The tunnels were vital in the expansion of the South Hills suburbs by providing a direct route to the city, eliminating the time-consuming task of navigating around or over Mt.

The design was two vertical wall horseshoe profile tunnels, each consisting of two lanes of traffic and a 4 ft (1.2 m) sidewalk.

The sidewalks, being minimally used and in disrepair, were removed during a renovation in the 1970s in order to widen and increase the height of the traffic lanes.

One plan, from engineer W. M. Donley, did not involve constructing tunnels, but called for a deep cut through the mountain.

The final plan for the tunnels shared the southern Bell Tavern portal, but followed a higher line to emerge on the north face of Mt.

Booth and Flinn, Ltd. were awarded the contract to build the new tunnels in January 1919, and construction began shortly thereafter.

On May 10, 1924, a traffic jam occurred due to a Pittsburgh Street Railway Company strike, which caused vehicles to be left stopped and idling in the tunnels.

The bridge and tunnel combination gave motorists and pedestrians a direct route to the city without traveling over or around Mt.