U.S. Route 12 in Washington

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U.S. Route 12 in Washington is a major east–west highway that runs from Aberdeen on the Pacific coast to the Idaho state line near Lewiston. It covers about 430 miles in Washington, making it the second-longest highway in the state. It’s the only numbered U.S. highway that crosses the state from west to east, and it climbs the Cascade Range via White Pass. Much of the route is not on an interstate, though it does share stretches with I-5, I-82, and I-182.

Route highlights
- The western end begins in downtown Aberdeen at U.S. Route 101, and US 12 follows the Chehalis River eastward. It bypasses Montesano and Elma, with a connection to State Route 107 in Elma.
- East of Elma, US 12 heads toward Olympia as State Route 8, then continues to Oakville and Rochester, before an interchange with I-5 near Grand Mound. The highway runs briefly with I-5 through Chehalis and Centralia, then splits south of Napavine.
- The route follows the Cowlitz River through Mossyrock, near the Mossyrock Dam and Riffe Lake, and meets State Route 7 at Morton.
- It climbs into the Cascade Range and crosses White Pass, about 4,500 feet up—the only year-round Cascade crossing between Snoqualmie Pass and the Columbia River Gorge.
- After White Pass, US 12 connects with State Route 410 near Naches, then heads toward Yakima. In Yakima it meets I-82 and U.S. 97, and the two share a stretch toward the Tri-Cities.
- West of the Tri-Cities, US 12 intersects Interstate 182 (I-182). US 12 and I-182 run together to the Tri-Cities area; in Pasco, I-182 ends and US 12 meets U.S. 395.
- The highway continues southeast to Wallula (where it intersects U.S. 730), then east to Walla Walla. In Walla Walla it mostly bypasses the city as the Inland Empire Highway and passes near historic sites such as Whitman Mission and Fort Walla Walla.
- US 12 crosses into Idaho near Lewiston over the Snake River.

History in brief
- The Washington portion developed from early 20th-century state highways. Parts were part of the U.S. Highway System since 1926 as U.S. Route 410, with some sections previously designated under other numbers.
- US 12 was extended west to Aberdeen in 1967, taking over much of what had been US 410. Where US 410 crossed the Cascades at Chinook Pass, US 12 uses White Pass instead.
- A major relocation near Mossyrock in 1967 moved US 12 onto a new stretch through Morton, and a long arch bridge over the Cowlitz River opened in 1968.
- In the Tri-Cities, the route was realigned onto I-182 in 1985.
- The White Pass Scenic Byway designation was given in 2009 for the mountain segment.
- Walla Walla bypass projects began in the 2000s and progressed through the 2010s, with a Touchet bypass phase opening in 2023. A final bypass around Wallula Junction was planned to begin in 2027 and finish by about 2030.