Central Freeway (Seattle)

Interstate 5 (I-5) is an Interstate Highway on the West Coast of the United States that serves as the region's primary north–south route. It spans across the state of Washington, from the Oregon state border at Vancouver, through the Puget Sound region, to the Canadian border at Blaine. Within the Seattle metropolitan area, the freeway connects the cities of Tacoma, Seattle, and Everett.

I-5 is the only interstate to traverse the whole state from north to south and is Washington's busiest highway, with an average of 274,000 vehicles traveling on it through Downtown Seattle on a typical day. The segment in Downtown Seattle is also among the widest freeways in the United States, at 13 lanes, and includes a set of express lanes that reverse direction depending on time of the day. Most of the freeway is four lanes in rural areas and six to eight lanes in suburban areas, including a set of high-occupancy vehicle lanes in the latter. I-5 also has three related auxiliary Interstates in the state, I-205, I-405, and I-705, as well as several designated business routes and state routes.

The freeway follows several historic railroads and wagon trails developed during American settlement of western Washington in the mid-to-late 19th century. The state legislature incorporated local roads into the Pacific Highway in 1913, connecting the state's southern and northern borders between Vancouver and Blaine. The Pacific Highway was built and paved over the next decade, and became the northernmost segment of the national U.S. Route 99 (US 99) in 1926.

The federal government endorsed the creation of a national expressway system in the 1940s, including several bypasses on US 99 that were built by the state in the early 1950s. The state's planned toll superhighway in the Seattle area was shelved in favor of a federally-funded freeway under the new Interstate Highway System, under which I-5 was created in 1957. Construction of I-5 was completed in 1969, and several segments of the highway have been widened or improved in the decades since.

Route description

Interstate 5 is the only Interstate to traverse Washington from north to south, serving as the primary highway for the western portion of the state. It is listed as part of the National Highway System and the state's Highways of Statewide Significance program, recognizing its connection to major communities. I-5 has three auxiliary Interstate Highways within Washington: I-205, an easterly bypass of Portland, Oregon, and Vancouver; I-405, bypassing Seattle via the Eastside; and I-705, a short spur into Tacoma., wit
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It was designated as the Purple Heart Trail in 2013 by the Washington State Transportation Commission to honor wounded military veterans.

The freeway runs through the most densely populated region of Washington state, with 4.6 million people living in the nine counties on the corridor, approximately 70 percent of the state's population. Several of the largest cities along the I-5 corridor are also connected by the parallel ''Cascades'', a regional train service between Eugene, Oregon, and Vancouver, British Columbia, operated by Amtrak and funded by the state governments of Oregon and Washington.

I-5 is maintained by the Washington State Department of Transportation (WSDOT), who conduct an annual survey of traffic volume that is expressed in terms of annual average daily traffic (AADT), a measure of traffic volume for any average day of the year. The stretch of I-5 through Downtown Seattle is the busiest highway in Washington state, with a daily average of over 274,000 vehicles in the mainline and express lanes. The least-traveled segment of I-5 is located at SR 548 in Blaine, with a daily average of 6,600 vehicles. I-5 through the Seattle metropolitan area is among the worst congested highways in the United States, with 78 percent of peak direction miles classified as "routinely congested" for seven to eight hours a day and an average annual delay of 55 hours for Seattle– Everett commutes. The freeway has a maximum speed limit of in rural areas and in urban and suburban areas, which includes a section between Tumwater and Marysville.

Southwestern Washington

I-5 enters Washington on the Interstate Bridge, a pair of vertical-lift bridges that span the Columbia River between Portland, Oregon, and Vancouver, Washington. The bridge is the only point on I-5 where vehicles have to stop for cross traffic, because of the lifts. On the north bank of the river, the freeway passes under a railroad viaduct carrying Amtrak's ''Empire Builder'' and intersects SR 14. The interchange with SR 14, located west of Pearson Field and the Fort Vancouver National Historic Site, also includes ramps serving downtown Vancouver. I-5 continues north through suburban Vancouver and into Hazel Dell, passing the Clark College campus and intersecting SR 501 at Fourth Plain Boulevard and SR 500 at Burnt Bridge Creek. I-5 intersects I-205, the eastern freeway bypass of the Portland metropolitan area, in Salmon Creek near the Vancouver campus of Washington State University.

From Salmon Creek, I-5 continues northwesterly and intersects SR 502 at the Gee Creek rest area west of Battle Ground. Its next interchange, in eastern Ridgefield, forms the eastern terminus of SR 501. The freeway passes the Ilani Casino Resort on the Cowlitz reservation and crosses the Lewis River into Woodland, where it intersects SR 503. Northwest of Woodland, the median of I-5 is used by freight trains and Amtrak's ''Cascades'' and ''Coast Starlight'' passenger trains, which follow the freeway for its entire length. I-5 continues along the east bank of the Columbia River, passing through Kalama on the way towards Longview and Kelso. At the south end of Kelso, near the confluence of the Columbia and Cowlitz rivers, the freeway intersects SR 432, which connects to Longview and the Lewis and Clark Bridge via SR 433. I-5 continues north along the Coweeman River to the Three Rivers Mall, located east of downtown Kelso, where SR 4 terminates. Between Vancouver and Kelso, the highway is part of the Lewis and Clark Trail, a state scenic highway that continues west along SR 4 towards the Pacific Ocean.

