Mount Baker Tunnel

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Mount Baker Ridge Tunnel
Murrow Memorial Bridge 07747.JPG
View of the Mount Baker Tunnel and the Lacey V. Murrow and Homer M. Hadley Memorial Bridges from the southwest
Mount Baker Tunnel is located in Washington (state)
Mount Baker Tunnel
Location I‑90, Seattle, Washington
Coordinates 47°35′25″N 122°17′55″W / 47.59028°N 122.29861°W / 47.59028; -122.29861Coordinates: 47°35′25″N 122°17′55″W / 47.59028°N 122.29861°W / 47.59028; -122.29861
Built 1940
Architect Bates & Rogers Construction Corp.
Architectural style Art Deco
MPS Historic Bridges/Tunnels in Washington State TR
NRHP reference # 82004243[1]
Added to NRHP July 16, 1982
Aerial view of I-90 in Seattle, including the Mount Baker Tunnel and the two parallel bridges to Mercer Island: the Homer M. Hadley Memorial Bridge (left) and the Lacey V. Murrow Memorial Bridge (right)

The Mount Baker Tunnel or Mount Baker Ridge Tunnel carries Interstate 90 under the Mount Baker neighborhood of Seattle, Washington. It is actually a group of three tunnels that carry eight lanes of freeway traffic, plus a separate path for bicycles and pedestrians. The two originals are twin tunnel bores completed in 1940 and rehabilitated in 1993. The newest tunnel was built north of the original tunnels and opened in June 1989.[2] The tunnel has a double-decked roadway with the bicycle/pedestrian path above the traffic lanes.

The entire tunnel is listed on the National Register of Historic Places (NRHP), ID #82004243.[1] The east portals of the tunnel, with murals designed by artist James FitzGerald, along with the Lacey V. Murrow Bridge, are an official City of Seattle landmark.[3]

The official length is 1,440 feet (440 m),[4] though the perceived length while driving is closer to a 1 kilometre (3,300 ft) because of a cut-and-cover "lid" between the western portal and the beginning of the actual tunnel under the Mount Baker ridge. The eastern end of the tunnel links to the Lacey V. Murrow Memorial Bridge and the Homer M. Hadley Memorial Bridge (collectively the I-90 floating bridge) on Lake Washington, to Mercer Island.

At 63 feet (19 m) in diameter, it is the world's largest diameter soft earth tunnel, having been bored through clay.[4][5][6]



When completed in 1940, the twin tunnels connected the I-90 floating bridge on Lake Washington and Rainier Avenue South in Seattle. The bridge and tunnels were part of US Highway 10, which used surface streets between the tunnels and downtown Seattle. US 10 was a four-lane, undivided highway. Each tunnel held two traffic lanes. The north tunnel normally carried two westbound lanes. The south tunnel normally carried two eastbound lanes. In the 1960s, a lane-use signal system was added to improve peak commute traffic. During peak commute times, the signals were used to make the center two lanes reversible. In the mornings, three lanes flowed west and one lane flowed east. In the evenings, three lanes flowed east and one lane flowed west. During morning and evening commutes, one tunnel carried two lanes of traffic in the same direction while the other tunnel carried opposing traffic separated by only dashed yellow lines.

The highway would later become part of Interstate 90, be expanded, and upgraded to a controlled-access freeway. The freeway expansion and upgrade was completed in 1993.


Eastern portal of 1989 tunnel
Cross section of 1989 tunnel. Top to bottom: pedestrian and bicycle trail, westbound motor vehicle traffic, Link light rail.

Upon completion of the Interstate 90 expansion program in the 1990s, the freeway capacity increased to three general-purpose lanes in each direction and two reversible HOV lanes. Eastbound traffic uses the original tunnels. Westbound traffic uses the upper deck of the northern new tunnel. A two-lane reversible roadway uses the lower deck. The reversible roadway served as express lanes for transit buses and carpools (2+ people) during peak commute periods until 2017. The express lanes flowed westbound on weekday mornings and eastbound on weekday afternoons and evenings. On weekends, the express lanes were open; traffic direction was variable depending on anticipated traffic volumes. When the express lanes were open to eastbound traffic, single-occupant vehicles headed to Mercer Island are allowed to use the lanes per an inter-local agreement signed in 1976.

Above the westbound traffic is the I-90 Trail tunnel. At 1,330 feet (410 m), it is the longest non-adapted trail tunnel in the world. On the western half, the trail emerges onto the lid and uses trails in Lid Park.[7]

In 2017, the Washington State Department of Transportation added a pair of high-occupancy vehicle lanes (HOV lanes) to Interstate 90 from Seattle to Bellevue, replacing the reversible express lanes. The express lanes were closed in June 2017 for the construction of the East Link light rail project, which will extend Link light rail service from Seattle to Bellevue and Redmond in 2023. On Mercer Island, WSDOT and Sound Transit are building direct access ramps to allow buses and HOVs to exit and enter I-90 from the HOV lanes.


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