01 / EXPLORE THE CORRIDOR
Highway 99 in California
From the Imperial Valley to the Oregon border: an introduction to the route and the places to research next.
A corridor with several chapters
US 99 crossed the Imperial Valley, the Los Angeles region, the Central Valley, and Northern California. The Sacramento Valley included separate 99E and 99W branches between Sacramento and Red Bluff.
I-5 replaced portions of the corridor, but it is not a complete substitute for tracing old US 99. Other successors include State Route 99, parts of I-10, and numerous local roads.
Sources: Historic Highway 99 Association: California routing
Explore by region
SOUTHERN CALIFORNIA
Desert & mountain crossings
Research focus: the Imperial Valley, Los Angeles, the original Ridge Route, and the Ridge Route Alternate.
CENTRAL CALIFORNIA
Valley towns & branches
Research focus: town-center routes, later bypasses, and the distinct histories of 99E and 99W in the Sacramento Valley.
NORTHERN CALIFORNIA
Canyons & the state line
Research focus: Red Bluff, Redding, the Sacramento River corridor, Yreka, and approaches to Oregon.
A useful first case study: the Ridge Route
The original Ridge Route opened in 1915, before US highway numbering. The Ridge Route Alternate followed in 1933, and later freeway construction changed the mountain crossing again. It is a useful place to examine how one corridor can contain several generations of road.
Sources: Ridge Route Preservation Organization
How this section will grow
Future entries will work town by town, separating documented street alignments from approximate corridors. A regional overview will never be treated as proof of an exact route line.
Continue researching
California Highways: Route 99 · SoCalRegion: virtual tour · All research links