From and after April 1, 1937, the width of thirty feet on each side of the center line of county roads, exclusive of such additional width as may be required for cuts and fills, is the necessary and proper right-of-way width for county roads, unless the board of county commissioners, shall, in any instance, adopt and designate a different width. This shall not be construed to require the acquisition of increased right-of-way for any county road already established and the right-of-way for which has been secured.