Washington State Route 508

SR 508 eastbound as Main Avenue in Morton near the SR 7 intersection.

SR 508 begins at a diamond interchange with Interstate 5 (I-5) concurrent with U.S. Route 12 (US 12) within Napavine city limits.[3] From the interchange, the highway leaves Napavine and travels southeast through an intersection with the Jackson Highway, formerly US 99,[4] 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.[5][6][7] 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;[8] the SR 7 intersection was the busiest section in 1970, with an estimated daily average of 3,200 motorists.[9]

A road paralleling the current route of SR 508 first appeared on a map in 1916, extending from Napavine to Cinebar.[10] By 1937, the road was extended east to Morton and signed as Secondary State Highway 5K (SSH 5K) in the same year.[11][12] SSH 5K ran from a branch of Primary State Highway 5 (PSH 5) in Morton west to PSH 1 east of Napavine.[12][13] In 1964, a highway renumbering created SR 508 to replace SSH 5K.[2][14][15] Between 1968 and 2008, SR 508 between the Interstate 5 (I-5) concurrent with U.S. Route 12 (US 12) interchange and Onalaska used Forest Road and the Jackson Highway to get to Onalaska,[15] but the route was later realigned.[6] In early November 2006, heavy rainfall at Bear Canyon, located east of Cinebar,[16] resulted in erosion of a cliff on the highway that caused portions of the roadway to fall into the Tilton River.[17] Construction of the repairs began April 23, 2007 and a limited opening happened on June 25.[18][19][20] On July 27, the new road was opened,[21] but during heavy snowfall between December 19 and 22, 2008 and the resulting floods in early January 2009, the area was washed out.[22] On January 29, 2009, SR 508 was reopened through Bear Canyon.[23] A bridge over the Tilton River west of Milton that was built in 1947 and three other bridges on the highway were classified as structurally deficient in 2008 and the Tilton River Bridge was partially closed in April 2009.[24][25] The bridge, like Bear Canyon, was damaged during floods in January 2009 and reopened on June 11.[26]

The entire route is in Lewis County.

Location	mi[1]	km	Destinations	Notes
Napavine	0.00	0.00	I-5 / US 12 – Chehalis, Kelso, Morton
​	2.44	3.93	Jackson Highway – Toledo, Chehalis	Former US 99
Morton	32.84	52.85	SR 7 (Second Street) – Elbe, Spanaway, Tacoma
1.000 mi = 1.609 km; 1.000 km = 0.621 mi

• Highways of Washington State
• Completed WSDOT Projects