The area of the Hudson known as the Tappan Zee (named for the Native American Tappan people who once lived in the area and the Dutch word for “sea”) is a natural widening of the river that’s 10 miles long and three miles wide in places. To stabilize a bridge that would cross the river at one of its widest points, engineers had to tackle a substantial problem: how to provide massive foundations for a 1,212-ft wide navigation channel when the bedrock needed to anchor them was at a depth of approximately 250 feet. The common solution available in 1950 was to use long steel piles, but the number of piles needed represented an astronomical — and unaffordable — cost.

Drawing on experience gained building temporary harbors for the Allied Forces during World War II, engineer Emil H. Praeger devised a novel solution that would eliminate the need to anchor everything to bedrock: use the river’s natural buoyancy to support some of the weight. So, when the nearly $81 million Tappan Zee Bridge (more than $760 million in today’s dollars) opened in 1955, it was the first permanent bridge in the U.S. to be supported in part by airtight floating concrete boxes (it was also the ninth longest span in the world). These eight giant buoyant caissons provided enough upward force to carry 70-80 percent of the weight of the span and its load of traffic over the navigation channel. The approach made engineering history and the bridge earned a place on the National Register of Historic Places.

The increasing spread of suburban communities through the Hudson Valley and the accompanying rise of automobile use made a river crossing north of New York City an urgent necessity by the start of the 1950s, although the need had been anticipated long before. In fact, 20 years before the bridge opened (and only four years after the opening of the George Washington Bridge in 1931), the U.S. Congress approved funding for a crossing between Nyack and Tarrytown . Over the years, many different sites were considered but, ultimately, the South Nyack/Tarrytown location was selected, in part, to avoid sharing toll revenue with New Jersey.

Beyond connecting population centers and regional highways in Rockland and Westchester counties, the Tappan Zee Bridge was a vital link between the upstate and downstate sections of the New York State Thruway. Approved in 1942, the Thruway started construction in 1946 near Syracuse. When the Tappan Zee Bridge opened nine years later, it linked the two portions of the roadway, giving the Thruway mainline a total length of 423 miles – the longest operating toll expressway in the nation. By the end of 1960, all 559 miles of the Thruway system were open, connecting Buffalo on the shores of Lake Erie, the state capital of Albany, and New York City. Prior to the Tappan Zee, the nearest Hudson River crossing to the George Washington Bridge  was 44 miles.

From the day it opened in December 1955, the Tappan Zee Bridge powered significant economic currents on both sides of the river. Its impact was most dramatic in Rockland County, which evolved rapidly from a predominantly rural and agricultural area to a diverse and bustling suburb. In the first five years after the bridge opened, the county’s population increased 50 percent.