[1] From the Turnpike's opening in 1940 until the realignment projects, the tunnels were bottlenecks; opposing traffic in the same tubes reduced speeds.

The Sideling Hill Tunnel's original plans date back to the year 1881, when surveying for the South Pennsylvania Railroad began.

[2][3][4] The Sideling Hill Tunnel was built before December 1884 by John O'Brien, an engineer from Rhinebeck, New York.

[6] Just sixteen days later, another blast occurred in the tunnel, taking the lives of a Hungarian worker, along with two African-Americans, two Italians, and one Irishman.

The studies looked into the major tunnels, the Laurel Hill and Allegheny Mountain, and the possibility of adding a second tube, or "twinning", to them.

[1] After the studies completed, the decision was made to go ahead with a US $100 million [a][8] construction project to work with the tunnels.

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.

[1] An engineering report dating back to 1961 suggested that building a bypass around the last two mainline tunnels was the best way to solve the congestion.

The system is called the Pike2Bike Trail and is planned to be an 18 miles (29 km) mountain bicycle loop that serves access to the Pennsylvania Department of Transportation's BicyclePA Route S.[9]