§ 308. Value of highway discontinued. If the necessity of such private road has been occasioned by the alteration or discontinuance of a public highway running through the lands belonging to a person through whose lands the private road is proposed to be opened, the jury shall take into consideration the value of the highway so discontinued, and the benefit resulting to the person by reason of such discontinuance, and shall deduct the same from the damages assessed for the opening and laying out of such private road.