In the new bridge's final configuration, the north span will carry the northbound and westbound automobile traffic of I-87 and I-287; it will also carry a shared-use path for bicycles and pedestrians. The south span will carry the southbound and eastbound automobile traffic of I-87 and I-287.
Tappan Zee Constructors began construction in 2013. The north span officially opened to westbound traffic on August 26, 2017; it also opened to eastbound traffic on October 6, 2017. Tappan Zee Constructors then began demolishing the old bridge. Eastbound traffic will be switched to the south span upon its completion. Both spans are expected to be operational by June 15, 2018.
The original Tappan Zee Bridge was a cantilever bridge built during 1952–1955. The bridge was 3 miles (4.8 km) long and spanned the Hudson at its second-widest point. The Tappan Zee river crossing was named by 17th century Dutch settlers. The Tappan Zee Bridge, along with the smaller Bear Mountain Bridge, are the only crossings of the stretch of the Hudson between Westchester and Rockland counties, both of which are part of New York City's populous northern suburbs.
The deteriorating structure bore an average of 138,000 vehicles per day, substantially more traffic than its designed capacity. During its first decade, the bridge carried fewer than 40,000 vehicles per day. Part of the justification for replacing the bridge stemmed from its construction immediately following the Korean War on a low budget of only $81 million. Unlike other major bridges in metropolitan New York, the Tappan Zee Bridge was designed to last only 50 years. The new bridge is intended to last at least 100 years.
The collapse of Minnesota's I-35W Mississippi River bridge in 2007 raised worries about the Tappan Zee Bridge's structural integrity. These concerns, together with traffic overcapacity and increased maintenance costs, escalated the serious discussions already ongoing about replacing the Tappan Zee with a tunnel or a new bridge. Six options were identified and submitted for project study and environmental review.