The route begins at SR 1 on the Pacific coast near San Gregorio. It then heads northeast through San Mateo County crossing the Santa Cruz Mountains. As it enters Woodside, it intersects SR 35, also known as Skyline Boulevard, then I-280. It then enters Redwood City, where it intersects SR 82, which carries El Camino Real through the South Bay. A few miles later, it interchanges with US 101, which it overlaps for a few miles. Upon routing eastward, it enters the city of Menlo Park as an expressway, called the Bayfront Expressway, which features traffic signals at Menlo Park streets, at driveways serving Facebook headquarters, and at intersections with SR 114 and SR 109. The Marsh Road intersection, in 2013, was the site where a car fatally struck cyclist Sam Felder, a Facebook employee. The SR 114 intersection was the site of a car crash in which author David Halberstam was killed on April 23, 2007. SR 84 then becomes a freeway at the south end of San Mateo County as it crosses as the Dumbarton Bridge over the San Francisco Bay. Midway over the bridge, it enters Alameda County. In Alameda County, it runs northward through the city of Newark, where it begins a concurrency southwards with I-880 for about one mile. Both interchanges with I-880 are partial cloverleaf interchanges. Upon separation, the route isn't upgraded to freeway standards as it enters the city of Fremont, following the streets of Thornton Ave, Fremont Blvd, Peralta Blvd, and Mowry Ave, which after, it has a short concurrency northwards with SR 238. SR 84 then leaves Fremont through the historic Sunol Valley. Parts of the valley are extremely narrow and are referred to as Niles Canyon; this part of the route is now officially a scenic route. After exiting the valley, it begins an overlap with I-680. After separating, it runs through Vallecitos Valley. It then goes over grasslands until it reaches a pass, then enters the city of Livermore with Ruby Hill development on the left. CA 84 then runs along Isabel Ave until it meets I-580, as the end of its southern section. SR 84 is unconstructed from I-580 to SR 12.
Northern section
The second section of SR 84 starts in Rio Vista at SR 12. It then follows the Ryer Island Ferry, which carries the route across the Cache Slough. When it leaves the ferry, it intersects SR 220. It continues northward and meets the end of state maintenance at the West Sacramento city limit, about six miles east of an intersection with CR E19. It then enters the city of West Sacramento in Yolo County. It then interchanges with I-80 Business, also signed as US 50. As it continues northward, it ends at I-80. From north of West Sacramento, SR 84 has been relinquished and given to the city of West Sacramento in 2003. SR 84 is part of the California Freeway and Expressway System, but is not 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. SR 84 is eligible for the State Scenic Highway System, and is designated as a scenic highway by the California Department of Transportation from SR 238 to I-680 in Alameda County, meaning that it is a substantial section of highway passing through a "memorable landscape" with no "visual intrusions", where the potential designation has gained popular favor with the community.
History
Original routing
Until recent years, all of CA-84 consisted of narrow, two-lane roads in California. This has remained almost unchanged except forurban areas and the CA-84 widening project in the Tri-Valley.
Modern history
Mid-State Tollway
Route 84 is legally defined to continue from I-580 to SR 4 in Antioch, but there are currently no plans in place to bridge the gap at this time. A plan to build a $600 million toll road called the Mid-State Tollway along the proposed route was suspended in 2001 due to local opposition.
CA-84 widening project
In the late 2000s and 2010s, a widening project began on Route 84 from I-680 near Sunol to I-580 in Livermore. This included a better connection between Stanley Boulevard and Isabel Avenue. The project was to be done in five phases:
Isabel Avenue/I-580 interchange
Jack London Boulevard to Concannon Boulevard, including connections to Stanley Boulevard
Concannon Boulevard to Vallecitos Road intersection and southern Ruby Hill entrance
Northern side of pass
Southern side of pass to I-680
Costs were estimated to be between $400 and $500 million. As of mid 2019, phases 1, 2, 3 and 4 are complete. This still leaves the southern section of CA 84 in its original condition.
