The Caldecott Tunnel is an east-west highway that cuts through the Berkeley Hills between Oakland and Orinda. The tunnel is made up of four bores which carry the California State Route 24. The tunnel opened in 1937with only two bores and was named after Thomas E. Caldecott, former mayor of Berkeley. The third bore opened in 1964 and the fourth bore in 2013. Bore 1 (the southernmost bore) and Bore 2 were completed in 1937 and are 3,610 feet (1,100 m) in length. Both carry two lanes each of eastbound traffic. Bore 3, completed is 3,771 feet (1,149 m) in length and bore 4 is 3,389 feet (1,033 m) in length. The active Hayward Fault runs just west of the western portals of the tunnel.
Before the tunnel was built, traffic over the Berkeley Hills had to go up Harwood Canyon, which is now known as Claremont Canyon. The idea of a tunnel through the hills began as early as 1860. The idea was proposed and rejected by the citizens of Alameda and Contra Costa Counties but the idea was revived in 1871. A franchise was granted to a group of developers who passed the franchise onto another group and nothing came of the proposal until the beginning of the 20th century.
A tunnel was finally built in 1903 above the present location of the Caldecott Tunnel, in the next canyon south of Claremont Canyon. This tunnel was known as the Kennedy Tunnel, the Inter-County Tunnel or the Broadway tunnel. The tunnel was very narrow and arched, such that two tall buggies could not pass each other. The tunnel height was increased in 1915 by 3 feet to accommodate larger vehicles. Once the new Caldecott Tunnel had been completed, the Kennedy tunnel was used mostly by pedestrians until it was sealed in 1947.
Construction of the first two bores of the Caldecott Tunnel began in 1929. They were completed in 1937. By the late 1950s, the Division of Highways upgraded the approach to the tunnel into a modern freeway. With Contra Costa County accessible by freeway, it quickly changed from a farming community and urban getaway into a major suburb of the city.
The third bore broke ground in 1960 and was completed by October 1964. When the third it opened, the other bores were closed one at a time for maintenance, including the installation of a continuous row of fluorescent lights. On April 7, 1982, an accident involving a gasoline tanker truck in the third bore set off the Caldecott Tunnel fire, an accident that caused major damage and closed the bore for several months.
To relieve traffic congestion in the reverse commute direction, the California Department of Transportation began planning for a fourth bore in 2000. In 2007, the California Transportation Commission approved the final funding needed to build the fourth bore. Construction was completed in 2013.