I-380 begins at a junction with I-280 in San Bruno. This junction was only partially built, with room to accommodate a proposed freeway extension west toward SR1. The freeway itself lacks overhead guide signs mentioning I-380. It then travels east through the city of San Bruno, intersecting with SR82 (El Camino Real) before reaching US101.
At its terminus at US101, the mainline lanes of I-380 continue to North Access Road. The ramps to and from southbound US101 provide connections to collector–distributor roads leading directly to San Francisco International Airport, allowing traffic between the Interstate and the airport to avoid merging with the main traffic lanes of US101.
There were plans to extend I-380 west to SR1 (Cabrillo Highway), but, due to the route's passing over the San Andreas Fault and opposition from members of the local community, this project was abandoned. The westbound lanes of I-380 branch off to the right at I-280, leaving a wide, paved space which would carry the freeway extension under I-280 toward SR1. This space is currently often used as storage space for equipment used in highway maintenance. A pair of unused bridges crossing over the I-280 south to I-380 east offramp remain as evidence.[citation needed]
Chapter2, Article3, Section608 of the California Streets and Highways Code legally defines Route380 as traveling from "Route1 near Pacifica to Route280 in San Bruno",[5] and the exit numbers assigned at I-280 start at 5 instead of 1 or 0.[6]
Exit list
Under the official exit list by Caltrans, mileage is measured from the unconstructed western terminus at SR1 near Pacifica. It starts at the location of the SR82 interchange, labeling the start of the I-280 ramps at that same point.[6]
The entire route is in San Mateo County.
Westbound exit and eastbound entrance; signed as exits 5A (north) & left 5B (south); west end of I-380; Exits 43B northbound & 43A southbound on I-280; stack interchange with westbound part being stub road
"Article 2 of Chapter 2 of Division 1". California Streets and Highways Code. Sacramento: California Office of Legislative Counsel. Retrieved February 6, 2019.
Federal Highway Administration (March 25, 2015). National Highway System: San Francisco–Oakland, CA(PDF) (Map). Scale not given. Washington, DC: Federal Highway Administration. Retrieved July 29, 2017.
Natzke, Stefan; Neathery, Mike & Adderly, Kevin (June 20, 2012). "What is the National Highway System?". National Highway System. Washington, DC: Federal Highway Administration. Retrieved July 1, 2012.