Variable message signs and emergency phones are also located throughout the entire tunnel. The tunnel has 15 emergency refuge areas located every 650 feet (200 m) with escape routes that lead to the north and south portals. Below the highway decks are utility lines and mechanical spaces for the tunnel's ventilation, lighting, and fire suppression systems. The decks are designed with banks of two degrees in turns and four-degree grades to facilitate designed speeds of 50 miles per hour (80 km/h).
Each deck has two 11-foot (3.4 m) lanes, an 8-foot (2.4 m) west shoulder, and a 2-foot (0.61 m) east shoulder. The SR 99 tunnel is a single tube that measures 9,270 feet (2,830 m) long and 52 feet (16 m) wide, carrying a double-decker highway that is 32 feet (9.8 m) wide and has two lanes in each direction. The completed south portal is seen behind Lumen Field in this aerial view. After several delays, tunnel boring was completed in April 2017, and the tunnel opened to traffic on February 4, 2019. The current plan emerged in 2009 when government officials agreed to a deep-bore tunnel.Ĭonstruction began in July 2013 using " Bertha", at the time the world's largest-diameter tunnel boring machine. Options for replacing the viaduct, which carried 110,000 vehicles per day, included either replacing it with a cut-and-cover tunnel, replacing it with another elevated highway, or eliminating it while modifying other surface streets and public transportation. Since the 2001 Nisqually earthquake, the replacement of the Alaskan Way Viaduct had been the source of much political controversy demonstrating the Seattle process. The 2-mile (3.2 km), double-decker tunnel carries a section of State Route 99 (SR 99) under Downtown Seattle from SoDo in the south to South Lake Union in the north. The State Route 99 tunnel, also known as the Alaskan Way Viaduct replacement tunnel, is a bored highway tunnel in the city of Seattle, Washington, United States.
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