Mount Baker Tunnel

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 tubes (or four—if counting the two original Mount Baker Tunnels separately) that carry eight lanes of freeway traffic, plus a separate path for bicycles and pedestrians, and a light-rail track that is currently under construction. The original tubes are twin tunnel bores completed in 1940 and rehabilitated in 1993. The new Mount Baker 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 titled Portal of the North Pacific 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 3,300 feet (1,000 m) 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.

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.

The expansion included construction of a new, parallel tunnel with a larger diameter, located north of the original tunnel. Construction began in 1983. When the overall I-90 project was completed in 1993, the twin 1940 tunnels would be connected to the southern I-90 floating bridge (the Lacey V. Murrow Memorial Bridge) and carry eastbound traffic only, while the 1989 tunnel would be connected to the northern floating bridge (the then-new Homer M. Hadley Memorial Bridge) and be westbound-only for general traffic.

The original pair of tunnels were each 29 feet (8.8 m) in diameter, while the new tunnel was 63 feet (19 m) in diameter. The new tunnel was built to carry traffic on three levels: two lanes for buses and carpools on the bottom level, three lanes of general traffic on the middle level, and cyclists and pedestrians on the top level. Excavation was completed in 1986, and the new tunnel opened to traffic in June 1989. Although designed to carry only westbound traffic, the new tunnel and then-new Homer Hadley Memorial Bridge temporarily carried all traffic—in both directions—when opened in 1989, while work continued on modifying the old Mercer Island Bridge where it connected to the 1940 Murrow Bridge and on rehabilitation of the latter.