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The Alaskan Way Viaduct is closed this weekend between 6 a.m. and 6 p.m. each day. The structure is closed for a full inspection twice a year to ensure it remains safe for drivers, according to WSDOT. Saturday morning WSDOT offered a press tour of the viaduct to highlight to aspects, the nine newly installed gates called the "Automated Closure System" and the "SR99 - S. Holgate Street to S. King Street Viaduct Replacement Project".

According to WSDOT's website, "WSDOT is installing a system designed to clear traffic from the aging and vulnerable structure as quickly as possible within two minutes of the first signs of a moderate to severe earthquake.

"The new system consists of nine traffic gates, similar to railroad crossing gates, located at all viaduct access points and controlled by an earthquake detection system.

"When the earthquake detection system detects a magnitude-5 earthquake or stronger, all of the gates will lower simultaneously, warning lights will flash, bells will ring and the system will automatically transmit a message alerting WSDOT's 24-hour traffic management center and SDOT that the viaduct is closed.

"The automated viaduct closure system will be in operation until we build a replacement for the viaduct, and demolish the existing structure. The project is expected to be fully operational in April."

Regarding the southern viaduct replacement project, WSDOT says, "We are committed to keeping people and goods moving during construction. We designed the project to keep at least two lanes of SR 99 open in each direction through the construction zone. To keep the lanes open, we will first build about half of the new SR 99 roadway west of the existing viaduct between S. Royal Brougham Way and S. Holgate Street. We'll then shift both directions of traffic to the new road before tearing down the existing viaduct.

"Additionally, we will build a temporary detour connecting the new roadway to the viaduct north of S. Dearborn Street."

SPOKESMAN ON THE VIADUCT TOUR TODAY

"The emergency auto gates are hooked up to earthquake sensors installed in the ground by the viaduct, explained Matt Preedy, Dir., South End Projects, Viaduct Replacement Project. "Three sensors have to register a 5.0 earthquake or greater. The gates come down automatically within about a minute. In about five minutes the vehicles on the (viaduct) will flush out. It keeps a whole bunch of people from going into a potentially dangerous situation.

"If we didn't have the gates it would take us up to two hours to close the corridor. The personnel that would have had to close the viaduct would be free to do triage in the greater Seattle area. The one structure you know is likely to take the most damage you might as well get something in place that takes care of controlling access to it so that the emergency crews can look at other things.

"We wanted to set the (5.0) threshold at the level low enough that there was potential damage it would certainly be closed (...) That is the trigger where you'd likely see additional damage. It would probably take a stronger earthquake than that to collapse (the viaduct). We've said that the Nisqually had lasted 10 seconds longer than it did the viaduct would not be here. That was a 6.8 but was centered near Olympia. A 6.8 here would be a lot more dramatic."