Environmental impacts of road systems and freeways

DEFINITION: Paved thoroughfares created for motor vehicle traffic

The construction and heavy use of highways can have severe impacts on the environment, including air and water pollution, land degradation, and loss of open space. Since motorized transportation is a common means of travel, compromise is often necessary to balance the need for additional roads and the need to protect the environment.

The era of the freeway began during the 1950s, when lobbying groups in the United States encouraged a political vision of a nationwide high-standard, high-speed road network. Retired U.S. Army general Lucius D. Clay led a committee that studied transportation needs across the United States and advised President Dwight Eisenhower that the nation needed what came to be called the National System of Interstate and Defense Highways. The costs of creating and maintaining the more than 69,000 kilometers (43,000 miles) of interstate highways built after 1956 have been shared by federal and state governments, with the federal government providing 90 percent of the funding. The federal share comes from the Highway Trust Fund, which receives revenues from federal taxes on fuels, lubricants, vehicles, and vehicle parts. Although the interstate system accounts for only 1 percent of the total road miles in the United States, it carries 20 percent of the traffic.

Studies of the U.S. road and highway systems as a whole—including local roads and services—have found that motor vehicle user fees cover only two-thirds of public expenditures, not including the substantial nonmonetary external costs of environmental impacts. Because many of the costs of using private vehicles are hidden, many Americans perceive driving their own cars to be less expensive than using public-transit alternatives. U.S. highway statistics for 2022 indicated the existence of more than 283 million registered motor vehicles and more than 4.1 million miles of public roads.

Traffic congestion is most severe in areas experiencing rapid growth in both total and number of vehicles in use. In fact, rapid population growth tends to offset the beneficial impacts of remedies adopted to reduce traffic congestion. Low-density settlement generates more total automotive vehicle trip miles per day, which consume more energy and cause greater of pollutants. One important development that has helped to reduce congestion, and thus motor vehicle emissions, on some urban and suburban freeways since the 1970’s is the so-called carpool lane—a lane reserved for the use of buses and other vehicles carrying more than one or two passengers.

Environmental Impacts

Air constitutes the most serious environmental impact caused by highway transportation. Developments that may generate traffic, such as parking lots for shopping centers, may be classified as indirect sources of pollution. Internal combustion and diesel engines are the principal sources of carbon monoxides and hydrocarbons, account for nearly one-half of the nitrogen oxides, and are the chief source of particulate lead in the atmosphere. Highway emissions are directly related to traffic volume and density, vehicle type, speed, and mode (idle, acceleration, cruise, or deceleration). Increased speed produces a demand on an engine for increased power, which leads to more fuel consumed and greater emissions, but the vehicle also passes through an area more quickly. Long trips by motor vehicle and traffic congestion both increase the emissions discharged into the atmosphere.

The building of roads and highways also consumes open space, affecting plant and animal life, as well as climate and water runoff. Highways facilitate the spread of urban areas and often lead to low-density developments, which are difficult to provide with services. Highways that connect developed areas usually follow valleys and other areas with flat terrain, and consequently highways are often built in close proximity to streams, lakes, and wetlands. Until very late in the twentieth century, hydrologic features that blocked proposed roads were seen primarily as obstacles to be bridged, filled, or moved at lowest cost. Laws that were put in place to protect endangered animal and plant species changed this approach, requiring highway developers sometimes to employ routes or construction procedures that are more expensive than those they would have used in the past.

Among the more subtle and probably more serious impacts of road construction are changes in local hydrologic patterns, such as changes in the that affect vegetation. Erosion and are also associated with road construction activities. Another environmental concern related to roads and highways involves the from street surfaces into waterways; such runoff can include salt (spread on roads to melt ice in winter) and products that contaminate the soil.

Highway noise has also been identified as an environmental problem. Noise—excessive or unwanted sound—has been found to be more than simply an annoyance; in the extreme, the sleep disturbances and other negative impacts of noise can be considered a danger to public health. Highway noise is troublesome to control, but some can be achieved through proper planning. Buffer zones and acoustical barriers, modifications of highway alignment, and traffic management measures can reduce the to noise for those who live or work near highways. The U.S. Federal Highway Administration specifies a methodology that should be used for the evaluation of traffic noise and abutting land use and provides guidance on when noise should be considered.

In balancing transportation needs and environmental goals, engineers must distinguish between mobility (simple movement) and access (the ability to reach destinations and desired services). Increasing mobility through more roads, more vehicles, and more traffic may actually reduce access over the long term. For example, older neighborhoods tend to have stores, schools, and transit services within walking distance of residential areas, whereas newer auto-dependent neighborhoods tend to be lower in with few local services and often no pedestrian facilities, so more trips require driving. Hence mobility increases but access declines.

Bibliography

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Hester, R. E., and R. M. Harrison, eds. Transport and the Environment. Cambridge, England: Royal Society of Chemistry, 2004.

"Interstate Frequently Asked Questions." Federal Highway Administration, 30 June 2023, highways.dot.gov/highway-history/interstate-system/50th-anniversary/interstate-frequently-asked-questions. Accessed 22 July 2024.

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