It’s hard to miss the four brick stacks of the Liberty Tunnel Fan House towering over the houses on Secane Avenue in Mount Washington.
“There are two exhaust shafts,” said Bill Lester, Assistant Director of Construction for PennDOT’s District 11, pointing them out. “And there are two intake shafts where we draw fresh air in from up here. We push it down into the tunnel and then we turn around and we drag the bad air back out.”
The fan house is the beating heart of the Liberty Tunnel: that exchange of air—out with the bad and in with the good—is what makes it possible to safely travel the 5,984 feet through the side of a mountain, says Lester.
“You can drive through the tunnel without passing out,” he said.
Every day, more than 34,500 cars pass through the Liberty Tunnel. When it was built, it was the world’s first tunnel specifically designed for automobile traffic and the first to be artificially ventilated.
Lester stands with Joe Edwards, the tunnel’s Lead Electrician, at the fenced edge of an exhaust shaft. It drops 270 feet into the outbound tunnel where cars are racing south. A giant fan pulls up lungfuls of acrid exhaust and pushes it out through the stack. Lester and Edwards only pause for a moment before heading for the door.
“I don’t like to spend a lot of time in here,” Lester said over his shoulder.
No one lingers in the exhaust fan rooms; it means breathing poison, something that became painfully clear when the tunnels opened in 1924.
More than two score of men and women are in a serious condition as the result of being overcome by fumes in the new Liberty Tunnels today…
… poison death swept its vapors through the packed tunnels…
On Saturday, May 10 1924, the front page of the evening edition of the Pittsburgh Press reported that that morning a record 649 cars jammed the Liberty Tunnel. Traffic officers told people to cut their engines. The order was largely ignored and carbon monoxide filled the tunnels, leaving people slumped at the wheel. There were no fatalities.
The project’s engineers had known all along that fumes would be a problem, so they’d designed a ventilation plant to pump in fresh air. But it wasn’t finished until after the tunnels opened, in 1925.
Now when something needs attention in the tunnels, an alarm alerts the fan house and the tunnel’s portal offices, says Edwards. He hits the switch to demonstrate and the mechanical hum rips through the air. He points out a carbon monoxide monitor.
“This is what we’re all watching,” he said. “At fifty parts per million, they start getting alarms. They start turning on fans. And going in and check to see what’s going on.”
“They” are the roughly fifty people in the city’s tunnel system who ensure everything runs smoothly in the Fort Pitt, Squirrel Hill and Liberty Tunnels. At the Liberty Tunnel, that’s just six electricians, said Lester.
“They’re the ones to make sure the lights are on and that the traffic’s moving. It goes all the way from people with flat tires in the tunnel to a major accident,” he said.
One of Edwards main concerns is keeping the lights on.
“People panic when the lights [go] out in that tunnel. It’s dark and they think something’s going on and even though they got headlights on they just panic,” he said.
Anyone that’s driven through a Pittsburgh tunnel and encountered the synchronized slowdown at its mouth can attest that humans in tunnels are panicky creatures. It was for just that reason that Pittsburgh City Engineer W.M. Donley promoted a non-tunnel plan in 1914, arguing, “people prefer the open air and views…not the isolation and fetid air of tunnels.”
He was overruled.
90.5 WESA Celebrates Inventing Pittsburgh is supported by UPMC.