The boards in their respective counties may accept the grant of rights-of-way for the construction of public highways over public lands of the United States, not reserved for public uses, contained in section 2477 of the Revised Statutes of the United States. Such rights-of-way shall henceforward not be less than sixty feet in width unless a lesser width is specified by the United States. Acceptance shall be by resolution of the board spread upon the records of its proceedings: PROVIDED, That nothing herein contained shall be construed to invalidate the acceptance of such grant by general public use and enjoyment, heretofore or hereafter had.