California Code, Streets and Highways Code - SHC § 111.5

Whenever a city street or a county highway and an existing state highway (1) are substantially parallel;  (2) each has been or is to be restricted to one-way traffic, proceeding in opposite directions as to each other, by ordinance of such city or county duly enacted and applicable thereto;  and (3) the commission determines that the existing state highway is inadequate and that state highway traffic conditions can be improved and state highway traffic more economically served than by confining improvements to the existing state highway, the commission may adopt any portion of the city street or county highway so substantially parallel with an existing state highway, as a part of the State Highway System without compensation to the city or county, and the department may improve the same and make necessary connections to route traffic one way on such city street or county highway and the existing state highway, respectively.

Whenever an ordinance of any city or county designating a particular highway as a one-way highway becomes applicable to a state highway, whether an existing state highway at the time of the enactment of such ordinance or a city street or county highway thereafter adopted as a state highway, such ordinance shall not be thereafter rescinded, repealed, revoked or amended by action of the legislative authority of such city or county without the consent in writing of the department with the approval of the commission first obtained, unless otherwise provided by the terms of any written agreement between the State and any city entered into prior to the effective date of this section.  If any city street or county highway which has become a portion of the State Highway System pursuant to this section ceases to be a one-way street because the ordinance declaring such street or highway to be a one-way street or highway is rescinded, repealed, revoked, or amended, with the consent of the department, as hereinabove provided, such city street or county highway, or the portion thereof that is no longer restricted to one-way traffic, shall by operation of law revert to the status of a city street or county highway and cease to be a portion of the State Highway System.