Humboldt County
The main roads of Humboldt County are US Route 101, which continues south from Del Norte County and passes through Eureka, Arcata, Fortuna and Garberville; State Route 299, which connects US 101 with Redding and Interstate 5 via the Trinity River Canyon; State Route 36, which connects US 101 with Red Bluff and Interstate 5 via the Van Duzen River Valley; and State Route 254, also known as the Avenue of the Giants, which parallels US 101 and showcases the redwood forests of Humboldt Redwoods State Park.
Del Norte County
The main roads of Del Norte County are US Route 101, which runs north-south along the coast and connects Crescent City with Eureka and other cities in Humboldt County; State Route 197, which branches off from US 101 and follows the Smith River to Oregon; and State Route 199, which connects US 101 with Interstate 5 in Oregon via the Redwood Highway.
Lake County
The main roads of Lake County are State Route 29, which runs north-south through the county and connects Lower Lake, Clearlake, Kelseyville and Lakeport with Napa County and Colusa County; State Route 20, which runs east-west through the county and connects Clearlake Oaks, Upper Lake and Nice with Mendocino County and Yuba County; State Route 53, which connects Lower Lake with Clearlake; and State Route 175, which continues from Mendocino County and passes through Cobb Mountain and Middletown before reaching Napa County.
Mendocino County
The main roads of Mendocino County are US Route 101, which continues south from Humboldt County and passes through Willits, Ukiah and Hopland; State Route 1, which runs along the coast and connects Fort Bragg, Mendocino, Point Arena and Gualala with Sonoma County; State Route 20, which connects US 101 with Fort Bragg via the Jackson State Forest; State Route 128, which connects US 101 with State Route 1 via the Anderson Valley and the Navarro River; and State Route 175, which connects US 101 with Lake County via the Hopland Grade.