Say there’s a fire. A fire caused by a car crash, inside a 2.5-mile tunnel under a major American city. It’s a terrifying idea, but if you want that kind of problem to ignite anywhere, it’s in the stretch of State Route 99 that, later this year, will start whisking traffic underneath downtown Seattle.

That’s because this bit of SR 99 is more than the hemisphere’s largest-diameter bored tunnel and the country’s longest roadway tunnel outside of Alaska, much of it dug by Bertha, formerly the planet’s largest boring machine. To go with such superlatives, it might be the world’s smartest tunnel, too.

The theoretical firefighting begins within moments of the flames’ first appearance, as the tunnel’s 8.3 miles of built-in heat sensors pick up the change in temperature. Of the 300 cameras, those closest to the problem zoom in on the flames. First into action is the deluge system, which can dump 17 inches of water per square foot into the tunnel, through 21 miles of piping. It’s so much water, the tunnel’s engineers had to run models to be sure the decks (those things that hold up the cars) could manage the weight. Eventually, 3.8 miles of drainage pipes will dry things out, ushering the water to a sewer treatment plant.

Next on the emergency procedure checklist comes clearing out the smoke and bringing in fresh air. Most tunnels use jet fans to push air from one end to the other, an understandable but less than nimble approach. SR 99 has vents every 100 feet, so the eight exhaust fans and 17 jet fans can move air into or out any part of tunnel, via the ventilation stacks at either end. “There are very few tunnels able to extract air,” says Susan Everett, the Washington State DOT project engineer in charge of commissioning the tunnel. “And there are very few, maybe one in Japan, that can extract at the pinpoint accuracy we can.”

Meanwhile, the tunnel’s operating system automatically changes the digital signs around the tunnel to stop incoming traffic, and instruct motorists already inside to clear out. They’ll do that on foot, via the world’s only self-sufficient tunnel emergency walkways, complete with their own pressurized air and fan systems.

While all this goes down, the latest info on what’s happening flows through 13 miles of fiber-optic cabling and 95 miles of electrical wiring, to the nearby Traffic Management Center. Inside the NASA-style control room, where monitors line the walls as well as rows of workstations, every movement in the tunnel gets noticed.

At the less catastrophic end of the scale, say a mattress falls of a truck. The tunnel’s sensors will detect the change as cars slow down and move out of the blocked lane (with time, they’ll get better at spotting what’s normal and what isn’t). From there, the cameras will zoom and tilt toward the problem, relaying their feeds to those monitors at mission control.

In case traffic starts to build up, environmental sensors on the roadway deck measure carbon monoxide, NOx, and particulate levels every minute, for a continuous five-minute average. If anything gets alarming, they can automatically turn on the jet fans and, if needed, the exhaust ventilation system to keep everyone safe. “If we see traffic backing up and raising carbon monoxide levels, we can start to extract air right at that location,” Everett says. “It is always equalizing, with more air rushing in.”

All these monitoring systems, of course, get their own lookout. Yet another system tracks recommendations for routine and preventative maintenance. When it’s not saving Seattleites from poisonous smoke (which should be all the time), the emergency passageway doubles as a maintenance corridor, allowing crews to stream up and down the tunnel in electric carts. Workers can pull equipment offline in sections, working on individual components without taking down the entire system.

That bit’s necessary because the watch never ends. Whether it’s the old-school loop detectors (those wires embedded in the roadway to detect vehicle movement) or the newfangled incident detection camera network spotting a flickering light or that wayward mattress, the computers, backed up by Washington State DOT personnel, keep tabs on the tunnel at all times. Because digging deep isn’t always enough—you’ve got to dig smart, too.

Follow Tim Newcomb on Twitter at @tdnewcomb.

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