Environmental Impacts

Highways

Travel by highways is still the prevailing mode of transportation in the United States and has negative effects from roadkill to vehicle emissions. Road construction and maintenance requires the use of land that can potentially impact the environment through the disruption of natural habitat, modified hydrology, and contaminated runoff. While much of the nation's highway construction has occurred over decades, the impact on many habitats can still be felt in the form of fragmentation. In expanding urban areas road construction is ongoing and, depending on the sensitivity of the habitat, can have [Page 513]significant consequences for local ecosystems such as wetlands. These wetlands may be made up of marsh, swamp, or mangrove and are beneficial for controlling floods and supporting species diversity. Construction site runoff can contribute to higher erosion rates accompanied by increased sedimentation, which can endanger aquatic habitats. Pollutants washed from roads can enter neighboring streams and lakes, affecting water quality. Repair shops, service stations, and maintenance facilities are sources of pollution in the form of gasoline, oil, and dirt runoff that enter waterways through the sewer system. Additionally, stormwater runoff is prevented from infiltrating paved surfaces and thus cannot replenish underground aquifers.

In large urban areas, 90 percent of the carbon monoxide (CO) and 50 percent of the ground-level ozone (O3), a major component of smog, is attributed to vehicle exhaust. Automobiles emit nitrogen oxides (NOx) and sulfur oxides (SOx), which contribute to the formation of acid rain as well as volatile organic compounds (VOCs) and particulate matter, which are carcinogenic. It is estimated that the annual number of premature deaths in the United States due to vehicle emissions is about the same as the number of deaths each year from traffic accidents. Overall, automobile exhaust can adversely impact the environment by damaging forests, grasslands, and croplands. Perhaps the most insidious emission in vehicle exhaust is carbon dioxide (CO2). CO2 is the primary greenhouse gas that causes global warming. Approximately one-third of the CO2 released in the United States is from the transportation sector and nearly one-quarter of this total comes from motor vehicles. The current global level of CO2 is 394 parts per million (ppm) with an annual rate of increase of 2.15 percent. CO2 emissions from the transportation sector have risen each year by approximately 2 percent since 1990. Carbon dioxide represents 95 percent of all the greenhouse gases released by the transportation sector, which includes methane, nitrous oxide, and various refrigerant compounds.

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