The route currently begins at Interstate 580 near Mills College in East Oakland and continues north as the Warren Freeway, named after former Alameda County District Attorney, California Governor and U.S. Supreme Court Chief Justice Earl Warren.

After going through an interchange with State Route 24 (upgraded August 1999) near the entrance to the Caldecott Tunnel, however, SR 13 takes drastic changes in its quality and capacity.

Upon reaching the foot of the hill, Tunnel Road becomes Ashby Avenue, a markedly urban (and very busy) two-lane street with an east–west routing.

In the 1964 state highway renumbering, it extended north to I-80 over part of SR 24, and was rerouted on its current routing.

This allowed road crews to drive street-sweeping vehicles along the shoulders throughout the week, rather than sending people to pick up trash by hand.

However, residents frequenting SR 13 have said that the median appears to be filled with fewer trees and bushes than prior to Caltrans construction.

These trees once densely lining the median have long supplemented the lush vegetation on both sides of the highway to beautify this scenic route.

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.