There was not yet a ventilation system when the tunnels opened, but authorities decided the flow of traffic would allow for a natural draft of ventilation. On May 10, 1924, a traffic jam which occurred due to a Pittsburgh Street Railway Company strike, causing vehicles to be stopped, idling in the tunnels. Several motorists passed out from the fumes emitted from the exhaust of the congested vehicles. For the safety of motorists, the number of vehicles permitted through the tunnels was regulated until a ventilation system was installed. With help from the U.S. Bureau of Mines, tunnel engineers designed a system to ventilate the tunnels to accommodate the high traffic flow. In 1928, construction of four vertical shafts was completed to continuously provide fresh air to the tunnels. A plant located on top on the mountain was constructed to pump the fresh air through the shafts, which also tower above the plant. After the ventilation system was put into service, the traffic restrictions were lifted. Liberty Bridge Following the completion of the Liberty Tunnels, the Liberty Bridge was completed and linked to the tunnels on March 27, 1928. The bridge and tunnel combination gave motorists and pedestrians a direct route to the city without traveling over or around Mt. Washington. Dimensions and specs in length Twin tunnels (1 inbound, 1 outbound) 2 lanes in each tunnel Vertical wall horseshoe profile 11 cross passages (Liberty Tunnels) posted vertical clearance (Liberty Tunnels) Each tunnel is wide and high to arch (Liberty Tunnels) Floor slopes upward 0.329 percent grade to Saw Mill Run portal higher (Liberty Tunnels) Opened in 1924 Between: Liberty Bridge [SR 3069]; P. J. McArdle Roadway (Mt. Washington Roadway) Saw Mill Run Blvd [PA 51]; West Liberty Ave [SR 3069] (Liberty Tunnels) Reception In 1941 it became the first tunnel in the world to provide AM radio reception throughout. Since August 1987 the tunnels have provided cellular phone reception. The Liberty Tunnels were not the first tunnel in the United States built exclusively for automobiles. As the Heinz History Center points out, the Liberty Tunnels were the longest tunnels at that time built exclusively for automobiles. The distinction of being the first automobile tunnels in the United States belongs to the Hill St. Tunnel in Los Angeles, which was built 12 years earlier. References Workmaster, Wallace F. Mt. Lebanon History and Information. (n.d.). Retrieved March 12, 2008, from History &