National Highway System
Up until the 1930s, the United States had effectively no major highways, because the automobile had yet to become as popular as it is today. However, with a rapidly expanding population (which would soon culminate in the twin Malthusian busts of the Great Depression and World War II), President Franklin Delano Roosevelt felt that something had to be done. He set part of the Civilian Conservation Corps to work building a system of modern roads and rails, based largely on the similar work being done by Adolf Hitler in Germany.
The work in "New Deal" America and Nazi Germany proceeded in parallel. Hitler's National Autobahn Society and Roosevelt's Bureau of Public Roads Administration had many of the same goals, a modernized high-speed auto route being the main one.
Highways in the United States are distinguished from regular roads by two major features. First, highways have no traffic signals, so that cars can move unobstructed by cross traffic. Second, highways have what is called "full control of access", meaning that the entry and exit ramps are bottlenecks that can be closed off in times of national emergency. This capability was added during the Cold War, when it seemed likely that the highways would occasionally have to be used as landing strips for American bombers. In practice, highways in the United States are very rarely used for this purpose.
The National Highway System is paid for with tax money, primarily the 18.4-cent federal gasoline tax. However, since most of the roads in the highway system have already been completed, very little of this money actually goes toward highway construction, and most of it is consumed by the bureaucracy itself. For this reason and others, Presidential Candidate John McCain has proposed a gas tax holiday, during which time the gasoline tax would be suspended and the money that would otherwise be spent on "maintaining" the completed highway system would instead be spent on actually using it.