The maximum speed limit on nonsurface-treated highways, which are roads that are comprised of an earth-aggregate or aggregate surface (i.e., dirt and gravel) that have not been stabilized with a bituminous or cementitious material, shall be 35 miles per hour. The maximum speed limit upon such highways may be increased or decreased by the Commissioner of Highways or other authority having jurisdiction over highways. However, such increased or decreased maximum speed limit shall be effective only when indicated by sign on the highway. For such highways upon which maximum speed limit is not indicated by sign, the maximum speed limit shall be 35 miles per hour.