Engineers are designing the SR 99 tunnel to withstand an earthquake, flooding or other disaster. The tunnel will also include state-of-the-art ventilation, fire detection and suppression, security and lighting systems.
Safe during natural disasters
Earthquakes
Geotechnical and structural engineers agree that tunnels can be designed as one of the safest places to be during an earthquake. The SR 99 tunnel is being designed to withstand an earthquake that only happens every 2,500 years on average (in the range of a 9.0 on the Richter scale) without collapsing.
Rising sea levels and tsunamis
The Seattle Department of Transportation is designing the Elliott Bay Seawall replacement to protect waterfront facilities, such as the SR 99 tunnel, against sea level rise and moderate storm surges.
In the event of a tsunami, it is highly unlikely that a wave would overtop the seawall and reach the tunnel. WSDOT and the City of Seattle found this could only happen during a very high tide - a combination of events estimated to occur only once every 6,000 to 24,000 years. Washington’s early warning system would allow us to restrict traffic from entering the tunnel during a tsunami. If necessary, drains and pump systems will help to quickly remove any water from the tunnel.
State-of-the-art safety features
Safe travel lanes
Two 11-foot travel lanes with an eight-foot safety shoulder and two-foot shoulder in each direction will ensure enough space for all vehicles and legal size trucks. Long, gentle curves will allow for safe sight distances.
Tunnel control center
The tunnel will have a 24-hour control center that allows quick response to changing travel conditions and emergencies. State-of-the-art control systems will provide real-time information to WSDOT’s tunnel operators. The control center will have direct lines to the Seattle Fire Department, Police Department and other emergency responders.
Incident response
Real-time traffic technology will minimize delays caused by collisions, stalled vehicles or other similar disruptions in the tunnel. If a collision occurs, incident detection systems would allow tunnel operators to view and respond to the incident. If one lane of the tunnel is blocked, overhead electronic signs would quickly close the lane to travelers, and variable speed limit signs would maximize traffic flow through the open lanes. Emergency vehicles would then enter the tunnel and remove the disabled vehicles.
Emergency exits and refuge areas
Safe and effective evacuation routes will be provided for motorists. Enclosed emergency walkways, with independent ventilation and fire control systems, will run parallel to both traffic levels in the tunnel. The walkways will be separated from the tunnel’s roadways by concrete walls and fire-rated doors.
Access to the walkways will be provided about every 650 feet. In an emergency, travelers would walk along the shoulders to reach an emergency doorway and a safe refuge area. A flight of stairs will connect the refuge area to the emergency exit walkway and the non-affected level of the tunnel.
Travelers unable to evacuate using the stairs would be protected by staying in the safe refuge areas, which will be equipped with fire-rated doors and lighting, ventilation and fire suppression systems. Refuge areas will also be monitored by cameras, provided with an emergency phone, and will be large enough to accommodate several people, including those with wheelchairs. Fire, police or WSDOT incident response vehicles would be dispatched to help those waiting in the refuge areas.
Additional tunnel safety systems
- Air monitoring and ventilation: The ventilation system will measure and reduce the levels of harmful vehicle emissions in the tunnel. It will also be able to control smoke and gases effectively in the event of a tunnel fire.
- Emergency power: Two independent power sources will ensure a reliable source of electricity. Generator systems will be provided for safe tunnel closure and the maintenance of critical systems during a regional power failure.
- Emergency notifications: In the event of an emergency, electronic signs, public address and AM/FM rebroadcast systems will provide advisory messages.
- Telephones: Cellular telephones will work in the tunnel and emergency phones will be located in refuge areas.