The road decks are finished, so final utility work and safety tests remain until the Highway 99 tunnel opens this fall.

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The new Highway 99 tunnel could be ready for drivers by October.

Crews were installing the final rows of lights over the upper, southbound deck and painting the walls white during a news-media tour Tuesday. Fireproof gypsum has been sprayed onto the arched ceilings, which now look like cappuccino-colored stucco rather than a vast grotto of trapezoids.

Contractor Seattle Tunnel Partners (STP) is closing in on a mid-August finish line. Tests of emergency signals and ventilation have started. Once the state confirms the lanes and safety systems are done, other contractors will take three weeks to connect the tunnel portals to their entry and exit ramps.

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During those three weeks, the Battery Street Tunnel, Sodo lanes and Alaskan Way Viaduct will be closed — bringing a period of detours and regional congestion — until the Highway 99 tunnel opens this fall.

“Probably, October would be the earliest,” said Dave Sowers, deputy Highway 99 administrator for the Washington State Department of Transportation (WSDOT).

Schedules must be coordinated with the city of Seattle, which is tearing open First Avenue to build new streetcar tracks and concrete lanes, along with nearby road, bus-lane and bicycle projects. Sowers thinks the public will receive six to eight weeks’ notice before the fall viaduct closure.

More traffic stress arrives in January, when demolition of the old viaduct will cause lane closures along waterfront Alaskan Way and affect pathways to the state ferry terminal.

The double-decked, 1.7-mile tunnel provides two lanes each direction. It has no exits at mid-downtown or Belltown, but it adds ramps next to the portals at Sodo and South Lake Union.

Construction workers this week are pouring a two-inch layer of concrete over the 1,152 recently installed lower-deck slabs. That layer covers what would otherwise be a seam every eight feet under passing cars and trucks.

Groundwater is still dripping into the tunnel entrance at Sodo, where STP is expected to inject more grout to seal scattered leaks.

A new WSDOT video calls the 57 foot, 4-inch-wide tube “One Smart Tunnel” because of its modern safety systems. These include jet fans at either end to pump in clean air. Along the east side of the highway, a series of ventilation ducts and fans suck out foul air during any fire.

Vents also would activate whenever traffic slowdowns, typically below 30 mph, causing vehicle fumes to accumulate, said Susan Everett, WSDOT’s design manager.

There are more than 300 cameras inside. Overhead traffic-information signs can display emergency messages, or instruct people to evacuate, via a walkway behind the west, or waterfront, wall. Fiber-optic heat sensors detect temperatures of 150 degrees, activating fire sprinklers and notifying tunnel operators.

The project is nearly three years late, and lawsuits continue over who pays for up to $600 million in overruns, beyond the $2.1 billion budget. Debates, design or construction have gone on 17 years, since the Nisqually earthquake of 2001 damaged the 65-year-old viaduct.

“I get goose bumps when I go into the tunnel because it’s such an accomplishment,” Sowers said. “Now it looks a little bit more routine, more like the I-90 tunnel or a typical highway tunnel … .

“I know there’s been a lot of work that’s gone to build this thing, a lot of labor, a lot of energy, a lot of years, a lot of politics. But at the end, it’s a monumental accomplishment, it’s a huge engineering feat, and I know all the guys who worked on it are very proud of what they’re seeing,” he said.

During the project, shifting commute patterns from Belltown and Interbay have kept the viaduct a crowded traffic artery, while detractors point to a shift toward transit to say the Legislature’s 2009 decision to build a tunnel replacement was a mistake.