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Freeway

Freeway

Rise of the freeway

Features of the freeway

Construction of a freeway

Safety features

Speed limits

Resources

Freeways, also called superhighways, motorways, expressways, and sometimes tollways (when fees are paid), are roads specifically designed to allow for the free flow of traffic. Freeways typically feature two or

more traffic lanes in each direction, medians to divide the opposing directions, full access control, a system of ramps to prevent merging and diverging traffic from interrupting the traffic flow, and grading to separate intersecting traffic on other roads. As of the 2000s, about one-third of the total number of miles driven on roads utilize the freeway system.

Rise of the freeway

The advent and eventual domination of the automobile created a corresponding demand for roads capable of handling the increasing traffic and loads. Increasing numbers of cars began to choke the cities with traffic. The need for linking cities to one another also became apparent, especially as the truck proved its flexibility and reliability for transporting goods, materials, and products.

The freeway was first conceived as a means for reducing the crush of traffic within the cities, and for linking the cities together. The first freeway was opened in the Grunewald Forest in Berlin, Germany, in 1921. The idea for a national highway system for the United States was also developed during this time. The first U.S. freeways appeared in 1940, when California opened the Arroyo Seco Parkway between Pasadena and Los Angeles, and when Pennsylvania opened the first section of the Pennsylvania Turnpike.

The numbers of automobiles in use skyrocketed in the years after World War II (19391945). With this increase came an alarming increase in traffic congestion and automobile accident fatalities. In 1956, legislation was passed creating the Federal Interstate Highway System (FIHS). The system was begun during the presidency of Dwight David Eisenhower (18901969), which is why it is often called the Eisenhower Interstate System. This network of freeways was meant to link nearly all cities in the United States with populations greater than 50,000 people. Although the original plans called for exactly 40,000 mi (64,630 m) of road, by the 1990s nearly 45,000 mi (72,405 m) of road surface had been completed, carrying more than 20% of all traffic in this country. The initial cost for the FIHS was $25 billion over a 12-year period. However, as of 1991, when the project was considered finished by the federal government, it had cost a total of over $114 billion. Freeways in the FIHS are constructed according to strict guidelines governing the materials and other elements of their design and construction.

Freeways dramatically changed the pattern of life in the United States. Access to the city by automobile allowed people to move beyond the traditional trolley and horse-drawn cart routes. The spread of people outside of the city created what is known as urban sprawl, in which the city extends farther and farther from its center. Meanwhile, the former centers of city life lost more and more manufacturers and other industries to the suburbs, draining the cities of vital resources and jobs. Although the freeway was originally meant to alleviate traffic, it actually increased traffic levels, by encouraging the use of the automobile over mass transportation methods such as trains and buses. The resulting increases in congestion brought problems of pollution and noise. What were once walking cities were now accessible only by car. Entire new communities, the suburbs, became so dependent on automobiles that most families found it necessary to have two or more. Meanwhile, the increased traffic on the roads brought corresponding increases in the number of traffic fatalities.

Nonetheless, the FIHS remains the most ambitious public works undertaking in U.S. history. The FIHS has made nearly every part of the country accessible by car. It has brought a greater flexibility and choice of places for people to live, work, and travel, and a greater mobility over longer distances and safer roads.

Features of the freeway

All freeways share a number of common features. A freeway has at least four lanes, two lanes in each direction. Many freeways, however, feature more than four lanes and many freeways now contain up to sixteen or eighteen lanes, especially as they near the cities. Lanes are required to be from 11 to 12 ft (3.35 to 3.66 m) wide. Shoulder lanes provided on each side of the driving lane for each direction of the freeway allow vehicles to safely leave the traffic stream in the event of an emergency. Shoulder lanes are generally 8 to 10 ft (2.4 to 3.0 m) wide. A median, or center strip, separates the opposing directions of traffic. Medians may vary from 16 to 60 ft (4.9 to 18.3 m) wide. The median improves safety by preventing head-on collisions of automobiles traveling toward each other.

Freeways are called controlled access highways. This means that traffic is limited in where it may come onto or leave the freeway. These entrance and exit points are referred to as interchanges. Minor roads and driveways are diverted away from the freeway so that their traffic does not interfere with the freeway traffic flow.

