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.

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.

[3] The SR 114 (Willow Road) intersection was the site of a car crash in which author David Halberstam was killed on April 23, 2007.

In Alameda County, it runs northward through the city of Newark, where it begins a concurrency southwards with I-880 for about one mile.

It then goes over grasslands until it reaches a pass, then enters the city of Livermore with Ruby Hill development on the left.

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.

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.

[17][18][19] The only road bridge connecting Ryer Island is on its north side, which results in longer commutes for residents if one or both ferries shutdown.

[20] Except where prefixed with a letter, postmiles were measured on the road as it was in 1964, based on the alignment that existed at the time, and do not necessarily reflect current mileage.