HighwaysEarly in the 20th century, most of the streets and roads in the U.S. were made of

Highways

Early in the 20th century, most of the streets and roads in the U.S. were made of dirt, brick, and cedar wood blocks. Built for horse, carriage, and foot traffic, they were usually poorly cared for and too narrow to accommodate(容纳)automobiles.

With the increase in auto production, private turnpike(收费公路)companies under local authorities began to spring up, and by 1921 there were 387,000 miles of paved roads. Many were built using specifications of 19th century Scottish engineers Thomas Telford and John MacAdam(for whom the macadam surface is named), whose specifications stressed the importance of adequate drainage. Beyond that, there were no national standards for size, weight restrictions, or commercial signs. During World War I, roads throughout the country were nearly destroyed by the weight of trucks. When General Eisenhower returned from Germany in 1919, after serving in the U. S. Army's first transcontinental motor convoy(车队), he noted: "The old convoy had started me thinking about good, two-lane highways, but Germany's Autobahn or motorway had made me see the wisdom of broader ribbons across the land."

It would take another war before the federal government would act on a national highway system. During World War II, a tremendous increase in trucks and new roads were required. The war demonstrated how critical highways were to the defense effort. Thirteen per cent of defense plants received all their supplies by truck, and almost all other plants shipped more than half of their products by vehicle. The war also revealed that local control of highways had led to a confusing variety of design standards. Even federal and state highways did not follow basic standards. Some states allowed trucks up to 36,000 pounds, while others restricted anything over 7,000 pounds. A government study recommended a national highway system of 33,920 miles, and Congress soon passed the Federal-Aid Highway Act of 1944, which called for strict, centrally controlled design criteria.

The interstate highway system was finally launched in 1956 and has been hailed as one of the greatest public works projects of the century. To build its 44,000-mile web of highways, bridges, and tunnels, hundreds of unique engineering designs and solutions had to be worked out. Consider the many geographic features of the country: mountains, steep grades, wetlands, rivers, deserts and plains. Variables included the slope of the land, the ability of the pavement to support the load, the intensity of road use, and the nature of the underlying soil. Urban areas were another problem. Innovative designs of roadways, tunnels, bridges, overpasses, and interchanges that could run through or bypass urban areas soon began to weave their way across the country, forever altering the face of America.

Long-span, segmented-concrete, cable-stayed bridges such as Hale Boggs in Louisiana and the Sunshine Skyway in Florida, and remarkable tunnels like Fort McHenry in Maryland and Mt. Baker in Washington, met many of the nation's physical challenges. Traffic control systems and methods of construction developed under the interstate program soon influenced highway construction around the world, and were invaluable in improving the condition of urban streets and traffic patterns.

Today, the interstate system links every major city in the U. S. , and the U. S. with Canada and Mexico. Built with safety in mind, the highways have wide lanes and shoulders, dividing medians or barriers, long entry and exit lanes, curves engineered for safe turns, and limited access. The death rate on highways is half that of all other U. S. roads(0.86 deaths per 100 million passenger miles compared to 1.99 deaths per 100 million on all other roads).

By opening the North American continent, highways have enabled consumer goods and services to reach people in remote and rural areas of t

A．Y

B．N

C．NG