Part I. Highway construction road

Road is the traveled way on which people, animals, or wheeled vehicles move. In modern usage the term road describes a rural, lesser traveled way while the word street denotes an urban roadway. Highway refers to a major rural traveled way; more recently it has been used for a road, in either a rural or urban area, where points of entrance and exit for traffic are limited and controlled.

The Romans were the first to construct roads scientifically. Their roads were characteristically straight, and the best ones were composed of graded soil foundation that was topped by four courses (layers): a bedding of sand or mortar; rows of large flat stones; a thin layer of gravel mixed with lime; and finally a thin wearing surface of flint like lava. Roman roads varied in thickness from 3 to 5 feet (0,9 to 1,5 m), and their design remained the most sophisticated until the advent of modern road building technology in the late 18^th and early 19^th centuries.

When interest in road building revived in Europe in the late 18^th century, engineers began designing roads that incorporated lighter surfaces, relying on the subsurface for load support. Roads could thus be built relatively cheaply and quickly. The most influential of the early engineers was John Loudon McAdam, inventor of the macadam road surface. His design comprised a compacted subgrade of crushed rock to support the load, and a surface covering of light stone to absorb wear and shed water to the drainage ditches.

By the end of the 19^th century the widespread use of the bicycle created a demand for roads with smoother surfaces. A pavement of natural rock asphalt was used in Paris as early as 1854, and Portland cement concrete was used in Scotland in 1865.

Two classifications of pavement have been developed: flexible and rigid. Flexible pavement is usually made of an asphalt-gravel aggregate that is laid in one or more courses over the subgrade. The aggregate can be mixed at the road-building site or at a central plant, and its quality varies with the production method used.

A cheap method of pavement, called surface treatment, is made by spraying hot asphalt or tar on a compacted stone base and then placing small stone chips on the tar; it is suitable for lightly traveled roads and can be built up in layers. Pavements made with a high-temperature plant mix are suitable for the heaviest loads and are made by laying the asphalt while it is hot and rolling it before it cools. A flexible pavement has the advantage of being easy to build and repair, its asphalt binder is both waterproof and plastic.

Rigid pavement made of Portland cement concrete, generally has greater strength but is susceptible to cracking. The cement, mixed with water and various grades of crushed stone called aggregate, is poured onto the built-up and graded foundation as a plastic mass. It shrinks as it dries, causing tensile stresses. The concrete also contracts and expands with temperature change, so that cracking is a constant problem. The best solution has been by pouring a continuous concrete slab in which a mesh of steel bars is embedded. The bars, running lengthwise through the concrete, absorb the tension of shrinkage and hold shut any cracks that form.

Modern highway design entails careful study of soil types, the topography of the intended route, and the drainage systems around the roadway. Where necessary, measures are taken to provide additional drainage facilities to prevent water from eroding the road base or freezing in cracks. The techniques of cutting and filling (excavating in one place and depositing it nearby to form a level roadbed) and switch-backing (zigzagging up a slope) have been used for centuries to obtain easy gradients in varied terrains.

The prototype of the modern superhighway was the Bronx River Parkway, which was completed in 1925 in New York City. It was a limited-access, high-speed highway designed to carry a large volume of traffic without disturbing the natural landscape. In the 1920s the Italians began the autostrada, and the Germans followed not long after with the autobahn. Military use was an important design feature of these highways, which could accommodate heavy traffic at speeds of 100 mi (160 km) per hour. In the United States the federal government created the national Interstate Highway System after World War II. It incorporated the toll-road network with other limited-access highways and linked all of the nation’s major cities. Most industrialized countries in the world built similar systems to facilitate automobile and truck traffic.