The economic and social complexion of life in the United States mirrors the country’s extraordinary mobility. A pervasive transportation network has helped transform the vast geographic expanse into a surprisingly homogeneous and close-knit social and economic environment. Another aspect of mobility is flexibility, and this freedom to move is often seen as a major factor in the dynamism of the U.S. economy. Mobility has also had destructive effects: it has accelerated the deterioration of older urban areas, multiplied traffic congestion, intensified pollution of the environment, and diminished support for public transportation systems.

Roads and railroads

Central to the U.S. transportation network is the 45,000-mile (72,000-km) Interstate System, officially known as the Dwight D. Eisenhower System of Interstate and Defense Highways. The system connects about nine-tenths of all cities of at least 50,000 population. Begun in the 1950s, the highway system carries about one-fifth of the country’s motor traffic. Nearly nine-tenths of all households own at least one automobile or truck. At the end of the 20th century, these added up to more than 100 million privately owned vehicles. While most trips in metropolitan areas are made by automobile, the public transit and rail commuter lines play an important role in the most populous cities, with the majority of home-to-work commuters traveling by public carriers in such cities as New York City, Chicago, Philadelphia, and Boston. Although railroads once dominated both freight and passenger traffic in the United States, government regulation and increased competition from trucking reduced their role in transportation. Railroads move about one-third of the nation’s intercity freight traffic. The most important items carried are coal, grain, chemicals, and motor vehicles. Many rail companies had given up passenger service by 1970, when Congress created the National Railroad Passenger Corporation (known as Amtrak), a government corporation, to take over passenger service. Amtrak operates a 21,000-mile (34,000-km) system serving more than 500 stations across the country.

Water and air transport

Navigable waterways are extensive and center upon the Mississippi River system in the country’s interior, the Great Lakes–St. Lawrence Seaway system in the north, and the Gulf Coast waterways along the Gulf of Mexico. Barges carry more than two-thirds of domestic waterborne traffic, transporting petroleum products, coal and coke, and grain. The country’s largest ports in tonnage handled are the Port of South Louisiana; the Port of Houston, Texas; the Port of New York/New Jersey; and the Port of New Orleans.

Air traffic has experienced spectacular growth in the United States since the mid-20th century. From 1970 to 1999, passenger traffic on certified air carriers increased 373 percent. Much of this growth occurred after airline deregulation, which began in 1978. There are more than 14,000 public and private airports, the busiest being in Atlanta and Chicago for passenger traffic. Airports in Memphis, Tennessee (the hub of package-delivery company Federal Express), and Los Angeles handle the most freight cargo.

Wilfred Owen The Editors of Encyclopaedia Britannica

Constitutional framework

The Constitution of the United States, written to redress the deficiencies of the country’s first constitution, the Articles of Confederation (1781–89), defines a federal system of government in which certain powers are delegated to the national government and others are reserved to the states. The national government consists of executive, legislative, and judicial branches that are designed to ensure, through separation of powers and through checks and balances, that no one branch of government is able to subordinate the other two branches. All three branches are interrelated, each with overlapping yet quite distinct authority.

The U.S. Constitution (see original text), the world’s oldest written national constitution still in effect, was officially ratified on June 21, 1788 (when New Hampshire became the ninth state to ratify the document), and formally entered into force on March 4, 1789, when George Washington was sworn in as the country’s first president. Although the Constitution contains several specific provisions (such as age and residency requirements for holders of federal offices and powers granted to Congress), it is vague in many areas and could not have comprehensively addressed the complex myriad of issues (e.g., historical, technological, etc.) that have arisen in the centuries since its ratification. Thus, the Constitution is considered a living document, its meaning changing over time as a result of new interpretations of its provisions. In addition, the framers allowed for changes to the document, outlining in Article V the procedures required to amend the Constitution. Amending the Constitution requires a proposal by a two-thirds vote of each house of Congress or by a national convention called for at the request of the legislatures of two-thirds of the states, followed by ratification by three-fourths of the state legislatures or by conventions in as many states.

In the more than two centuries since the Constitution’s ratification, there have been 27 amendments. All successful amendments have been proposed by Congress, and all but one—the Twenty-first Amendment (1933), which repealed Prohibition—have been ratified by state legislatures. The first 10 amendments, proposed by Congress in September 1789 and adopted in 1791, are known collectively as the Bill of Rights, which places limits on the federal government’s power to curtail individual freedoms. The First Amendment, for example, provides that “Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.” Though the First Amendment’s language appears absolute, it has been interpreted to mean that the federal government (and later the state governments) cannot place undue restrictions on individual liberties but can regulate speech, religion, and other rights. The Second and Third amendments, which, respectively, guarantee the people’s right to bear arms and limit the quartering of soldiers in private houses, reflect the hostility of the framers to standing armies. The Fourth through Eighth amendments establish the rights of the criminally accused, including safeguards against unreasonable searches and seizures, protection from double jeopardy (being tried twice for the same offense), the right to refuse to testify against oneself, and the right to a trial by jury. The Ninth and Tenth amendments underscore the general rights of the people. The Ninth Amendment protects the unenumerated residual rights of the people (i.e., those not explicitly granted in the Constitution), and the Tenth Amendment reserves to the states or to the people those powers not delegated to the United States nor denied to the states.

The guarantees of the Bill of Rights are steeped in controversy, and debate continues over the limits that the federal government may appropriately place on individuals. One source of conflict has been the ambiguity in the wording of many of the Constitution’s provisions—such as the Second Amendment’s right “to keep and bear arms” and the Eighth Amendment’s prohibition of “cruel and unusual punishments.” Also problematic is the Tenth Amendment’s apparent contradiction of the body of the Constitution; Article I, Section 8, enumerates the powers of Congress but also allows that it may make all laws “which shall be necessary and proper,” while the Tenth Amendment stipulates that “powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.” The distinction between what powers should be left to the states or to the people and what is a necessary and proper law for Congress to pass has not always been clear.

Between the ratification of the Bill of Rights and the American Civil War (1861–65), only two amendments were passed, and both were technical in nature. The Eleventh Amendment (1795) forbade suits against the states in federal courts, and the Twelfth Amendment (1804) corrected a constitutional error that came to light in the presidential election of 1800, when Democratic-Republicans Thomas Jefferson and Aaron Burr each won 73 electors because electors were unable to cast separate ballots for president and vice president. The Thirteenth, Fourteenth, and Fifteenth amendments were passed in the aftermath of the Civil War. The Thirteenth (1865) abolished slavery, while the Fifteenth (1870) forbade denial of the right to vote to formerly enslaved men. The Fourteenth Amendment, which granted citizenship rights to formerly enslaved people and guaranteed to every citizen due process and equal protection of the laws, was regarded for a while by the courts as limiting itself to the protection of formerly enslaved people, but it has since been used to extend protections to all citizens. Initially, the Bill of Rights applied solely to the federal government and not to the states. In the 20th century, however, many (though not all) of the provisions of the Bill of Rights were extended by the Supreme Court through the Fourteenth Amendment to protect individuals from encroachments by the states. Notable amendments since the Civil War include the Sixteenth (1913), which enabled the imposition of a federal income tax; the Seventeenth (1913), which provided for the direct election of U.S. senators; the Nineteenth (1920), which established woman suffrage; the Twenty-fifth (1967), which established succession to the presidency and vice presidency; and the Twenty-sixth (1971), which extended voting rights to all citizens 18 years of age or older.