Interstate 80 in California

Interstate 80 (I-80) is a transcontinental Interstate Highway in the United States, stretching from San Francisco, California, to Teaneck, New Jersey. The segment of I-80 in California runs east from San Francisco across the San Francisco–Oakland Bay Bridge to Oakland, where it turns north and crosses the Carquinez Bridge before turning back northeast through the Sacramento Valley. I-80 then traverses the Sierra Nevada, cresting at Donner Summit, before crossing into the state of Nevada within the Truckee River Canyon. The speed limit is at most 65 mph (105 km/h) along the entire route instead of the state's maximum of 70 mph (110 km/h) as most of the route is in either urban areas or mountainous terrain. I-80 has portions designated as the Eastshore Freeway and Alan S. Hart Freeway.

Throughout California, I-80 was built along the corridor of US Route 40 (US 40), eventually replacing this designation entirely. The prior US 40 corridor itself was built along several historic corridors in California, notably the California Trail and Lincoln Highway. The route has changed from the original plans in San Francisco due to freeway revolts canceling segments of the originally planned alignment. Similarly in Sacramento, the freeway was rerouted around the city after plans to upgrade the original grandfathered route through the city to Interstate Highway standards were canceled.

The entirety of Interstate 80 in California is defined in section 380 of the California Streets and Highways Code as Route 80:

Route 80 is from:
(a) Route 101 near Division Street in San Francisco to Route 280 near First Street in San Francisco.
(b) Route 280 near First Street in San Francisco to the Nevada state line near Verdi, Nevada, passing near Oakland, via Albany, via Sacramento, passing near Roseville, via Auburn, via Emigrant Gap, via Truckee and via the Truckee River Canyon.

The section of Route 80 defined in subdivision (a) is not considered an Interstate Highway according to the Federal Highway Administration (FHWA)'s route logs, but is still signed as I-80 by Caltrans.

I-80 is recognized as the Dwight D. Eisenhower Highway in the Western United States and a Blue Star Memorial Highway for its entire length. In California, it follows the original corridor of the Lincoln Highway from Sacramento to Reno (with minor deviations near Donner Summit). I-80 is part of the California Freeway and Expressway System and is part of the National Highway System, a network of highways that are considered essential to the country's economy, defense, and mobility by the Federal Highway Administration (FHWA). I-80 is also known as the Dutch Flat and Donner Lake Wagon Road from Emigrant Gap to Donner Lake. The segment of I-80 from Emigrant Gap to Truckee also forms part of the Yuba–Donner Scenic Byway, a National Forest Scenic Byway.

According to the California Streets and Highways Code, most maps, and local signs, I-80 begins at the interchange with US 101 in San Francisco. However, federal records place the western terminus of I-80 at the western approach to the San Francisco–Oakland Bay Bridge, at the location of the Fremont Street offramp (previously known as the Terminal Separator Structure that once connected it to the Embarcadero Freeway). The federal and state governments disagree as to whether this westernmost segment of the signed Interstate, known as the San Francisco Skyway or Bayshore Viaduct, is actually part of the Interstate Highway System, although it is consistently shown as I-80 on most maps of San Francisco.

The Eastshore Freeway is a segment of I-80 and I-580 along the northeast shoreline of San Francisco Bay. It runs from the MacArthur Maze interchange just east of the eastern end of the San Francisco–Oakland Bay Bridge to the Carquinez Bridge. I-580 splits from the Eastshore Freeway at an interchange known locally as the "Hoffman Split" in Albany. The section of the Eastshore Freeway between the MacArthur Maze and I-580 (Hoffman) split between Albany is a wrong-way concurrency where the northbound direction is signed as I-80 east and I-580 west, while the southbound direction is signed as westbound I-80 and eastbound I-580. This segment suffers from severe traffic congestion during rush hour due to the merger of three freeways (I-80, I-580, and I-880) at the MacArthur Maze.