Interstate 5 in Washington

Interstate 5
Purple Heart Trail
A map of Western Washington with I-5 highlighted in red
Route information
Maintained by WSDOT
Length	276.62 mi[1][2] (445.18 km)
Existed	August 14, 1957[3][4]–present
History	Completed in 1969[5]
Tourist routes	Cascade Loop Scenic Byway Lewis and Clark Trail Scenic Byway Skagit Valley Agricultural Scenic Corridor
NHS	Entire route
Major junctions
South end	I-5 at the Oregon state line in Vancouver
Major intersections	I-205 in Salmon Creek US 12 near Centralia US 101 in Tumwater SR 16 in Tacoma I-405 / SR 518 in Tukwila I-90 in Seattle SR 520 in Seattle I-405 / SR 525 in Lynnwood US 2 in Everett SR 20 in Burlington
North end	Highway 99 at the Canadian border in Blaine
Location
Country	United States
State	Washington
Counties	Clark, Cowlitz, Lewis, Thurston, Pierce, King, Snohomish, Skagit, Whatcom
Highway system
Interstate Highway System Main Auxiliary Suffixed Business Future State highways in Washington Interstate US State Scenic Pre-1964 1964 renumbering Former
← SR 4 → SR 6

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 277 miles (446 km) 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.