California State Route 84

State Route 84 (SR 84) is a state highway in the U.S. state of California that consists of two unconnected segments, one in the San Francisco Bay Area and the other primarily in the Sacramento–San Joaquin River Delta area.

The first section is an east–west arterial road running from SR 1 in San Gregorio to Menlo Park, across the Dumbarton Bridge through Fremont and Newark and ending at I-580 in Livermore. The segment between Marsh Road and the Dumbarton Bridge has been upgraded to an expressway and is known as the Bayfront Expressway. The segment from the eastern end of the Dumbarton Bridge to the interchange with I-880 has been upgraded to a freeway.

The other section is a north-south arterial road that begins at SR 12 in Rio Vista, passes through Ryer Island (where it connects to SR 220), and ends in West Sacramento. The Ryer Island Ferry provides the crossing over Cache Slough from Rio Vista to Ryer Island. The ferry is a diesel-powered boat operated by Caltrans, and is in operation twenty-four hours per day and charges no toll.

There are no plans to connect the two unconnected segments of SR 84 at this time. A proposed toll road called the Mid-State Tollway along the proposed route was abandoned in 2001 due to local opposition.

The route begins at SR 1 on the Pacific coast near San Gregorio. It then heads northeast through San Mateo County, following San Gregorio Road and La Honda Road and crossing the Santa Cruz Mountains. As it enters Woodside, it intersects SR 35, also known as Skyline Boulevard. From the juncture of La Honda Road with Portola Road to US 101 it follows Woodside Road. It then passes through downtown Woodside. After intersecting I-280, it 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 struck cyclist Sam Felder , a Facebook employee, after he ran a red light. He died 3 months later. The SR 114 (Willow Road) 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, however, the route is not built to freeway standards as it enters the city of Fremont, following the streets of Thornton Avenue, Fremont Boulevard, Peralta Boulevard, and Mowry Avenue, 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 officially a scenic route. The section is known as Niles Canyon Road, and there are legends of a Niles Canyon ghost inhabiting the area.