Wherever the state proposes to improve a section of an existing two-lane undivided public road, the width of the right-of-way of which cannot be ascertained totally or partially by instruments of conveyance, court orders or otherwise, there shall be a presumption that the unascertained width is twenty-five feet (25′) on either side of the centerline of the traveled portion of the road. This presumption is rebuttable only and if necessary in the judgment of the commissioner of transportation to effect the intent of this part, the state shall acquire the adjoining property by negotiation or by eminent domain. Fences in place for the prescriptive period shall be considered ownership.