Main article: New Tappan Zee Bridge
The superstructure was constructed during a period of material shortages during the Korean War. The deteriorating structure, which bears far more traffic than it was designed for, has led to plans to repair the bridge or replace it with a tunnel or a new bridge. These plans and discussions were whittled down to six options and underwent environmental review. Part of the justification for the replacement of the bridge has been that it was constructed during material shortages during the Korean War and designed to last only 50 years. The collapse of the I-35W Mississippi River bridge in Minnesota on August 1, 2007 has renewed concerns about the bridge's structural integrity.
The Metropolitan Transportation Authority (MTA) is studying the feasibility of either including a rail line across the new bridge or building the new bridge so a new rail line can be installed at a future date. The rail line, if built, will be located on a lower level, beneath the roadway. Commuter rail service west of the bridge in Rockland County is limited, and the MTA is studying expansion possibilities in Rockland County that would use the new bridge to connect with Metro-North's Hudson Line on the east side of the bridge along the Hudson River for direct service into Manhattan.
On September 26, 2008, New York state officials announced their plan to replace the Tappan Zee Bridge with a new bridge that includes commuter-train tracks and lanes for high-speed buses. The bridge would cost $6.4 billion, while adding bus lanes from Suffern to Port Chester would cost $2.9 billion. Adding a rail line from the Suffern Metro-North station and across the bridge, connecting with Metro-North’s Hudson Line south of Tarrytown, would cost another $6.7 billion. The plan is being reviewed for environmental impact.
Meetings by the New York State of Transportation with local communities were held in December 2009. They revised the replacement cost including road, rail, and bus to be $16 billion.
After years of public comment in favor of including public transportation on the Tappan Zee Bridge replacement, and after all previous replacement alternatives studied by the New York State Department of Transportation (NYSDOT) as part of the required environmental review included transit, New York Governor Andrew Cuomo and the Federal Highway Administration in October 2011 abruptly reversed course. The new project they advanced lacked transit and transit advocates assert that, if built as proposed, the replacement bridge would lock in new auto- and truck-only infrastructure for years. Transit advocates have complained that NYSDOT has consistently over-estimated the cost of transit in the replacement, in some cases claiming that the cost could be $10–15 billion for transit alone. Transit supporters have argued that a Bus Rapid Transit (BRT) system incorporated into the bridge can cost less than the state's projected range of $900 million to 2.5 billion. By comparison, the bridge replacement project without transit is estimated to cost $5.2 billion.