State Route 99 was constructed along the Territorial military roads that linked the settlements of the Puget Sound and the Columbia River regions. In the southern section, it approximates also the route of a branch of the Oregon Trail, blazed northward around the middle of the nineteenth century.
The northern section of the tour passes through the most popular part of the state. From many points, it affords sweeping views of the island-dotted Puget Sound and the jagged Olympic Mountains to the west, and of the Cascade Mountains, a blue-green ridge tipped here and there with white, in the east. At intervals the route crosses sluggish rivers flowing through fertile bottom lands, or skirts the edges of high, wooded bluffs. Large well-cultivated farms, truck gardens, orchards and berry fields, dairy farms, and poultry ranches alternate with areas of mature forest. Between the small towns strung out along the highway are the usual roadside inns and gasoline stations, joined since by RV parks and fast-food joints.
South of the Puget Sound Basin, the tour crosses gently rolling prairies, broken by small areas of evergreen, maple, and alder. Dominating the eastern horizon is Mount Rainier, while southward Mount Adams and Mount St. Helens seem less majestic, though only by comparison. In the lowlands, the large farms, gardens, and poultry ranches of yesteryear have given way to pulp processing plants, auto dealerships and the occasional farmhouse. Logging operations have been largely reduced. For miles, the tour follows the Cowlitz and Columbia Rivers and, finally, crosses the Columbia into Oregon.