39 of the Oregon state highway system,[2] named after the river alongside its westernmost segments.

From there it winds eastward though the coast range along the Salmon River, past Rose Lodge and through a stand of timber known as the Van Duzer Corridor.

Emerging from the coast range, it enters the fringes of the Willamette Valley in the community of Grand Ronde.

It is briefly joined by Oregon Route 22 at a location known as Valley Junction, and overlaps OR 22 in a four-mile (6 km) stretch between Grand Ronde and Willamina, at which point OR 22 splits southeast towards Salem and OR 18 continues northeast towards the Portland area.

Two interchanges along OR 18 provide access to McMinnville, including Three-Mile Lane, a spur of the highway.

Oregon Route 18 provides direct access to the McMinnville Airport and the Evergreen Aviation Museum, home of the Spruce Goose.

This section was proposed by the Oregon State Highway Commission in 1955 as part of the adoption of new routes bypassing city centers.

For several years, Route 18 has had a notorious reputation with Oregon motorists as an extremely unsafe highway with a relatively high number of fatal accidents.

In 1995, the opening of two casinos in Grand Ronde and Lincoln City respectively, have greatly increased traffic on OR 18.

One is a project which widened Oregon Route 18 to a four-lane expressway in the stretch between Grand Ronde and Willamina; an area where increased traffic (largely due to the casino) has caused significant delays.

The second project is a bypass around the Newberg-Dundee area intended to alleviate severe congestion that occurred along 99W.