Automobiles are a major form of transportation that enclose people and their cargo in a sealed compartment with a roof, doors and windows. Modern automobiles also have safety features such as seat belts, airbags and crumple zones that help protect passengers from injury or death in case of a collision. There are an estimated 1.4 billion automobiles in operation worldwide.
The automobile revolutionized modern life in the 20th century, changing industries, technology and culture. Henry Ford introduced the assembly line, making it possible to make automobiles cheap enough for middle-class families. It became one of the most popular vehicles in history, and its use caused new businesses like petroleum and gasoline, rubber, and then plastics to blossom. It also created jobs for those who built the cars, drove them and maintained them, as well as those who provided services such as gas stations and convenience stores.
Many automobiles have engines that use chemical energy stored in fuel—gasoline, diesel, or kerosene—to turn the wheels. Others use electrical motors to convert electrical energy into mechanical energy, which turns the wheels. The amount of energy that the motor sends to the wheels is measured in watts or horsepower.
Many factors affect the design of an automobile, including its intended use and manufacturing costs. Automobiles designed for off-road or other special use must have robust systems that can withstand severe overloading and extreme operating conditions. They must have good ground clearance and handling characteristics that optimize high-speed performance. They should have clear visibility, a pleasant shape and color, and a practical layout.