Cal. Sts. & Hy. Code § 81
Except as is provided in Section 2109, whenever there exists between the termini of, and approximately on, any route included in the state highway system a traversable highway, the commission shall adopt the same as the state highway between such termini. The commission may adopt a portion of any such road if it determines that such portion is constructed to adequate standards and if such portion is contiguous to a portion of the state highway system presently being maintained by the department. If more than one such traversable highway exists, the commission shall determine and designate which of such highways shall constitute the state highway. The traversable highways thus selected and adopted shall be state highways in all respects the same as if originally constructed or acquired by the state, subject to all laws applicable to state highways. All acts and actions of the commission and the department with respect to the taking over and maintenance of such highways heretofore taken are hereby approved and ratified.
Ca. Sts. and High. Code § 81