To help celebrate the 100 year anniversary of the 1924 opening of Liberty Tunnels in Pittsburgh, the ASCE Pittsburgh Section History & Heritage Committee, in collaboration with the Pennsylvania Department of Transportation (PennDOT), nominated in 2024 the Liberty Tunnels Ventilation System for recognition as an ASCE National Historic Civil Engineering Landmark (NHCEL). This particular feature of the Liberty Tunnels was selected for nomination after extensive consultation with the ASCE National History & Heritage Committee. The nomination was approved by the National History & Heritage Committee and final approval was provided by the ASCE Board of Direction at their March 2025 meeting. The NHCEL plaque was subsequently installed on the Fan House building entrance above the tunnels and officially dedicated on September 4, 2025. The dedication ceremony, which was covered by various Pittsburgh television and print media organizations, involved PennDOT Secretary Michael Carroll, PennDOT District 11 Assistant Executive Lori Musto, PennDOT District 11 Tunnel Manager Todd Caddy, Pittsburgh Section President Taylor DaCanal, Pittsburgh Section Past President Alma Rettinger, and the members of the Pittsburgh Section History & Heritage Committee: David DiGioia, David Dzombak, Greg Scott, and Todd Wilson. In her remarks at the dedication ceremony, Pittsburgh Section President Taylor DaCanal said: “The recognition will now be a permanent reminder of the impact that the City of Bridges and Tunnels has had on infrastructure worldwide. It will also serve as an inspiration to future engineering generations to drive improvement of infrastructure forward through innovation.” David Dzombak, Chair of the Pittsburgh Section History & Heritage Committee, concluded his overview of the path-breaking features of the Liberty Tunnels and the ventilation system with: “Thanks to the forward-looking design of the Liberty Tunnels, and the committed effort of Allegheny County – the constructor and original owner – and the Pennsylvania Department of Transportation – owner since 1962 – the Liberty Tunnels have provided important and safe service to our city and region for the past 100 years and are poised to do the same for the next 100 years.”

Historic Significance

The twin-bore, four-lane Liberty Tunnels opened to automobile traffic in January 1924. This was America’s first twin-bore auto-exclusive tunnel, and one of the first automobile-only tunnels. At over a mile long, the Liberty Tunnels comprised America’s longest auto tunnel upon opening. The need for new kinds of public safety systems, including cross-passages and a ventilation system, for such a long tunnel were recognized from the start of the design. As little was known about human health impacts of gases discharged by motor vehicles in tunnels, a significant research and development effort was initiated to support the design of a ventilation system that would protect the public. The tunnel design engineers, led by civil engineer Almos D. Neeld (an 1875 civil engineering graduate of the University of Pittsburgh), engaged and collaborated with the U.S. Bureau of Mines research lab, which was located in Pittsburgh. The collaborative research and design effort developed knowledge and methods that advanced tunnel ventilation engineering nationally.

Novel Design

To ventilate the tunnels, the design included a shaft with four compartments above each tunnel. The ventilation shafts extend 200 feet vertically from the top of the tunnels to the Liberty Tunnels Ventilation System Fan House building floor located in the Mt. Washington neighborhood above the tunnels. Assisted by the air flow induced from moving vehicles, fresh air is pulled in from each entry portal and pulled to the center where the polluted tunnel air is drawn into the ventilation shafts via the fan-house-induced draft flow and ultimately discharged through a tower 110 feet above grade at the Fan House building. Fans also push fresh air through a different compartment of the same shaft down into the tunnel, with discharge above the moving traffic at a point 50 feet from the location of the intake vents in the direction of the exit portal. The moving traffic induces air flow toward the exit portal, providing a second location for discharge of polluted air. Wind breaks at the exit portals reduce interference of cross currents in outside air from interfering with the air flow exiting the tunnels.

ASCE National Historic Civil Engineering Landmark Program: Recognizes historically significant national and international civil engineering projects, structures, and sites. For more information, visit www.asce.org/about-civil-engineering/history-and-heritage/historic-landmarks

ASCE NHCEL Plaque Inscription

Liberty Tunnels Ventilation System

The 5,889-foot-long Liberty Tunnels were the longest automobile tunnels in America when they opened to traffic January 30, 1924. The concentration of exhaust in the long tunnels from early 20th Century motor vehicles created hazardous conditions for the vehicle occupants. Tunnel design engineers, led by civil engineer Almos D. Neeld, collaborated with the U.S. Bureau of Mines in Pittsburgh to develop a solution. The pioneering Liberty Tunnels Ventilation System incorporated a unique combination of induced air draft and forced air exhaust and intake. The research on exhaust emissions informed the design of subsequent vehicular tunnels.