In many regards, the Interstate's effect has been positive. By allowing people, goods and services to move much quicker than was ever possible on the narrow, two-lane highways and congested railways of pre-Interstate America, it has created a level of prosperity and personal freedom that in many ways has been unparalleled in the history of humanity. It revolutionized the way people live and work. The allowed people to take advantage of cheaper housing in the suburbs while still being able to travel quickly to their jobs. It allowed manufacturing and distribution companies to locate themselves anywhere; they were no longer bound by a required proximity to railheads or ports. Trucking, in fact, replaced railroads as the primary means of shipment. It brought economic opportunity and social interaction to previously remote or otherwise forgotten areas of the country. The Interstate made the family road trip a staple of the summertime and allowed access to a new range of entertainment and leisure options, from remote national parks to distant beaches to Walt Disney World. Due to their high enginerering standards, the Interstate made automobile travel safer as well.
At the same time, the Interstate has also had a lot of profoundly negative affects on the nation. Rural Interstates divided farmlands and replaced scenic vistas with unsightly agglomerations of billboards, truck stops and fast-food restaurants. Urban Interstates divided cities, destroyed older, walkable neighborhoods, led to the decline of downtowns and facilitated automobile-oriented suburban sprawl. As I explain in my report about highway aesthetics, they irrevocably redefined the American cityscape and the way it is experienced. The Interstates promulgated the dominance of the private automobile over all other forms of transportation, a dominance that elevated the automobile from a luxury to an absolute necessity and brought with it negative externalities such as poor air quality and dependence on foreign oil. Today, anybody who does not own a car is, for all practical purposes, a second-class citizen. Even the congestion problems that Interstates were supposed to solve did not disappear; in fact, as the United States added more people, houses and jobs, highway congestion actually grew worse.
I say all this not to praise or denounce the Interstate, but simply to reflect the fact that, for better or for worse, our lives today would be unimaginable without it. With a stroke of a pen exactly half a century ago today, Dwight D. Eisenhower redefined the American Experience.
EDIT: this interesting commentary from the Christian Science Monitor compares the Interstate network to another network that has transformed our lives, the Internet.