Question: What year was the Cumberland Gap Tunnel, one of two mountain vehicular tunnels in the United States, opened which extended U.S. 58 from Harrogate, Tennessee to Virginia Beach, Virginia?

Context articles: Elizabeth River Tunnels Project
Chesapeake Bay Bridge–Tunnel
U.S. Route 58
Blue Ridge Tunnel
Holland Tunnel
Interstate 77 in Virginia
Cumberland Gap Tunnel
Cumberland Gap, Tennessee
Pembroke Manor, Virginia
Eisenhower Tunnel

The Elizabeth River Tunnels Project, a series of transportation projects in the South Hampton Roads region of Virginia, comprises the rehabilitation of the Downtown and existing Midtown Tunnels, the construction of a new parallel Midtown Tunnel, and the extension of the MLK Freeway/U.S. 58 to I-264.
 Intended to reduce congestion on area surface streets and arterial roads, the project is being administered by Elizabeth River Crossings (ERC) and Virginia Department of Transportation as part of a 58-year public-private partnership concession that will cost about $2.1 billion (2012) to complete.
The Chesapeake Bay Bridge–Tunnel (CBBT) is a 23 mi fixed link crossing at the mouth of the Chesapeake Bay in the U.S. state of Virginia.
 It connects Northampton County on the Delmarva Peninsula with Virginia Beach, which is part of the Hampton Roads metropolitan area.
 The bridge–tunnel originally combined 12 mi of trestle, two 1-mile-long (1.6 km) tunnels, four artificial islands, four high-level bridges, approximately 2 mi of causeway, and 5.5 mi of approach roads—crossing the Chesapeake Bay and preserving traffic on the Thimble Shoals and Chesapeake shipping channels.
 It replaced vehicle ferry services that operated from South Hampton Roads and from the Virginia Peninsula from the 1930s.
 Financed by toll revenue bonds, the bridge–tunnel was opened on April 15, 1964, and remains one of only ten bridge–tunnel systems in the world, three of which are located in Hampton Roads, Virginia.
U.S. Route 58 is an east–west U.S. Highway that runs for 508 mi from U.S. Route 25E just northwest of Harrogate, Tennessee, to U.S. Route 60 in Virginia Beach, Virginia.
 Until 1996, when the Cumberland Gap Tunnel opened, US 58 ran only inside the commonwealth of Virginia.
 It was then extended southwest along a short piece of former US 25E, which no longer enters Virginia, to end at the new alignment in Tennessee.
The Blue Ridge Tunnel (also known as the Crozet Tunnel) is a historic railroad tunnel built during the construction of the Blue Ridge Railroad in the 1850s.
 The tunnel was the westernmost and longest of four tunnels engineered by Claudius Crozet to cross the Blue Ridge Mountains at Rockfish Gap in central Virginia.
 At 4237 ft in length, the tunnel was the longest tunnel in the United States at the time of its completion in 1858.
 The tunnel was used by the Virginia Central Railroad from its opening to 1868, when the line was reorganized as the Chesapeake and Ohio Railroad (renamed Chesapeake and Ohio Railway in 1878).
 The Chesapeake and Ohio routed trains through the tunnel until it was abandoned and replaced by a new tunnel in 1944.
 The new tunnel was named the "Blue Ridge Tunnel" as well, although the original tunnel still remains abandoned nearby.
 The old Blue Ridge Tunnel has since been named a Historic Civil Engineering Landmark.
The Holland Tunnel is a highway tunnel under the Hudson River between Manhattan in New York City and Jersey City, New Jersey.
 Its two tubes carry eastbound and westbound Interstate 78.
 In New Jersey, it is also designated NJ 139.
 An integral conduit within the New York Metropolitan Area, it opened in 1927 as the first of two vehicular tunnels under the river, the other being the Lincoln Tunnel.
 Both are operated by the Port Authority of New York and New Jersey.
In the U.S. state of Virginia, Interstate 77 is a north–south highway along the U.S. Route 52 corridor, serving Hillsville, Wytheville, and Bland.
 Along I-77's 67 mi length in Virginia, it passes through the Big Walker Mountain Tunnel and East River Mountain Tunnel, the latter on the West Virginia state line and one of only two land vehicular tunnels to cross a state line.
The Cumberland Gap Tunnel is a tunnel that carries U.S. Route 25E under Cumberland Gap National Historical Park near the intersection of Kentucky, Tennessee, and Virginia.
 According to United States Geological Survey maps of the area, the tunnel does not enter Virginia.
 It is one of only two mountain vehicular tunnels in the United States that cross a state line, the other being the East River Mountain Tunnel on Interstate 77 between Virginia and West Virginia.
Cumberland Gap is a town in Claiborne County, Tennessee, United States.
 Its population was 494 at the 2010 census.
 The town is located near the Cumberland Gap, a historic mountain pass for which the town is named, and is surrounded on all sides by the Cumberland Gap National Historical Park.
 The town is also a part of the park.
Pembroke Manor (also known as Pembroke) is an area in the independent city of Virginia Beach, Virginia, United States located around the intersections of Virginia Beach Boulevard (U.S. Route 58) and Independence Boulevard (State Route 225).
 The community's name comes from "Pembroke Manor", a plantation built in 1764.
 The house was donated by the Aragona family to the Princess Anne Historical Society, but currently is under private ownership and serves as Ivy League Academy, a private Christian school for children K4 through 5th grades.
 The structure itself was placed on the National Register of Historic Places in 1970 as #70000887 and is one of the oldest and most significant structures in the City of Virginia Beach.
The Eisenhower Tunnel, officially the Eisenhower–Edwin C. Johnson Memorial Tunnel, is a dual-bore, four-lane vehicular tunnel approximately 60 mi west of Denver, Colorado, United States.
 The tunnel carries Interstate 70 under the Continental Divide in the Rocky Mountains.
 With a maximum elevation of 11158 ft AMSL, it is one of the highest vehicular tunnels in the world.
 The tunnel is the longest mountain tunnel and highest point on the Interstate Highway System.
 Completed in 1979, it was one of the last major pieces of the Interstate Highway system to be completed.
 The westbound bore is named after Dwight D. Eisenhower, the U.S. President for whom the Interstate system is also named.
 The eastbound bore is named for Edwin C. Johnson, a governor and U.S. Senator who lobbied for an Interstate Highway to be built across Colorado.