Designers equipped the deep tube with a 21-mile network of sprinkler pipes, along with jet fans, surveillance cameras and instant message signs, that together reflect the world's newest safety features.

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Sooner or later a crash or fire will occur inside the new Highway 99 tunnel, where the vehicle exits are a daunting two miles apart.

To quell the dangers, designers equipped the deep tube with a 21-mile network of sprinkler pipes, along with jet fans, surveillance cameras and instant message signs, that together reflect the world’s newest safety features.

“This is one of the best,” said Brian Russell, regional vice president of HNTB, an engineering firm for builder Seattle Tunnel Partners.

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The need for preparedness was driven home last month, when a charter bus carrying the Stanford University track team burst into flames on northbound Interstate 5 near Columbian Way. What if a vehicle catches fire in the tunnel?

Temperature sensors on the walls would trip alarms. Water would automatically start spraying in 87 seconds, or sooner if engineers at the regional traffic-control center in Shoreline turn them on. Tunnel decks are fully covered by security cameras that make a quicker response likely, said Susan Everett, tunnel design manager for the Washington State Department of Transportation (WSDOT).

Water would fall at a rate of one-third of an inch per minute. This rate should quench an open car fire, but not a fire under the hood, Everett said. In either case, sprinklers will keep flames from spreading until firefighters arrive, said Brian Parker, an HNTB mechanical engineer.

Equally important, the deluge would avert extreme heat buildup. This matters because a prolonged fire near 2,000 degrees can make concrete explode like popcorn. That has happened in European tunnels, including one under Mont Blanc between France and Italy in 1999, where smoke and heat killed 39 people.

Seattle fire code bans petroleum trucks within the Highway 99 tunnel, and requires tunnel systems that hold temperatures below 716 degrees. To supplement its sprinklers, tunnel ceilings and upper walls were sprayed with a fireproof calcium liner that looks like stucco.

Smoke would be sucked sideways through wall-mounted baffles, into a closed passage. Huge 500-horsepower fans would propel smoke out the north or south end, each of which has four yellow smokestacks. The smoke would be replaced by outdoor air pumped in by 17 jet fans mounted at the tunnel ends.

People could escape through emergency doors, marked in green every 650 feet, on the Puget Sound side of the tunnel — the right going southbound, left going northbound.

The door leads to a refuge area, equipped with a camera, emergency phone and vents where clean air is pumped in. Stairs lead to an emergency corridor, where people can walk to either Sodo or South Lake Union. Those in wheelchairs or who are injured might wait in the refuge zone until help arrives.

WSDOT tunnels average only about one small car fire per year, Everett said.

Police and firefighters drilled in September for a worst-case crash involving two buses, one fire and 24 injuries. The exercise turned up one problem — a 3- to 5-minute delay until information was relayed from the Shoreline control center through emergency dispatchers to firefighters, said fire Battalion Chief Brady O’Brien. A direct hotline connection was created to solve that, he said.

People in fender-bender, noninjury collisions should drive out of the tunnel. “It doesn’t matter if your flashers are on, when you’re stopped in the roadway it’s a hazardous situation, somebody could not be paying attention and rear-end you,” said Capt. Eric Sano of the police traffic unit.

Failing that, try to park on the 8-f0ot-wide shoulder. In most situations, authorities say drivers should stay in vehicles until a state incident-response truck arrives to shield the area, or until police or firefighters show up. WSDOT traffic-control specialists would activate lane-closure signs, warning messages, or a tunnel closure. They might tell drivers to evacuate, over public-address speakers. Cellphones and radios work in the tunnel.

During a blockage, police and paramedics might drive and stop on the opposite tunnel deck, then use the emergency stairs to reach the victims. Or they could wait a couple of minutes for traffic to exit, then drive the “wrong way” toward the scene.

Other safety features include extra lights at the portals, so drivers’ eyes adjust easier between outdoor and indoor roadways, and systems to detect and ventilate carbon monoxide when gases accumulate during a traffic jam.

HNTB engineers designed the reinforced tube with two-foot-thick walls, as WSDOT required, to withstand the worst quake expected in 2,500 years, such as a magnitude 9 rupture of the Cascadia Subduction Zone off Washington’s Pacific coast.

Local emergency managers think a worst-case tsunami in Puget Sound, combined with 1 meter of sea-level rise from climate change, would total roughly 16 feet. This level would barely reach the Sodo entrance, where saltwater would flow beneath car decks and travelers.