There are 4 million miles of roads in the United States. One hundred years ago, roads were primarily unpaved and had half the number of miles of the present . road system. As the system grew, roads became wider and more complex structurally to provide for more and heavier traffic. New construction technology and greater structural stability were needed to improve the road system. All phases of road development—from construction and use by vehicles to maintenance—affect physical and chemical soil conditions, water flow, and air and water quality. Roads alter habitats, increase wildlife mortality, and disperse nonnative pest species of plants and animals. At larger scales, roads affect wildlife migration.
