Highway 1
From Santa Cruz County history wiki
State Highway 1 (officially "California State Route 1") is the longest state highway in California, running near the Pacific coast for the entire northwest-southeast length of the state. It enters Santa Cruz County from Monterey County, in the southeast near Watsonville, and exits to San Mateo County in the northeast past Waddell Creek.
Only one state highway originates elsewhere on both ends and passes through Santa Cruz County: Highway 1. Five highways originate in Santa Cruz County and pass out of it: Highway 9, Highway 17, Highway 35, Highway 129, Highway 152. One begins and ends within the county: Highway 236.