2005 California Streets and Highways Code Sections 1850-1852 HIGHWAY SYSTEMS

1850.  Any city may, by ordinance or resolution, permit the use of
its streets and highways by the board of supervisors of the county,
for the purpose of constructing and maintaining thereon any highway
or boulevard, as part of a county system of roads within any portion
of its incorporated limits.
1851.  The legislative body of any city may establish the grades of
those portions of any county highway lying within the city
boundaries.
1852.  No city shall change the grade of any portion of a state or
county highway where such grade has been established prior to the
incorporation of the city, without first obtaining the written
consent of the State or county authorities, as the case may be.