I-5 is the backbone of California, running from Mexico to the Oregon border. Counties traversed: San Diego, Orange, Los Angeles, Kern, Kings, San Benito, Merced, Sanislaus, San Joaquin, Sacramento, Yolo, Colusa, Glenn, Tehama, Shasta, and Siskiyou. #2 in total length.

Route 99 is what is left of US 99, running from I-5 N of Los Angeles, through Bakersfield, Fresno, and Sacramento, to rejoin I-5 in northern California. Counties traversed: Kern, Tulare, Fresno, Madera, Mariposa, Stanislaus, San Joaquin, Sacramento, Sutter, and Butte. #5 in total length.

US 101 runs from downtown Los Angeles, through Ventura, Santa Barbara, San Luis Obispo, Monterey, San Jose, San Francisco, Marin, and Redwood Country. Counties traversed: Los Angeles, Ventura, Santa Barbara, San Luis Obispo, Monterey, San Benito, Santa Clara, San Mateo, San Francisco, Marin, Sonoma, Mendocino, Humboldt, Del Norte. #1 in total length.

I-80 runs to the Nevada border from San Francisco, through Sacramento. Counties traversed: San Francisco, Alameda, Contra Costa, Napa, Solano, Yolo, Sacramento, Placer, Nevada, and Sierra. Not in the top 10 for total length.

Route 1 runs from southern Orange County to US 101 near Leggett, hugging the coast most of the way. Counties traversed: Orange, Los Angeles, Ventura, Santa Barbara, San Luis Obispo, Monterey, Santa Cruz, San Mateo, San Francisco, Marin, Sonoma, and Mendocino. #3 in total length.

Route 49 runs from Route 41 near Oakhurst to Route 70 near Vinton through the Sierra gold country. Counties traversed: Madera, Mariposa, Tuolumne, Calaveras, Amador, El Dorado, Placer, Nevada, Yuba, Sierra, and Plumas. #8 in total length.

Route 65 was a planned freeway E of Route 99 between Bakersfield and Route 99 near Yuba City. Counties traversed: Kern, Tulare, Fresno, Madera, Mariposa, Merced, Stanislaus, San Joaquin, Sacramento, Placer, and Sutter. #7 in total length.

Route 33 runs from Ventura in various segments up to I-5 near Vernalis. Counties traversed: Ventura, Santa Barbara, San Luis Obispo, Kern, Kings, Fresno, Merced, Stanislaus, and San Joaquin.. #10 in total length.

Route 89 runs from US 395 near Coleville to I-5 near Mt. Shasta. Counties traversed: Mono, Alpine, El Dorado, Placer, Nevada, Sierra, Plumas, Tehama, Shasta, and Siskiyou. Not in the top 10 for total length.

US 395 currently runs from north of San Bernardino to the Oregon border, with a break when it passes through Nevada. Counties traversed: San Bernardino, Kern, Inyo, Mono, Sierra, Lassen, and Modoc. #4 in total length.