A driver heading from Maple Leaf to Bellevue came up with a few theories: “To lessen gusts, deal with pollution during traffic jams, or something completely different?”

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The whirring of the giant fans in the Mount Baker Ridge tunnel along Interstate 90 caused Eric Clark to take notice.

So he wrote to Traffic Lab, asking about their purpose.

“I was wondering what the large fans/turbines in the I-90 eastbound tunnel are for?” asked Clark, who drove through the tunnel when he traveled from Maple Leaf to Bellevue for work at the T-Mobile headquarters.

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He came up with a few theories: “To lessen gusts, deal with pollution during traffic jams, or something completely different?”

While those are plausible ideas, Harmony Weinberg, spokeswoman for the Washington State Department of Transportation, said fans are installed to ventilate the tunnel in the event of a fire.

Fire inside a tunnel can be particularly dangerous. A burning car or truck can lead to searing air temperatures, accumulation of toxic gases and low oxygen levels — all potentially deadly.

And the darkness and narrowness of the tunnels can make it difficult for firefighting crews to quickly get to the fire.

That’s where the fans come in.

Supply fans bring in clear air from the outside, while exhaust fans push dirty air out of the tunnel.

The Mount Baker tunnel uses a system that blows fresh air down the tunnel in the direction of traffic.

“If there’s a fire at a location, people behind it get stuck there,” a worker says in a WSDOT video about the ventilation upgrades in I-90 tunnels. “People in front of it can drive out. So then you blow fresh air from behind the people stuck there, so they have fresh air all the time, and the smoke goes down the tunnel out.”

The jet fans in the Mount Baker tunnel were installed in 2015 as part of a WSDOT and Sound Transit project on I-90 that added high-occupancy vehicle lanes to the outer roadways in both directions from Mercer Island to Rainier Avenue South in Seattle.

The plan also traded the center express lanes, which carried reversible traffic, for Sound Transit’s East Link light-rail line to Bellevue, scheduled to open in 2023.

As part of the project, road shoulders were converted to HOV lanes, making fire rescue in the tunnel more difficult.

A 2014 report authored by representatives from WSDOT and the American Society of Heating, Refrigerating and Air-Conditioning Engineers found the previous ventilation system in the Mercer Island and Mount Baker tunnels, built almost 30 years ago, was outdated.

An analysis said the system “may not be able to maintain tenable conditions ensuing from a fire involving heavy goods or flammable liquids cargo vehicles,” the report said.

In the eastbound Mount Baker tunnel, 16 new jet fans were added in the median between the twin bore tunnels.

“If you were to drive eastbound through the Mount Baker tunnel, about halfway along, the tunnel divides into two tunnels,” said Douglas Haight, an engineering manager at WSDOT. “Right at that division space, called the ‘gore,’ [is where] we placed 16 of the jet fans.”

There are also additional tunnel fans that are not visible from the roadway, Haight said.

“They provide the same function as the ones you can see in the roadway, providing high speed, high volume fresh air to the tunnels as needed during an incident,” he said.

Crews also installed new fire and carbon-dioxide-detection devices, added cameras, and upgraded lighting to help drivers and first responders better see inside the tunnel.

In addition to I-90’s tunnels, the section of I-5 under the Washington State Convention Center in downtown Seattle is equipped with fire-suppression systems.

Got a question?

Do you have a question about transportation for Traffic Lab? We’d like to try to answer it. Send your questions to trafficlab@seattletimes.com or tweet at us @STtrafficlab, and we may feature them in an upcoming column.