valley to its headwaters near Spirit Lake, passing several trailheads, and turns west to continue its ascent. The treeless landscape is home to winter herds of Rocky Mountain elk and deer. The highway turns east once again near the Loowit Viewpoint and reaches the Johnston Ridge Observatory, the volcano's main visitor center open from spring to autumn and situated at an elevation of . SR 504 terminates downhill from the observatory at its parking lot, located north of the Mount St. Helens crater. SR 504 is maintained by the Washington State Department of Transportation (WSDOT), which conducts an annual survey on state highways to measure traffic volume in terms of average annual daily traffic. Average traffic volumes on the highway in 2016 ranged from a minimum of 500 vehicles near Coldwater Lake to a maximum of 14,000 vehicles at the I-5 interchange in Castle Rock. SR 504 is the main route for a majority of the 3 million annual visitors to the Mount St. Helens area. It is designated by the state government as the Spirit Lake Memorial Highway, a state scenic byway, and a National Forest Scenic Byway by the United States Forest Service. History Early history and state acquisition The Spirit Lake Highway was completed in 1903 by the Cowlitz County government, connecting Castle Rock to the shore of Spirit Lake by following the North Fork Toutle River. The highway was initially unpaved and characterized as rough and narrow, and several sections were improved or bypassed in the 1910s. It was added to the state highway system in 1937 as Secondary State Highway 1R (SSH 1R), terminating at the western boundary of the Columbia National Forest near Spirit Lake. The state government paved sections of the highway in the late 1930s and early 1940s, with only a short section at the east end remaining unpaved by 1944. The remaining section of the highway within Columbia National Forest was improved and paved by the federal Bureau of Public Roads in the late 1930s for $1.2 million (equivalent to $ in dollars). Expanded logging and a planned ski resort on Mount St. Helens hastened the construction of a modern highway linking Spirit Lake to the timberline on the mountain's northern slopes in the 1950s. Despite support from the state government, the Forest Service, and Bureau of Public Roads, the highway project was put on hold for several years due to