Planet money writes:
So I wanted to answer a simple question: Why did they build the Tappan Zee where they did, rather than building it a few miles south?
I started digging through newspaper clippings from the 1940s and 1950s. It turns out, the bridge was part of a much larger project: The New York State Thruway, one of the first modern highway systems.
The clippings also reveal something suspicious. There was an alternate proposal for a bridge at a narrower spot nearby. The proposal was put forward by top engineers at the Port Authority of New York and New Jersey.
But that proposal was killed by New York governor Thomas E. Dewey. The New York Thruway was his baby; in a 1954 speech he proclaimed that it would be "the world's greatest highway."
I called historians, and libraries and historical societies. No one seemed to know for sure why Governor Dewey did what he did. Then I found Jim Doig, a professor emeritus at Princeton.
Doig interviewed some of the key government officials involved in the project, and knew the answer.
The Port Authority — the body that proposed putting the bridge further south — had a monopoly over all bridges built in a 25-mile radius around the Statue of Liberty.
If the bridge had been built just a bit south of its current location — that is, if it had been built across a narrower stretch of the river — it would have been in the territory that belonged to the Port Authority.