U.S. Route 101 in Washington

The Astoria-Megler Bridge carries US 101 over the Columbia River.

Start of US 101 in Washington.

US 101 enters Washington from Oregon on the Astoria–Megler Bridge, a four-mile (6 km) bridge that carries vehicles across the Columbia River near its mouth; it is the longest continuous truss bridge in North America and among the longest of its kind in the world.[2] At the north end of the bridge is an intersection with State Route 401 (SR 401), where US 101 turns west to follow the north bank of the river, forming part of the Lewis and Clark Trail Scenic Byway. The highway passes under Fort Columbia State Park in a tunnel and continues along the Willapa Hills through Chinook towards Baker Bay. At the head of the bay, near Port of Ilwaco Airport, US 101 intersects a short alternate route that bypasses the coastal section of the highway. US 101 then dives southwesterly along the Wallacut River into Ilwaco, where it runs through town as Spruce Street before turning north onto 1st Avenue after an intersection with SR 100.[3]

From Ilwaco, US 101 travels northwest into Seaview, where it intersects SR 103 at the foot of Long Beach and turns east onto 40th Street. The highway intersects the north end of the alternate route and crosses northeast over the Bear River and enters the Willapa National Wildlife Refuge. US 101 continues north along the east side of Willapa Bay and crosses the Naselle River before turning east towards Johnston's Landing, where SR 4 terminates.

Soon, US 101 turns north towards Raymond. In Raymond, US 101 intersects the western end of SR 6 and southern end of SR 105, which it will intersect again in Cosmopolis.

After intersecting with SR 105 and SR 107, US 101 crosses the Chehalis River, which leads the highway to the western end of US 12 in Aberdeen. The roadway turns southwest and splits into two streets for each direction until it reaches Hoquiam, where it intersects the southern end of SR 109 and the eastern end of SR 109 Spur.

US 101 approaching downtown Aberdeen

US 101 then turns north to the Quinault Indian Reservation and then into the isolated area bordered by Olympic National Park and the Quinault Indian Reservation. After turning east at Lake Quinault and entering the Quinault Indian Reservation again, US 101 goes north to the Olympic National Park on the Pacific coast.

The road then curves north to Forks and north to Beaver, where it turns east after intersecting with SR 113 and touching the shore of Lake Pleasant. After a mile of going east, US 101 enters its second segment in the Olympic National Park, about one-mile (1.6 km) east of the west shore of Lake Crescent and then exits at the eastern shore of the same lake.

After exiting park boundaries, US 101 intersects SR 112. Shortly later, it intersects SR 117 (the Tumwater Truck Route) in an odd interchange/intersection hybrid. It then enters Port Angeles and turns onto Lincoln Street. It then turns onto a one-way couplet of First and Front streets, which lasts until Golf Course Road. There, the roads merge into one four-lane road, divided only by a two-way left turn lane, and leaves Port Angeles.

Following the dangerous Morse Creek S-Curves is a Rest Area and viewpoint at the Deer Park Loop interchange. The road then becomes a divided expressway until it reaches Carlsborg and crosses the Dungeness River. It enters the town of Sequim and turns into a super-two expressway, bypassing the town. East of Sequim, the road turns back into a standard two-lane road and goes to Sequim Bay and then Discovery Bay, where it intersects SR 20 and turns south. After a short distance, US 101 intersects the western end of SR 104. The road then turns south to the town of Quilcene. Exiting Quilcene, the road heads southwest into Olympic National Forest and crosses a 227 m (744 ft 9 in) pass between Mount Walker and Buck Mountain, after which it proceeds sharply downhill southeast towards Dabob Bay, where it continues following the shore and reaches Hood Canal.

At the Hood Canal, US 101 follows the western shore to SR 106, where it turns directly south to the east end of SR 106 and Shelton, to meet the west end of SR 3. After some time going south, US 101 meets SR 108, the second-to-last major junction. After going southeast, US 101 finally joins SR 8 and turns into a freeway to its easternmost point of US 101 in Washington, the so-called "north end" at I-5 near Tumwater.[3]

Before U.S. Route 101 was created, two roads existed in the place of US 101. They were State Road 9 and State Road 12. These were two roads that were part of the early Washington State Road system that was created in 1923. US 101 was created in 1926 and was co-signed with these two routes. US 101 took part of State Road 9's route and part of State Road 12's route.

In 1937, the Washington State Legislature established a new system, the Primary and secondary system.[4] State Road 9 became Primary State Highway 9 or PSH 9. State Road 12 became PSH 12, and they both held the co-signed designation with US 101.[5]

There was nothing changed about US 101's route from 1937 to 1956. In 1956, President Dwight D. Eisenhower signed the Interstate Highway Act into law. This eliminated part of the U.S. Route system and encouraged Washington's Legislature to once again change the highway system.

In January 1964, the Washington State Legislature and the Washington Department of Highways (renamed WSDOT) approved a new state route system with no branches or secondary route with names, but a full system that lives on today. PSH 9 and PSH 12 became part of U.S. Route 101 with no co-designation and parts of them became other new state routes.[6]