Washington State Route 508

State Route 508
SR 508 is highlighted in red.
Route information
Auxiliary route of I‑5
Defined by RCW 47.17.675
Maintained by WSDOT
Length:	32.84 mi (52.85 km)
Existed:	1964 – present
Major junctions
South end:	I‑5 / US 12 in Napavine
North end:	SR 7 in Morton
Highway system
State highways in Washington Interstate US State Scenic Former PSH 1964 renumbering Former ← SR 507 SR 509 →

State Route 508 (SR 508) is a 32.84-mile (52.85 km) long state highway located in Lewis County within the U.S. state of Washington, extending from an interchange with Interstate 5 (I-5) concurrent with U.S. Route 12 (US 12) in Napavine to SR 7 in Morton. By 1916, a road between Napavine and Cinebar was constructed on the current route of SR 508 and was signed in 1937 as Secondary State Highway 5K (SSH 5K) after being extended to Morton. SSH 5K became SR 508 in 1968 and since then, a segment in Bear Canyon has been reconstructed twice between 2007 and 2009 and a bridge over the Tilton River was reconstructed in 2009.

SR 508 begins at a diamond interchange with Interstate 5 (I-5) concurrent with U.S. Route 12 (US 12) within Napavine city limits. From the interchange, the highway leaves Napavine and travels southeast through an intersection with the Jackson Highway, formerly US 99, to bridge and parallel the South Fork of the Newaukum River, passing Onalaska and Alpha before unparalleling the river and continuing east to Cinebar. East of Cinebar, the roadway starts to parallel the Tilton River through Bear Canyon to Morton, where the road becomes Main Avenue and ends at SR 7, named Second Street. An estimated daily average of 5,000 motorists used the I-5 / US 12 / SR 508 interchange in 2007, making it the busiest section of the highway; the SR 7 intersection was the busiest section in 1970, with an estimated daily average of 3,200 motorists.