[8][9] Below the highway decks are utility lines and mechanical spaces for the tunnel's ventilation, lighting, and fire suppression systems.

[10] The tunnel has 15 emergency refuge areas located every 650 feet (200 m) with escape routes that lead to the north and south portals.

[9] The tunnel is monitored by over 300 security cameras that are fed into a WSDOT traffic control center in Shoreline that can dispatch incident response teams.

[7][14] The SR 99 tunnel is tolled with a variable rate that changes based on time of the day, number of vehicle axles, and payment method.

[17][18] The start of tolling was delayed by several months due to vendor issues and the viaduct demolition running behind schedule.

[21] The Alaskan Way Viaduct was a double-decked elevated freeway that ran along Elliott Bay on the Downtown Seattle waterfront and, until January 11, 2019, when it was permanently closed, carried a section of State Route 99 (SR 99).

The 1989 Loma Prieta earthquake destroyed the similarly designed Cypress Street Viaduct in Oakland, California, with the loss of 42 lives.

[28] The 2001 Nisqually earthquake damaged the viaduct and its supporting Alaskan Way Seawall and required the Washington State Department of Transportation (WSDOT) to invest US$14.5 million in emergency repairs.

Many Seattle leaders, including then-Mayor Greg Nickels and state and city transportation officials, favored building a tunnel.

[33] The tunnel was envisioned to solve not only the viaduct's traffic limitations and safety problems, but also to allow better uses for the waterfront real estate, including parks, housing, and retail developments.

While future development of the Alaskan Way real estate corridor may provide tax revenue for the city, many state lawmakers objected to the cost of the proposed six-lane tunnel.

[citation needed] One criticism compared the plan to Boston's Big Dig project, which was said to illustrate the schedule and budget challenges of a large cut-and-cover tunnel.

[35] Many prominent leaders and organizations opposed the elevated structure and believed this was a unique opportunity to remove the viaduct and connect downtown Seattle to the waterfront.

A proposal to remove the viaduct and replace it with surface street and transit improvements was backed by former King County Executive Ron Sims,[39] the People's Waterfront Coalition,[40] and the Congress for the New Urbanism.

Proponents further argued that this plan had the potential to improve the tourist economy, create jobs, and encourage a denser and more residential downtown through the offering of a generous waterfront park.

[44] However, several of the viaduct "stakeholders committee" brought on board to advise the city indicated that the tunnel option should remain on the table.

[50][51] Under the ruling, the referendum would only challenge one section of the original 140-page City Council ordinance that approved agreements over street use, utilities, design and liability for the tunnel following completion of the project's ongoing environmental review process.

[52] The Federal Highway Administration completed its analysis of the project's final EIS and issued its record of decision with WSDOT later that month, allowing pre-construction activities to begin.

[55] A chairman of a local Sierra Club chapter argued that the large investment in automobile transport did not take into account global warming concerns.

[62] The $80 million tunnel boring machine (TBM) Bertha was created for this project by Hitachi Zosen Corporation near Osaka, Japan.

The 326 ft (99 m), 6,700-short-ton (6,100 t) TBM was disassembled into 40 pieces and shipped to Seattle, where it was reassembled in the launch pit near the south end of the future tunnel.

Three days prior to stopping, the machine mined through a hollow, 8-inch steel well-casing and pipe used to measure groundwater in 2002 around Alaskan Way, which was drilled as part of the planning phases of the project.

[79] Even though contractors filled the hole with 250 cubic yards (190 m3) of material, the ground above the tunnel-boring machine continued to sink, according to the Washington State Department of Transportation.

[80] Bertha passed under the Alaskan Way Viaduct in early May, closing the roadway for 11 days as the machine had 15 feet (4.6 m) of vertical clearance under the structure's pilings.

[86][87] Fragments of the steel well casing struck in December 2013 and cited as a possible cause of Bertha's breakdown were stored as evidence at the construction site and subsequently went missing in 2014.

[90] Following the closure of the viaduct, sections of SR 99 were closed to traffic for three weeks to allow for the excavation of buried tunnel ramps at the north and south portals, as well as other preparations.

The closure was widely expected to cause increased traffic congestion, and local government agencies responded by implementing expanded transit service and making other transportation changes to accommodate the displaced drivers.

[98] The first full day of tunnel operations saw lower traffic volumes, with only 22,145 vehicle trips, due to an ongoing snowstorm, as well as confusion at the left-side exits.

[99][100] Within days, reports of smoke and haze within the tunnel prompted WSDOT to investigate possible faults in the exhaust ventilation system.

[104] WSDOT measured traffic before and after the start of tolling and reported a 26 percent decline in average weekday volumes in the tunnel, with no noticeable spike on nearby I-5.