The highway commissioner of any road district may in his discretion reduce the width of any existing township or district road to a width of 40 feet, if the reduction is petitioned for by a majority of the landowners along the line of such road within the district. When possible the land vacated by reducing the width of the road shall be taken equally from each side of the road. In cases of natural obstruction on one side of the road or where the road extends along the right-of-way of any railroad, river or canal, the commissioner is authorized to reduce the width of road on one side only.