[greenyoda]

Some other differences between the new bridge and the old bridge that would account for higher cost:

- The new bridge is designed to handle a much larger capacity than the old one, which was built starting in 1952, when there were many fewer commuters from distant suburbs trying to get to the NYC area.

- The new bridge has a bicycle/pedestrian path.

- The new bridge had to be built while the old bridge was still standing. Its construction costs presumably included the demolition of the old bridge and the rerouting of the highway, first from the old bridge to the first completed span of the new bridge (which initially handled traffic in both directions), then to the second span when that was completed.

- The old bridge was designed with a a life-span of only 50 years, so it was built using cheaper materials. 50 years seems like a very short life-span for a bridge. For example, the Brooklyn Bridge, opened in 1883, is now well into its second century of operation (although it has undergone various renovations, it hasn't required demolition).

Reference: https://en.wikipedia.org/wiki/Tappan_Zee_Bridge_(2017%E2%80%...