Construction started in March 1952 and the bridge opened to traffic on December 15, 1955, along with a 27-mile (43 km) long section of the New York State Thruway from Suffern to Yonkers. New York State Governor W. Averell Harriman signed a bill on February 28, 1956, to officially name the structure the Tappan Zee Bridge. In 1994, the name of Malcolm Wilson was added to the bridge's name upon the 20th anniversary of his leaving the governor's office in December 1974, though it is almost never used when the bridge is spoken about colloquially.

When was the original tappan zee bridge built? OK. Let's think. My stream of consciousness:
The relevant sentence in the passage is: Construction started in March 1952 and the bridge opened to traffic on December 15, 1955, along with a 27-mile (43 km) long section of the New York State Thruway from Suffern to Yonkers. The answer is started in March 1952 and the bridge opened to traffic on December 15 , 1955.