California Code, Streets and Highways Code - SHC § 1721

The board of supervisors by resolution may determine that it is of general county interest that an existing street in the county extending in whole or in part within a city be made a county highway within the city or that a new county highway be laid out, acquired, constructed and maintained in whole or in part within a city.  The resolution shall describe the proposed location or relocation and the general nature of construction proposed to be undertaken by the county.  A copy of such resolution shall be forwarded to the governing body of the city within which is included any portion of the highway.