Automobiles are motor vehicles that are designed to carry people or cargo over a road. Modern cars are almost always powered by an internal combustion engine that converts gasoline or diesel fuel into energy to move the vehicle forward. Some are also equipped with a range of air and road-conditioning systems to make driving comfortable and safe for passengers.

Automobile technology has been rapidly developed since the early 1900s, spurred by competitive market pressure to produce affordable automobiles. The large-scale, assembly line manufacturing introduced by Henry Ford revolutionized industrial production and made the car affordable to middle-class consumers. This led to the rise of large automakers like Ford and General Motors that dominate the world automotive industry today.

In the United States, cheap raw materials and a lack of tariff barriers encouraged new firms to enter the market, producing a wide variety of automobiles in very high volumes. The availability of inexpensive labor and a national network of railroads facilitated the shipment of these products to distant markets. The development of the automobile mirrored America’s economic transformation into an industrial powerhouse.

Historically, automobiles have served as the dominant mode of personal transportation in the United States. Currently, more than three trillion miles (about five trillion kilometers) are driven in the country each year. A wide range of automotive designs exist, from the utilitarian Model T to the artful mid-century modern models that cruised U.S. highways in the 1950s.

The automotive industry has become a major component of the economy, with the total value of all vehicles in operation in the United States exceeding $800 billion. Automobiles are a major source of air pollution and contribute to climate change, but they also provide a convenient way for individuals to commute to work or school. Public transportation such as buses, passenger trains, trams and subways can help reduce automobile traffic congestion and pollution.

Although many people in urban areas depend on automobiles for daily transportation, the commuting habits of Americans vary widely. In cities and suburban communities with poor public transportation options, a car may be the only practical means of travel for people who cannot afford or do not want to use public transit. In sparsely populated rural areas, where public transportation services are limited or nonexistent, automobile ownership can save families time and money by enabling them to travel greater distances for recreation and shopping.

However, automobiles can be dangerous to their owners and others when they crash, and their emissions can cause air pollution. Moreover, they can be a nuisance to their neighbors when they block driveways or park on the street. For these reasons, many governments regulate the design and operation of automobiles to limit their impact on human life and property. These regulations include regulating engine performance, safety, and environmental impact, and setting speed limits. They also require manufacturers to install seat belts and airbags, as well as mandate that drivers take periodic driver’s tests.