Extensive river systems like the MississippiûMissouri provided early transportation networks for the USA. Today, it has the world's cheapest, most extensive internal air network and a good system of interstate highways. The rail network is poorly developed, by European standards, and carries mostly freight. Since Henry Ford began mass production in Detroit nearly 90 years ago, Americans have been wedded to the car. In 1919, Ford sold one million cars. Today, there are over 255 million cars in the USA. Many cities, such as Los Angeles, have come to depend on the car; the USA now accounts for more than half of the world's car trips. Cheap gasoline has underpinned this growth. In the long term, the prospect of dependence on oil imports, as domestic supplies run out, could force a review of the car's role.
