Secondary State Highway 1R
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| Spirit Lake Memorial Highway | ||||
SR 504 highlighted in red.
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| Route information | ||||
| Auxiliary route of I‑5 | ||||
| Defined by RCW 47.17.655 | ||||
| Maintained by WSDOT | ||||
| Length: | 51.76 mi (83.30 km) | |||
| Existed: | 1964 – present | |||
| Major junctions | ||||
| West end: |
   I‑5 / SR 411 / I‑5 Bus. in Castle Rock
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 SR 505 near Toutle  SR 504 Spur in Gifford Pinchot NF
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| Eastern end: | Johnston Ridge Observatory in Mount St. Helens NVM | |||
| Highway system | ||||
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| Location: | Gifford Pinchot NF |
| Existed: | 1991–present |
State Route 504 (SR 504, also known as the Spirit Lake Memorial Highway) is a state highway in Cowlitz and Skamania counties, in the U.S. state of Washington. It extends 51.76 miles (83.30 km) from an interchange with Interstate 5 (I-5), SR 411 and Interstate 5 Business (I-5 Bus) in the city of Castle Rock, east to end at the Johnston Ridge Observatory. The route serves as a connector from Castle Rock to Mount St. Helens NVM.
The highway was Secondary State Highway 1R (SSH 1R) from 1937 until 1964. SSH 1R ran from Castle Rock to the border of the Columbia National Forest (now known as the Gifford Pinchot National Forest) from 1937 until 1961. In 1961, the Washington State Legislature extended SSH 1R east to Spirit Lake. In 1964, SSH 1R became SR 504. When Mount St. Helens erupted on May 18, 1980, SR 504 was closed and was heavily damaged from SR 505 to Spirit Lake. The highway was rebuilt and partially opened in 1991 on a higher and safer route to Coldwater Ridge. The new highway was completed when the road to the Johnston Ridge Observatory was opened in 1994.
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