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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 four tunnels that carry eight lanes of freeway traffic, plus a separate tunnel for bicycles and pedestrians. The two original tunnels are twin tunnels bores, completed in 1940 and rehabilitated in 1993. Two doubled-decked tunnels and the bicycle/pedestrian tunnel were built north of the original tunnels in 1991.
The entire tunnel is listed on the National Register of Historic Places (NRHP), ID #82004243. The east portals of the tunnel, along with the Lacey V. Murrow Bridge, are an official City of Seattle landmark.
The official length is 1440 feet, though the perceived length while driving is closer to a kilometer (3280 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 I-90 floating bridge on Lake Washington.
At 63 feet (19 meters) 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 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 be upgraded to a controlled access freeway. The freeway expansion and upgrade was completed in 1993.
Seattle - Portal to the PacificUpon completion of the I-90 expansion, 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 serves as express lanes for transit buses and carpools (2+ people) during peak commute periods. The express lanes flow westbound on week day mornings and eastbound on week day afternoons and evenings. On weekends, the express lanes are open; traffic direction is variable depending on anticipated traffic volumes. When the express lanes are open to eastbound traffic, single occupant vehicles headed to Mercer Island are allowed to use the lanes.
The Washington State Department of Transportation is renovating I-90 between Interstate 5 and the East Channel Bridge to add one HOV lane in each direction. WSDOT is also modifying the Bellevue Way exit and entrance ramps. 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 future HOV lanes. When the HOV lanes and direct access ramps are complete, the express lanes will be dedicated to rapid bus transit use. The two-lane reversible roadway would be converted to carry one lane in each direction 24-hours a day, 365 days per year.
Sound Transit would like to build light rail rapid transit in the current I-90 Express Lanes for its Seattle to Bellevue and Redmond light rail line. Voters in urban and suburban King, Pierce, and Snohomish counties rejected the measure on November 6, 2007, as part of an $18 billion "Roads and Transit" construction package, but approved the plan a year later as part of a transit-only initiative.