Automobiles (or motor cars) are wheeled passenger vehicles that carry their own engines and run primarily on roads. There are now over 590 million automobiles worldwide, and 140 million in the United States alone. The modern automobile is powered by an internal combustion engine that produces power for the wheels through a system of drives, axles and tires. The engine is usually water-cooled and mounted in front of the wheels, although air-cooled engines are used for heavy vehicles such as trucks and buses.
The development of the automobile triggered a host of social and economic changes in industrialized countries, which restructured whole societies around the ability to make long-distance travel fast and easy. Families discovered the ability to vacation in new places and shop in towns, while rural people gained access to better medical care and schools. New industries, such as roadside restaurants and motels, opened to meet the needs of travelers. The automobile also encouraged sprawl, a form of low-density urban development that degrades landscapes and produces traffic congestion that can immobilize the very cars that created sprawl.
The first automobiles were built in the nineteenth century, but it was American industrialist Henry Ford who made them affordable to the middle class by using assembly lines. Since then, technological innovation has slowed considerably. Today, the automotive industry relies more on consumer expectations and marketing than engineering research and design. The automobile is the most widely used machine in history, and its impact continues to be felt far beyond the boundaries of its industrialized nations.