The SR 99 tunnel is one of the safest tunnels ever constructed, built to withstand strong earthquakes and equipped with smart safety systems designed to keep traffic moving and people safe. In the rare event an emergency requires drivers and passengers to leave the roadway, there are doors every 650 feet leading to safe areas to wait or exit the tunnel. An exit pathway runs the entire length of the tunnel and is equipped with its own state-of-the-art ventilation and fire suppression systems. Tunnel operators will monitor the SR 99 tunnel 24 hours a day, 7 days a week, utilizing an extensive camera system able to detect anything unusual inside the SR 99 tunnel.
Safety shoulder for pulling over
The tunnel features two 11-foot travel lanes in each direction. Long, gentle curves provide good sight distances. Each roadway has an eight-foot-wide shoulder, providing space for vehicles to pull over in case of a breakdown or collision and for emergency vehicles to travel.
Ventilation for safe air
The tunnel has three different types of fans to keep fresh air flowing in the tunnel and remove fumes from engines or fires. Under normal operations, vehicles traveling through the tunnel act like pistons, pushing fresh air into and through the tunnel. When environmental monitoring stations inside the tunnel detect increasing particulates in the air, jet fans automatically activate and supplement the air flow. In the event of a vehicle fire, the tunnel’s eight enormous extraction fans pull smoke out via vents in the tunnel wall while the jet fans push fresh air inside. The extraction fans push the smoke out the tunnel’s yellow ventilation stacks, located atop the operations buildings at both ends the tunnel.
Testing the SR 99 tunnel's ventilation system with a smoke bomb in a garbage can. pic.twitter.com/Lwaw4IWiul— SR 99 (@BerthaDigsSR99) October 18, 2018
Inside the tunnel’s emergency exit corridor, maintenance fans provide slight pressurization (much like an elevator shaft) so when emergency doors are opened, smoke is kept outside the exit areas.
Emergency exits and refuge areas
Enclosed emergency walkways with independent ventilation and fire control systems run parallel to both levels of roadway in the tunnel. Concrete walls and fire-resistant doors separate the emergency areas from the tunnel’s roadways.
In an emergency, travelers can leave the roadway area by following signs that point toward the closest emergency door. There are emergency push-to-talk phones inside the corridor where someone can immediately connect with a 9-1-1 operator and hear emergency instructions over a loudspeaker. Travelers will be instructed to shelter in place in these refuge areas or take the stairs to reach the exit corridor and walk to the nearest end of the tunnel. Emergency signs point to the nearest tunnel exit point.
Travelers unable to use stairwells will be safe in the refuge areas. These areas can accommodate several people, including those with wheelchairs. Emergency phones and cameras allow responders to aid those inside the refuge area, if needed.
In case of fire
The tunnel’s fire response system is state-of-the-art. Incident detection cameras can spot a fire and alert tunnel operators in the tunnel’s control center, who monitor the tunnel 24/7. The fire notification triggers a countdown for the deluge sprinklers. Tunnel operators cans activate the sprinklers or the sprinklers can activate automatically. The deluge system works inside a fire zone – so fire response is targeted within the tunnel. Vents will open along the wall in the area of a fire so powerful extraction fans can pull smoke out of the tunnel and up through the yellow ventilation stacks at the tunnel’s north and south portals. Fire extinguishers and emergency phones are also located throughout the tunnel.
A suite of safety systems makes the SR 99 tunnel one of the “smartest” tunnels ever built. The tunnel has a 24-hour control center for quick response to changing travel conditions and emergencies. More than 300 incident and security cameras provide real-time information to WSDOT’s tunnel operators.
Real-time traffic technology instructs drivers via overhead traffic signs – minimizing delays caused by collisions or stalled vehicles. In the event of an emergency, electronic signs, a public address system and an AM/FM override system provide advisory messages. The control center has direct lines to the Seattle fire and police departments and other emergency responders.
Two independent power sources ensure a reliable, continuous source of electricity. During a regional power failure, generators will power critical tunnel systems and help close the tunnel.
The SR 99 tunnel is designed to withstand a 2,500-year quake, which is roughly a 9.0-magnitude earthquake along the Cascadia Subduction zone. Geotechnical and structural engineers agree that tunnels are among the safest locations during an earthquake. This is because earthquake waves are most severe above the surface. Underground structures like tunnels move with the soil, while above-ground structures sway back and forth.
The tunnel is designed to remain watertight after a seismic event, allowing for the safe evacuation of people. Joints between the tunnel’s rings provide flexibility so the tunnel can move with an earthquake. Every piece of equipment installed in the tunnel has undergone seismic analysis.
The Seattle Seawall Project provides additional earthquake and tsunami resilience. It is highly unlikely that a tsunami could overtop the seawall and reach the tunnel (WSDOT and City of Seattle analysis found this could only happen if a tsunami coincided with a very high tide – a combination of events estimated to occur only once every 6,000 to 24,000 years). If that were to happen, Washington’s early warning system and the tunnel’s real-time traffic technology would immediately restrict vehicles from entering the tunnel. If water entered the tunnel, drains and pump systems capable of pumping up to 300 gallons per minute would help to quickly remove the water.