Studies were completed for the remaining five mainline tunnels to determine the cost benefit of twinning of the tunnels, or complete bypass. After the studies completed, the decision was made to go ahead with a US $100 million construction project to work with the tunnels. Construction began on September 6, 1962, with the first subproject being a bypass of the Laurel Hill Tunnel. On October 30, 1964, just two years after construction began, the Laurel Hill Bypass was completed and opened. The Laurel Hill Tunnel was permanently closed on that day. However, this was not the last bypass to occur for tunnels along the turnpike. Two years after the closing of the Laurel Hill, the Allegheny Mountain became the first tunnel to be twinned, and opened on August 25, 1966. With the twinning of the Blue Mountain, Kittatinny Mountain and Tuscarora Mountain tunnels under construction, the Commission turned its focus to the remaining two mainline tunnels, the Rays Hill Tunnel (the shortest tunnel) and the Sideling Hill Tunnel (the longest tunnel).