each county.  Thereafter the board of each county may take the proceedings it considers proper under the provisions of this article.