The freeway continues north, following the Cowlitz River to Castle Rock, where it meets SR 411 and a short business route that loops back to an interchange with SR 504, the main highway to Mount St. Helens. North of Castle Rock, the Cowlitz River turns away from I-5 as the latter enters Lewis County, intersecting SR 506 west of Toledo and SR 505 east of Winlock. Between the two interchanges is the Gospodor Monument Park, a roadside attraction with four sculptures of up to in height commemorating religious and indigenous figures. After their installation in 2002, the sculptures caused backups on the freeway due to rubbernecking by passing drivers who slowed down near them. Near Napavine, I-5 becomes concurrent with US 12, which continues east across White Pass to Yakima.

The two highways intersect SR 508 and cross over the Newaukum River near the Uncle Sam billboard, a politically conservative message board and roadside attraction. I-5 and US 12 turn northwest to follow the river and pass along the western edge of Chehalis, where they intersect SR 6. After passing the Chehalis-Centralia Airport, the freeway follows the Chehalis River to the western side of Centralia. I-5 and US 12 then intersect SR 507 and gain a set of collector–distributor lanes as the freeway crosses the Skookumchuck River and a set of railroad tracks on the northwest side of the city. US 12 leaves the concurrency at Grand Mound, heading west towards Aberdeen while I-5 continues north into Thurston County.

South Sound region

North of Grand Mound, I-5 passes two interchanges with SR 121, which forms a loop between two of the exits to serve Millersylvania State Park. The freeway travels through the suburb of Tumwater, passing the Olympia Regional Airport and several state office parks before reaching the terminus of US 101, a major highway that encircles the Olympic Peninsula, on the south side of Capitol Lake. After the interchange, I-5 enters Olympia and turns eastward after passing the Washington State Capitol campus and downtown Olympia. The freeway passes through Lacey and the Hawks Prairie industrial area, where it intersects SR 510, a highway serving the Nisqually Indian Community and Yelm. I-5 continues northeast through the Billy Frank Jr. Nisqually National Wildlife Refuge towards DuPont on the east side of the Nisqually River in Pierce County.

Immediately east of DuPont, I-5 travels through Joint Base Lewis–McChord, a major military installation that encompasses land on both sides of the freeway and its parallel railroad. Near American Lake, an interchange with Thorne Lane marks the proposed western terminus of SR 704, a new highway that would travel between the boundaries of Fort Lewis and McChord Air Force Base (the two components of Joint Base Lewis–McChord) to Spanaway. Continuing past the bases, I-5 passes through Lakewood and intersects SR 512, a freeway connecting east to Puyallup, before it reaches Tacoma.

In Tacoma, the freeway passes the Tacoma Mall, turns east, and splits into collector–distributor lanes that run through central Tacoma and serve two interchanges: the terminus of SR 16, which continues northwest over the Tacoma Narrows Bridge to the Kitsap Peninsula; and I-705 and SR 7, which serve downtown Tacoma, the Tacoma Dome, Tacoma Dome Station, and the Pacific Avenue corridor. East of the Tacoma Dome area, I-5 intersects SR 167 and crosses over the Puyallup River and a railroad carrying Sounder commuter trains. The freeway reaches Fife on the Puyallup Indian Reservation and intersects SR 99, a section of former US 99, at 54th Avenue East near the Emerald Queen Casino. After crossing Hylebos Creek, I-5 turns north and ascends from the Puyallup River Valley, entering King County and the city of Federal Way while parallel to SR 99.

After passing under SR 161 at Kitts Corner near the Wild Waves Theme Park, I-5 intersects SR 18, a freeway that connects to Auburn and Maple Valley. I-5 continues north past the former corporate headquarters of Weyerhauser to central Federal Way, where the freeway's high-occupancy vehicle lanes (HOV lanes) have a direct off-ramp to the Federal Way Transit Center and The Commons at Federal Way shopping mall. The freeway travels north into western Kent, intersecting SR 516 near Highline College. North of Angle Lake, I-5 tracks eastward between the cities of SeaTac and Tukwila, passing east of Seattle–Tacoma International Airport. At the Westfield Southcenter shopping mall in Tukwila, I-5 intersects SR 518, the primary means of access to the airport and Burien, and I-405, the eastern freeway bypass of Seattle that travels through Renton and the Eastside. The interchange includes several left-hand ramps, necessitating the separation of the thru HOV lanes from the mainline. For a short distance, the light rail tracks of the Link 1 Line, which followed SR 518 from Tukwila International Boulevard station, join I-5 and run on its west side until the next interchange at SR 599, a short freeway that connects to SR 99. From the SR 599 interchange, I-5 makes a gradual turn to the northwest while crossing over the Duwamish River