Cal. Sts. & Hy. Code § 790
The board of supervisors of any county may, by a vote of not less than three-fifths of its membership, petition the department to cooperate, under the provisions of this article, in the improvement of an existent highway or the construction of a proposed highway in that county when such existent highway connects, or such proposed highway will connect, a forest highway system road or national park road which is already built or under construction, with any State highway. Such connecting highway shall not exceed fifty miles in length.
When any highway sought to be improved or constructed under the provisions of this article is situated in two or more counties, a petition, or concurrence in such a petition, passed by a three-fifths vote of the membership of the board of supervisors of each of such counties is required to initiate the proceeding under this article.
Ca. Sts. and High. Code § 790