Over half a century has passed since President Dwight D. Eisenhower signed the Federal-Aid Highway Act, thereby initiating construction on the largest public works project in the history of the world (only later surpassed by China’s Three Gorges Damn). 35 years and $114 billion later (1), the Interstate Highway System (IHS) has dramatically changed the way many Americans choose to live and has had a significant environmental impact on this country. While it is difficult to separate the effects of Interstate highways from other factors such as increasing American prosperity or changes in technology, some interesting research has been done on their impact. A general disclaimer on the data and trends presented here: correlation is not causation. It is clear that the IHS is not the sole contributor to changes in American lifestyle since 1960. However, I believe (as so many others, far more knowledgeable that myself) that it has played a significant role.

Firstly, it is impossible to deny that Americans are driving more than they were 50 years ago. In 1960, the population of the United States was 177 million persons and Americans drove 1.3 trillion miles annually. By 2000, those numbers increased to 281 million persons and 4.4 trillion miles, meaning miles per capita have more than doubled (2, 3). Given that interstate highways account for 24 percent of all U.S. driving (while making up less than 1 % of the total highway mileage) (1), it is highly likely that a substantial portion of the increase in American driving has come as a result of the IHS. The environmental costs of this increase in driving come in the form of higher emissions of air pollutants, CO 2 and other greenhouse gasses, and greater exposure to lead, until its 1986 phasing out as an additive in gasoline. Particularly harmful was the 70 percent increase in driving during the 1960s and 1970s (2, 3), before the federally mandated use of catalytic converters to reduce emissions and the implementation of higher mileage standards to lower overall gas consumption. Additionally, the IHS has had a dramatic impact on the price of overland shipping. Between 1960 and today, the real cost of overland freight hauling has fallen 35 percent (4). Nearly half of America’s trucking miles are traveled on interstate highways (5). Apart from the general environmental impacts of more trucks on the road, diesel engines from heavy trucks are one of the leading sources of the nitrogen oxides that play a critical role in the many chemical pathways that contribute to air pollution.

There is also evidence that shows patterns in population change can be linked to the completion of an Interstate highway. An extensive study that examined all American counties over the course of the 1950s, 60s, 70s, and 80s showed that there was a positive correlation with the completion of an interstate with net migration (6). Counties with an interstate consistently grew more than those without one. Moreover, in counties experiencing population losses, a greater fraction of those connected to an interstate experienced a migration turnaround in the decade following construction (36.5 %) than those that did not (17.8 %) (6). While it is true that interstate highways were constructed in counties that already possessed above average population densities and rates of growth, the study claims that use of statistical methods (beyond my understanding) show that counties with an interstate still outperformed non-interstate counties when their initial advantage is taken into consideration. Another unknown is whether highway construction fostered growth or growth fostered highway construction. This could not be answered conclusively, but the study’s authors suggested that it was most likely some of both: highway construction led to growth, which lead to more highway construction.

The environmental impact of an increase in population density is obvious: an increase in vehicle emissions, water usage and runoff, and waste coupled with a decrease in open space and other wild habitat. Therefore, as the IHS has had an impact on the distribution of the American population, it has also created new pockets of environmental contamination and destruction.

Additionally, not all lifestyles have the same environmental impact. A general trend of the last 40 years has been an increase in the fraction of Americans enjoying a suburban lifestyle. In 1960, one third of all Americans lived in the suburbs; by 2000, that fraction had climbed to slightly more than half (and it still grows today) (2). For all urban areas, population grew by 92 percent between 1950 and 1990; during this same time, urban land area increased 245 percent (7). Additionally, populations have spread westward into previously low density areas. A study published in 2000 attempted to quantify the environmental impact of suburbanization relative to life in city centers as well as the impact of westward expansion relative to life in the rustbelt (7). It found that suburbanites drive 31 percent more than their urban counterparts and that western households drive 35 percent more than those in the northeast. Suburbanites also use more land. The average household lot size is 58 percent larger in the suburbs than in cities; if the affect of higher density apartment buildings and condos were included, the per capita land use difference would be even greater. The study showed that while there was little difference in household energy use between urban and suburban populations (when you discount gas consumption from increased driving), there were substantial increases in energy consumption outside of the Rust Belt. As westward and southern migration continues, so will household energy consumption.

Again, not all these differences are a direct consequence of the Interstate Highway System. However, they are all tied to the IHS, highways in general, and the changes in American automobile use, all of which are interconnected as a single advent in transportation. It is not a coincidence that changes in American driving habits coincided with the construction of IHS (which is one reason why the automobile industry was one of its principal proponents). There is little doubt in my mind that today’s urban sprawl (and its associated environmental impact) would be impossible without modern interstate highways.

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