The width of all county highways, other than bridges, alleys, lanes, and trails, shall be at least 40 feet, except as provided by Sections 906.5 and 969 and except that the board of supervisors of any county may, by a resolution passed by a unanimous vote of its members, determine that the public convenience and necessity demand the maintenance of a highway of less than 40 feet and may after such determination proceed with the maintenance of any such highway.
This section shall not increase or diminish the width of any county highway established, dedicated, and maintained as such prior to January 1, 1945.