Both are immersed tubes, constructed by sinking precast concrete segments to a trench in the Estuary floor, then sealing them together to create a tunnel.

The Oakland Estuary (then known as San Antonio Creek) was first crossed by the Webster Street swing bridge for narrow gauge rail and road traffic, completed in 1871.

[6] The replacements were prompted by the Secretary of War, who stated the swing spans each needed to be at least 150 feet (46 m) to accommodate marine traffic in 1896.

[8] During the construction of the replacement Webster bridge, county supervisors initially rejected an offer to use the old Alice bridge as a detour for road traffic,[9] but later accepted, avoiding a more distant route through the eastern part of Alameda,[10] and teamster traffic moved to Alice in December.

[12] By 1916, the War Department had declared the replacement Webster and Harrison bridges[13] were a menace to deep-water navigation and an obstacle to continued development of Oakland Harbor in 1916.

[14] As an example, SS Lancaster rammed the Webster Street bridge in January 1926, causing the swing section to fall into the Estuary and forcing road traffic to be rerouted.

[17] After the completion of the Posey Tube, the Webster Street bridge was sold to Sacramento County for US$3,100 (equivalent to $60,000 in 2023) in November 1928.

[20] However, the entry of the United States into World War I delayed the plans for a new connection between Oakland and Alameda.

[22] After the passage of the bond issue, test borings were taken in the Estuary, and bids were received for the work on March 23, 1925; the construction contract was awarded to the California Bridge and Tunnel Company (CB&TC) with a low bid of US$3,882,958 (equivalent to $67,460,000 in 2023), and excavation started from the Oakland end on June 15, 1925.

In 1952, the Posey Tube was handling 30,000 to 36,000 cars per day; that same year, Coronado, California, and its mayor Lloyd Harmon contemplated the laying of a similar tunnel to connect Coronado with San Diego, when Alameda mayor Frank Osborne wrote a letter to Harmon, which stated from the time it was completed the tube was never adequate for the purpose for which it was built ...

[27]The ventilation buildings that house the exhaust and fresh air fans are built in an art deco style;[28] local architect Henry H. Meyers is credited with the design of both portals.

[41] The roadway within the Webster Street Tube is 24 feet (7.3 m) wide, and the minimum vertical clearance is 15 ft 1+3⁄8 in (4.607 m).

Each portal building contains four blowers and four exhaust fans, and they are capable of providing nearly 1,000,000 cu ft/min (470 m3/s) of airflow in total.