The Secretary is further authorized to convey to the State of Tennessee, subject to such conditions as he may deem necessary to assure administration and maintenance thereof by the State and to preserve the existing parkway character of the conveyed lands, the rights-of-way heretofore conveyed to the United States for the purposes of the Gatlinburg Spur of the Foothills Parkway together with any and all parcels of land heretofore conveyed by the State of Tennessee to the United States for the control and stabilization of landslides along said Gatlinburg Spur, except such lands as the Secretary determines may be necessary to provide for (1) the interchange between the road known as the Gatlinburg bypass and United States 441, (2) the interchange between United States Highway 441 and the Foothills Parkway in the vicinity of Caney Creek, and (3) the management and administration of the Foothills Parkway: Provided, That such reconveyance shall not be effected until construction of the Gatlinburg bypass and of two rock retaining walls to control erosion on the Gatlinburg Spur are completed, and Interstate Route 40 is open to public travel from Newport, Tennessee to United States Route 19 near Waynesville, North Carolina.