The
Mount Baker Tunnel or
Mount Baker Ridge Tunnel carries
Interstate 90Interstate 90 is the longest Interstate Highway in the United States at . It is the northernmost coast-to-coast interstate, and parallels U.S. Route 20 for the most part. Its western terminus is in Seattle, Washington, at 4th Avenue S. next to Safeco Field and Qwest Field, and its eastern terminus...
under the
Mount Baker neighborhoodMount Baker is an upscale neighborhood in southeast Seattle. A geographically diverse area, it offers a mix of parklands, lakefront, wooded hills, and quiet residential streets and boulevards. The neighborhood's name comes from the view of Mount Baker in Whatcom County, that is seen by looking...
of
Seattle, WashingtonSeattle is located in the Pacific Northwest region of the United States. Situated in the western part of Washington State on an isthmus between Puget Sound and Lake Washington, about south of the Canada – United States border, it is named after Chief Sealth, of the Duwamish and Suquamish tribes...
. 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 PlacesThe National Register of Historic Places is the United States government's official list of districts, sites, buildings, structures, and objects deemed worthy of preservation...
(NRHP), ID #82004243.
The
Mount Baker Tunnel or
Mount Baker Ridge Tunnel carries
Interstate 90Interstate 90 is the longest Interstate Highway in the United States at . It is the northernmost coast-to-coast interstate, and parallels U.S. Route 20 for the most part. Its western terminus is in Seattle, Washington, at 4th Avenue S. next to Safeco Field and Qwest Field, and its eastern terminus...
under the
Mount Baker neighborhoodMount Baker is an upscale neighborhood in southeast Seattle. A geographically diverse area, it offers a mix of parklands, lakefront, wooded hills, and quiet residential streets and boulevards. The neighborhood's name comes from the view of Mount Baker in Whatcom County, that is seen by looking...
of
Seattle, WashingtonSeattle is located in the Pacific Northwest region of the United States. Situated in the western part of Washington State on an isthmus between Puget Sound and Lake Washington, about south of the Canada – United States border, it is named after Chief Sealth, of the Duwamish and Suquamish tribes...
. 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 PlacesThe National Register of Historic Places is the United States government's official list of districts, sites, buildings, structures, and objects deemed worthy of preservation...
(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 bridgeThe Lacey V. Murrow Memorial Bridge is a floating bridge that carries the eastbound lanes of Interstate 90 across Lake Washington from Seattle to Mercer Island, Washington. It is the third longest floating bridge in the world at , whereas the second longest is the Evergreen Point Floating Bridge...
on
Lake WashingtonLake Washington is the largest natural lake in King County and the second largest in Washington . It is bordered by the cities of Seattle on the west, Bellevue and Kirkland on the east, Renton on the south and Kenmore on the north, and surrounds Mercer Island...
.
At 63 feet (19 meters) in diameter, it is the world's largest diameter soft earth tunnel, having been bored through
clayClay is a naturally occurring material composed primarily of fine-grained minerals, which show plasticity through a variable range of water content, and which can be hardened when dried and/or fired...
.
Original
When completed in 1940, the twin tunnels connected the
I-90 floating bridgeThe Lacey V. Murrow Memorial Bridge is a floating bridge that carries the eastbound lanes of Interstate 90 across Lake Washington from Seattle to Mercer Island, Washington. It is the third longest floating bridge in the world at , whereas the second longest is the Evergreen Point Floating Bridge...
on
Lake WashingtonLake Washington is the largest natural lake in King County and the second largest in Washington . It is bordered by the cities of Seattle on the west, Bellevue and Kirkland on the east, Renton on the south and Kenmore on the north, and surrounds Mercer Island...
and Rainier Avenue in Seattle. The bridge and tunnels were part of
US Highway 10U.S. Route 10 is an east-west United States highway formed in 1926. Though it was never the cross-country route suggested by the "0" in its route number, US 10 was one of the original long-haul highways, from Detroit, Michigan, to Seattle, Washington, before losing much of its length to the...
, 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.
Current
Upon 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.
Future use
The
Washington State Department of TransportationThe Washington State Department of Transportation , was established in 1905. The agency, led by a Secretary and overseen by the Governor, is a Washington governmental agency that constructs, maintains, and regulates the use of the state's transportation infrastructure...
is renovating I-90 between
Interstate 5Interstate 5 is the main Interstate Highway on the West Coast of the United States, paralleling the Pacific Ocean from Canada to Mexico and serving some of the largest cities of that part of the U.S., including Seattle, Tacoma, Portland, Sacramento, San Francisco/Oakland, Los Angeles, and San Diego...
and the
East Channel BridgeThe East Channel Bridge is a bridge carrying Interstate 90 from Mercer Island, Washington, to Bellevue, Washington, over the East Channel of Lake Washington, which separates Mercer Island from the rest of the Eastside....
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 TransitSound Transit has been the popular name of Washington state's Central Puget Sound Regional Transit Authority since September 19, 1999. It was formed in 1996 by the Snohomish, King, and Pierce County Councils...
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 eastside link light rail use.
Sound TransitSound Transit has been the popular name of Washington state's Central Puget Sound Regional Transit Authority since September 19, 1999. It was formed in 1996 by the Snohomish, King, and Pierce County Councils...
will soon begin to build light rail rapid transit in the current I-90 Express Lanes for the
East LinkEast Link is an eastern extension of the Sound Transit Link Light Rail system spanning about 22 miles from Downtown Seattle to the suburb of Redmond...
light rail line connecting Seattle with the Bellevue and Redmond suburbs. 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.