Interstate 680 (California)

Interstate 680 (I-680) is a north–south auxiliary Interstate Highway in Northern California. It curves around the eastern cities of the San Francisco Bay Area from San Jose to I-80 at Fairfield, bypassing cities along the eastern shore of San Francisco Bay such as Oakland and Richmond while serving others more inland such as Pleasanton and Concord.

Built in the 1920s as SR 21 and designated in 1955, I-680 begins at a junction with I-280 and US Route 101 (US 101/Bayshore Freeway) and heads northeast and north-northwest through the northeast part of San Jose. After passing State Route 237 (SR 237) in Milpitas and SR 262 in Fremont, I-680 abruptly turns northeast (where a connection to a SR 238 freeway was planned) and enters the hills and valleys of the California Coast Ranges. The highway crosses over Mission Pass, also known as the Sunol Grade, and descends into the Sunol Valley, where it meets SR 84 near Sunol. From Sunol, I-680 again heads north-northwesterly through valleys, including the San Ramon Valley, along the Calaveras Fault. Junctions along this portion include I-580 in Dublin and SR 24 in Walnut Creek. Beyond the latter interchange, a three-way directional junction with the SR 24 freeway west to Oakland, I-680 heads north into Pleasant Hill, where SR 242 splits and I-680 again heads northwesterly. After the junction with SR 4 in Martinez, the highway crosses the Carquinez Strait on the Benicia–Martinez Bridge, immediately meeting the east end of I-780 on the Benicia end. The remainder of I-680, from Benicia to I-80 at Fairfield, lies between a hilly area to the west representing the southwestern tip of the Vaca Mountains, and a marshy area (along the Suisun Bay and Cordelia Slough) to the east.

The entirety of I-680 is defined in section 620 of the California Streets and Highways Code as Route 680, and that the highway is from:

(a) Route 101 near San Jose to Route 780 at Benicia passing near Warm Springs, Mission San Jose, Scotts Corners, and Sunol, and via Walnut Creek.
(b) Route 780 at Benicia to Route 80 near Cordelia.

This definition roughly corresponds with the Federal Highway Administration (FHWA)'s route logs of I-680.

I-680 begins at US 101 at the Joe Colla Interchange, where it acts as a continuation of I-280 eastward. From here, it begins its journey northward through San Jose, where it meets the Capitol Expressway, signed as County Route G21 (CR G21), about a mile (1.6 km) northeast of I-680's southern terminus. The next exit northbound is SR 130, which is also known as Alum Rock Avenue, unsigned at the intersection. As it continues through Santa Clara County, it meets numerous local roads before interchanging with the Montague Expressway (CR G4). Here, it exits San Jose and enters the city of Milpitas, where it meets SR 237, often referred to as Calaveras Boulevard. After one more intersection, I-680 exits Santa Clara County and enters Alameda County.

In Alameda County, the freeway begins in the city of Fremont, where it intersects SR 262, which was unsigned until 2000. Continuing through the city, it meets Mission Boulevard at SR 238 before exiting the city. Prior to 2002, two ghost ramps existed here, remains of an abandoned freeway project replacing Mission Boulevard. Amid Alameda County, it abruptly turns northeastward and enters a hilly area, where it crosses over Mission Pass, and descends into the Sunol Valley, where it runs concurrently with SR 84 for a short while. Afterward, it enters Pleasanton and intersects with I-580, currently California's longest auxiliary Interstate providing access to Oakland and the Central Valley. It enters Dublin for a short segment before exiting the county and entering Contra Costa County.

Upon entering Contra Costa County, the route meets numerous local roads through the cities of San Ramon, Danville, and Alamo before entering Walnut Creek, where it meets SR 24. I-680 then enters Pleasant Hill for a short time and Concord, where it meets SR 242. Upon exiting Concord, it meets SR 4. It then enters Martinez, where it follows the Benicia–Martinez Bridge over the Carquinez Strait, on which the route crosses the county line and enters Benicia in Solano County. On the Benicia–Martinez Bridge, I-680 northbound is tolled, while I-680 southbound is free. In Benicia, I-680 interchanges with I-780. It then exits the city and, after passing through rural areas, routing parallel to the San Joaquin Delta, it enters Fairfield, where it meets I-80, which is the route's northern terminus.