There was once a time when most Americans took streetcars to work every day. Nowadays, 85 percent of workers drive.

And although a few different factors fueled this transition, the biggest one may have been a $425 billion investment over half a century in the world's most advanced network of highways: the Interstate Highway System.

The 48,000 miles of interstate highway that would be paved across the country during the 1950s, '60s, and '70s were a godsend for many rural communities. But those highways also gutted many cities, with whole neighborhoods torn down or isolated by huge interchanges and wide ribbons of asphalt. Wealthier residents fled to the suburbs, using the highways to commute back in by car. That drained the cities' tax bases and hastened their decline.

So why did cities help build the expressways that would so profoundly decimate them? The answer involves a mix of self-interested industry groups, design choices made by people far away, a lack of municipal foresight, and outright institutional racism.

"There was an immense amount of funding that would go to local governments for building freeways, but they had little to no influence over where they'd go," says Joseph DiMento, a law professor who co-wrote Changing Lanes: Visions and Histories of Urban Freeways. "There was also a racially motivated desire to eliminate what people called 'urban blight.' The funds were seen as a way to fix the urban core by replacing blight with freeways."

How freeways became "free"

The roots of the interstate system go back to the 1930s, when General Motors, AAA, and other industry groups formed the National Highway Users Conference to influence federal transportation policy.

These groups realized the nation's transportation system needed to be reframed entirely — as a public responsibility. After all, most cities had just ripped up their streetcar networks because they were privately owned systems that weren't making money. The auto industry didn't want the same thing to happen to highways. So "there was a really successful effort by people with a stake in the automotive industry to characterize road-building as a public responsibility," says Peter Norton, a historian at the University of Virginia and author of Fighting Traffic: The Dawn of the Motor Age in the American City.

The first step was changing how roads were funded. In the 1930s, there were already privately owned toll roads in the East, and some public toll highways, like the Pennsylvania Turnpike, were under construction. But auto groups recognized that funding public roads through taxes on gasoline would allow highways to expand much more quickly.

They also decided to call these roads "free roads," a term that was later replaced by "freeways." Norton argues that this naming shift was essential in persuading the federal government — and the public — to shift away from tolls. "It started with calling the roads drivers pay for 'toll roads,' and calling the ones that taxpayers pay for 'free roads,'" he says. "Of course, there's no such thing as a free road."

Those terms were officially enshrined in a 1939 congressional planning document called "Toll Roads and Free Roads" that roughly outlined what would become the interstate system for the first time.

Even though gas taxes have never fully paid for highways — a recent Public Interest Research Group report found they've covered between 43 and 74 percent of costs through the interstate system's history — the widespread perception that highways and freeways are somehow self-funding has stuck around.

The design of the Interstate system

Around the same time, auto industry groups began envisioning an ambitious network of wide, smooth highways, accessible only by on-ramps, that would crisscross the country.

These highways would link distant cities but also thread through downtowns, allowing people to drive as quickly as possible from home to work and back. This vision was distilled in a massive, one-acre diorama GM built for the 1939 World's Fair in New York called Futurama:

World War II delayed progress in this highway system, but policymakers in Washington, DC, began working on a plan afterward.

The paths of the highways that would become the interstates were laid out in a 1947 map, followed by a 1955 Department of Commerce document — often called the "yellow book" — that specified the paths these highways would take through city centers:

The plan's key contributors included members of the auto industry (including General Motors CEO Charles Erwin Wilson) and highway engineers. Curiously, urban planners were absent — the profession barely existed at the time.

"Highway engineers dominated the decision-making," says DiMento. "They were trained to design without much consideration for how a highway might impact urban fabric — they were worried about the most efficient way of moving people from A to B."

As a result, the official plans dictated that highways cut directly through the core of virtually every major city in order to bring commuters from newly growing suburbs in and out:

This document was the basis for the Federal Aid Highway Act of 1956, which birthed the interstate system. The bill stipulated that the rest of a massive, nationwide highway system be toll-free, with 90 percent of the construction cost borne by the federal government through both gas taxes and other funding sources.

This was an unheard-of amount: previous federal highway bills had the cost split 50-50 or 60-40 between federal and state governments. But this new arrangement had the backing of President Eisenhower, who was especially interested in seeing the system built, partly so it could be used for troop movements and mass evacuations in the event of a nuclear attack.

The new bill essentially gave states highways for free — provided they consented to the paths created in the yellow book, which had highways running through every city center in the US:

The racial politics of "urban renewal"

State and city politicians accepted these plans for a variety of reasons. In an era when suburbs had just begun to grow, DiMento says, "local politicians saw urban freeways as a way of bringing suburban commuters into city." Some local businesspeople supported them for similar reasons.

But an unmistakable part of the equation was the federally supported program of "urban renewal," in which lower-income urban communities — mostly African-American — were targeted for removal.

"The idea was 'let's get rid of the blight,'" says DiMento. "And places that we'd now see as interesting, multi-ethnic areas were viewed as blight." Highways were a tool for justifying the destruction of many of these areas.

The new freeways also isolated many other neighborhoods, ushering in their demise. Combined with federal housing bills that paid developers to tear down existing housing stock and replace it with high-rises, they resulted in the continued decimation of huge swaths of many cities.

"Many neighborhoods, predominantly black, were wiped out and turned into surface parking and highways," Norton says, noting Black Bottom and Paradise Valley in Detroit, historical neighborhoods that were torn down to make way for I-375.

The same pattern was repeated over and over, leading to cities pockmarked with empty neighborhoods and destructive highways. People displaced from the destroyed areas moved to others, leading to overcrowding and increases in crime, while most people with the means fled to the suburbs — commuting on the new highways, and siphoning money away from these cities' tax bases.

But not all the highways got built. Many city governments opposed them from the beginning — and in San Francisco, DC, and elsewhere, key segments were blocked by a coalition of local officials and residents. In New York, activists led by Jane Jacobs successfully prevented construction of I-78 through Lower Manhattan, which would have torn up much of Greenwich Village, SoHo, Little Italy, Chinatown, and the Lower East Side.

"The explanation, in almost every case, is that the relatively well-off, influential people in those cities were able to stop the urban highways that would have gone through their neighborhoods," Norton says, pointing to Wisconsin Avenue, in DC, which was slated to become a highway but never did due to the protest of wealthy residents of the city's Northwest quadrant.

"The destruction mostly happened in the most disenfranchised neighborhoods," he says. "It's astounding how selective it was."

This article is part of a series about the past, present, and future of commuting in America.