Whenever in the judgment of the board of county commissioners any part of the road in addition to being of public utility, shall be of general importance to the county, the improvement of which by reason of sand, creeks, heavy grades or other reasons would necessarily incur unusual expense, the board of county commissioners may make an order to that effect and charge not exceeding sixty percent of the expense of the improvement to the county, the expense to the lands within the benefit district to be decreased in proportion.