The National Road, which was the first major highway constructed by the Federal Government of the United States beginning in 1811, was a part of a much broader political program to improve American infrastructure and therefore trade, by building bridges, damns, waterways and roadways. Like the Eerie Canal, the National Road became an alternate route for trade that went east to west, instead of north to south. Before the construction of east-west routes like the National Road, the westward expansion away from the Atlantic Coast was severely limited because the trek into states such as Pennsylvania, Ohio and Illinois was so long, difficult and treacherous.
Yet the American System, envisioned and championed by Henry Clay and other influential politicians after the War of 1812, helped to accelerate trade and migration between states on the Atlantic Coast and the newer, more interior states, such as Ohio and Illinois. Once these states and others like it came into the union, their political power began to rival that of the southern states, and their economies, based on family farming, fur and lumber trading, as well as manufacturing, began to rival the economies of the southern states. The southern states in turn saw the increased political clout of non-slave states as a grave threat to their ability to national policy, and to advocate for the expansion of slavery in the west.
Many of the political dogfights that took place for the next forty years (Missouri Compromise, Fugitive Slave Act, Homestead Act, Compromise of 1850, et cetera) involved issues related to westward expansion, the future of slavery in the west, and the waning influence of Plantation owners and slave states in Congress.
From an environmental perspective, the National Road and the railroads that followed decimated animal populations, particularly buffalo, upon which many Plains Indians relied for sustenance. The road also decimated forests and polluted rivers. As the National Road carried more and more American settlers into the heartland of the country, those settlers ground that rugged and pristine terrain into farmable land, but they didn't know much about farming, so they often used methods that dried up water tables and cut off or rerouted fresh water supplies (like rivers and streams) in order to increase their crop yields. In the process, these settlers caused mass extinctions of countless species of plants and animals, whose habitats were destroyed before they could be catalogued.
The land that these settlers took had for thousands of years been sparsely populated and lightly farmed. Native Americans had taken great care to preserve it. Within one hundred years, much of that land had been rendered unrecognizable. Moreover, a political system that had been designed to ensure that citizens lived close to their representatives so that they could hold them accountable had shifted to a system in which citizens were governed by a federal government that most of them had never seen, and would never see. This lack of contact between the federal government and its citizens led to a chasm of understanding and a lack of responsiveness to people’s daily needs, which undercut the very notion of a representative democracy. It took another century for the Progressive Movement to give some semblance of control over the government back to the people.