Mount Baker Tunnel
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Mount Baker Ridge Tunnel
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View of the Mount Baker Tunnel and the Lacey V. Murrow and Homer M. Hadley Memorial Bridges from the southwest
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| Location |
 I‑90, Seattle, Washington
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| Coordinates | 47°35′25″N 122°17′55″W / 47.59028°N 122.29861°WCoordinates: 47°35′25″N 122°17′55″W / 47.59028°N 122.29861°W |
| Built | 1940 (original; parallel tunnel built 1989) |
| Architect | Bates & Rogers Construction Corp. |
| Architectural style | Art Deco |
| MPS | Historic Bridges/Tunnels in Washington State TR |
| NRHP reference # | 82004243 |
| Added to NRHP | July 16, 1982 |
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. The tunnel has a double-decked roadway with the bicycle/pedestrian path above the traffic lanes.
The tunnel was listed on the National Register of Historic Places in 1982 (ID #82004243). 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.
The official length is 1,440 feet (440 m), though the perceived length while driving is closer to 1 kilometer (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 former west portal, now located well inside the tunnel, is no longer discernible and its two arch structures were removed during 1989–1993 modification work. 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.
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 only by dashed yellow lines.
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Wikipedia