The state plans to equip the crumbling Alaskan Way Viaduct with sensors that will close the elevated roadway at the first sign of seismic activity, Gov. Chris Gregoire said Saturday.

The state plans to equip the crumbling Alaskan Way Viaduct with sensors that will close the elevated roadway at the first sign of seismic activity, Gov. Chris Gregoire said Saturday.

The announcement came as Gregoire and Seattle Mayor Greg Nickels met near Qwest Field to sign an agreement to replace the structure with a deep-bore tunnel.

The system will use sensors, signs and gates on entrance ramps to close the structure in an emergency until engineers know whether it is safe to travel on, Gregoire said.

“It is a measure of how seriously we take safety now until we can get the replacement activated,” Gregoire said.

The $4.2 billion replacement project is scheduled to open in 2015, but until then, the viaduct, which carries more than 100,000 cars per day, will remain the city’s second-busiest north-south arterial after Interstate 5. The structure was damaged in a 2001 earthquake, prompting debate about how to replace it.

In 2007, Seattle voters overwhelmingly rejected a proposal to replace the viaduct with a tunnel and more narrowly rejected a plan to rebuild it as an elevated structure. Transportation officials and politicians deemed the remaining option — tearing it down and shifting traffic onto city streets — impractical, and Nickels pushed ahead with plans for the tunnel.

“There are issues that are worth losing elections over,” he said Saturday. “This is one of them.”

The state will install the new warning system by the end of next year, Gregoire said, and the federal government will pick up the $5 million to $10 million tab.

The tunnel — the most expensive of options for replacing the viaduct — remains controversial, and about a dozen protesters showed up for Saturday’s ceremony, some carrying signs that said “No Big Dig.”

“We’ve had nearly 10 years … of public meetings, town halls, interest-group briefings, thousands of public comments,” Gregoire said. “We have ended the debate, we have made the decision, we have selected the option that will forever change the face of downtown Seattle.”

Nickels, who fought hard for the tunnel replacement, said the signing was historic.

“This will make a huge difference in the face of the city. It is one of the most beautiful cities in the world,” the mayor said. “In six years we will able to stand on or near this spot and look across and be able to see the waters of Elliott Bay; you’ll be reconnecting Pioneer Square with the water, downtown with the water; we’ll have a great place for our public to come.”