SR 510 begins as Marvin Road at exit 111, a diamond interchange on in southern Lacey. The highway travels south by the Hawks Prairie Village Mall, home of the Hawks Prairie Center, a division of the South Puget Sound Community College. South of the mall is the Martin Way intersection, located west of River Ridge High School and one of the busiest intersections on the roadway at a daily average of 24,000 motorists in 2008, and the Pacific Avenue roundabout, where SR 510 turns east, renamed Pacific Avenue, northeast of Long Lake. After leaving Lacey city limits, the roadway begins to parallel the Quadlok line owned by Tacoma Rail south towards the Old Pacific Highway. At the Old Pacific Highway, the road becomes the St. Clair Cutoff Road, named for nearby Saint Clair Lake, and crosses the railroad tracks twice. After turning northeast, parallel to Saint Clair Lake's shoreline, the highway dips southeast, now parallel to the Nisqually River, into Fort Lewis and the Nisqually Indian Community. Outside of the community, SR 510 is renamed the Yelm Highway and passes Southworth Elementary. After intersecting Mudd Run Road, future western terminus of the Yelm Loop, the roadway enters Yelm city limits. After turning southeast, SR 510 becomes Yelm Avenue and serves Yelm High School. Shortly thereafter, the roadway serves Yelm Middle School, crosses the Yelm-Tenino Trail, a long rail trail in operation since 1993, and ends at First Street, signed as, which continues southeast as Yelm Avenue.
History
SR 510 began as an unsigned county-maintained road that ran from the Saint Clair Lake area to the Northern Pacific Railway station at Yelm, constructed by 1916. The road later extended west to Tumwater and designated Secondary State Highway 5I in 1937. The old route followed present-day Yelm Highway on the southern side of Saint Clair Lake and Patterson Lake to Tumwater. In 1959, SSH 5I was moved to a northern route to the and freeway in Lacey. SR 510 officially replaced SSH 5I after the 1964 highway renumbering; US 99 and US 410 also became in 1968. SR 510 between I-5 and Pacific Avenue was reconstructed and widened in late 2003, with the addition of a roundabout at the Pacific Avenue intersection. The I-5 interchange will be replaced in 2020 with a diverging diamond interchange that began construction in October 2018. The interchange will be the first diverging diamond constructed in Washington and is estimated to cost $72 million, with funding provided by the state legislature's 2015 Connecting Washington package.
Alternate route
State Route 510 Alternate, also known as the Yelm Loop, is a partially completed bypass of Yelm. The first, section opened to traffic in October 2010 and cost $4.3 million to construct. The wide, two-lane highway begins at a roundabout with SR 510 near the current Mudd Run Road intersection and travels east through a residential and industrial area, ending at Cullins Road. The bypass was designed in the 1990s in response to increasing traffic congestion and was funded by the Washington State Legislature in 2009. The second phase of the Yelm Loop project, which would finish the loop and extend it to SR 507, remained unfunded after the completion of the first phase. The state legislature's 2015 "Connecting Washington" transportation package will fund the $67 million second phase beginning in 2019.
