The east–west tunnel is signed as a part of California State Route 24 and connects Oakland to central Contra Costa County.

It is named after Thomas E. Caldecott (1878–1951), who was mayor of Berkeley in 1930–1932, a member of the Alameda County Board of Supervisors in 1933–1945, and president of Joint Highway District 13, which built the first two bores.

[3][4] Just before the end of the 19th century, the tunnel idea was revived and discussed by business and government representatives of Contra Costa and Alameda Counties.

[10] In 1929, Alameda and Contra Costa County formed Joint Highway District 13 to accomplish this goal.

[14] The project included an extension of Broadway from its existing termination point below the hills some two miles from the planned west portal of the new tunnel.

This included filling in one of the inlets of Lake Temescal as well as part of the upper canyon where Broadway was to run.

The approach to the east portal on the other side of the Berkeley Hills was via Mount Diablo Boulevard, also at that time part of State Highway 24.

With Contra Costa County accessible by freeway, its change from an agricultural community into a major suburb accelerated.

However, with the traffic count exceeding 50,000 vehicles per day, the state embarked on a project to build a third bore at the Caldecott Tunnel.

With the touch of a button, water would fill tubes to cause the delineators to pop up from the pavement; the middle bore was closed to one direction of travel, while opening it to another.

Ashby Avenue and Tunnel Road were redesignated State Highway 13 and aligned with the new Warren Freeway through the Montclair District of Oakland.

On April 7, 1982, an accident involving a gasoline tanker truck in the third (then-northernmost) bore set off the Caldecott Tunnel fire.

As a result of the fire, it is now illegal to transport hazardous material in a tanker truck through the tunnel, except between the light-traffic hours of 3:00 am and 5:00 am.

The Caldecott Tunnel was designated a City of Oakland Landmark in 1980, and received a Preservation Award from the Art Deco Society of California in 1993.

To relieve traffic congestion in the reverse commute direction, the California Department of Transportation began planning for a fourth bore in 2000.

[23] Commencement of the project was stalled as a result of the financial crisis of 2007–2008, but was quickly back on track as a beneficiary of President Obama's American Recovery and Reinvestment Act of 2009 (ARRA).

In summer, for example, motorists may enter the tunnel from the east where it is sunny and warm, and emerge on the west end into fog and cold.