Many roads, from small local roads and streets to other highways in the freeway system, intersect with a freeway. Grade separation prevents the intersection of two roads traveling crossways to each other from interrupting each others traffic flow. Generally, one road, usually the road minor to the freeway, is raised on a grade, or slope, so that it is higher than the freeway, and allowed to cross it over a bridge. Ramps are constructed to lead the crossing road to the grade separation. Additional access ramps, often called on-ramps and off-ramps, connect the freeway to the intersecting road. They allow vehicles entering the freeway to accelerate to the proper speed before merging with the freeway traffic; the ramps allow vehicles leaving the freeway to decelerate to the slower speeds of the crossing road.

As part of the FIHS, freeways are designated by red, white, and blue signs in the shape of shields. Freeways are also numbered, with the numbering system used to indicate the direction of the road. Freeways traveling in an east-west direction are given even numbers; those traveling north-south are given odd numbers.

Construction of a freeway

Planning

With the completion of the FIHS, few new freeways may be expected to be built in the United States. Existing freeways, however, will continue to be expanded and improved. In all cases, work on a freeway must be carefully planned, its route laid out, and its impact on the environment and surrounding area thoroughly investigated. Engineers design the freeways, following government specifications. In addition, geographical and geological features are examined, including the grade, or slope of the land, and the type of soil found along different sections of the proposed roadway. The type of soil will affect the nature of the pavement to be laid, so soil samples are analyzed both in the field and in the laboratory.

Many questions must be answered when designing a freeway. The expected volume of traffic must be estimated, with minimum and maximum levels established. The expected use of the freeway is another consideration, and takes into account where people live and work and how they currently travel; and, also, the location and type of industry in the area, the types of goods that are produced, the markets or destinations of those goods, and how those goods have been transported in the past. These questions will affect the volume of traffic on the proposed freeway; they will also affect the type of vehicles that will use it. A freeway that will serve heavy trucks will require different surfacing, lane widths, and bridge heights than freeways serving mostly or only automobiles.

Clearing, grading, and drainage system

Work begins by clearing the right-of-way, the path, of the freeway. Vegetation will be removed, and the course for the freeway will be laid out. The use of modern construction equipment, including bulldozers and other specifically designed heavy equipment, has made this process much easier and faster than in the past. At this time, hills and valleys along the freeway route may be smoothed out, to minimize the variability of the route.

At the same time, features of the water drainage systeman important part of any roadwayare formed. These include the slope of the road, and ditches and culverts alongside of the road. The drainage may be the single most costly part of constructing a freeway; yet, if the water is not properly guided away from the road, the road will quickly weaken. The cleared right-of-way, including the shoulders and drainage ditches, will next be compacted in order to provide a firm underbed for the freeway. Any bridges to be placed along the freeway will then be constructed, before the freeway itself is paved.

Paving

Paving a freeway may actually take place in several phases, adding layer upon layer of road surface over a long period of time, even years, until the freeway has achieved its final form. This allows weaknesses, and the effects of settling, in the roadway and drainage system to be detected and corrected.

Roads, including freeways, are generally composed of three layers: the subbed, or subgrade; the bed, or base; and the surface, or pavement or wearing course. The subbed is the soil on which the freeway is built. It is prepared by leveling and compacting the soil, and may be treated with asphalt, tar, or other substances to provide greater firmness. Next, the base is laid, consisting of crushed stone, gravel, or concrete pieces in a variety of sizes ranging from dust to 3-in (8-cm) rocks mixed in exact proportions. This allows the base to remain porous, so that moisture will not build up beneath the pavement. This course is also compacted, then sprayed with a thin, liquid layer of tar or asphalt to fill in the gaps and spaces between stones and make this surface even.

The pavement is then laid on top of the base. A layer of tar or asphalt is added and then covered with gravel or stones, which are all the same size. The gravel layer is compacted into the asphalt so that they are firmly mixed together. This process, which forms the pavement, may be repeated several times, until the road surface reaches the proper thickness. Each layer is rolled with special machines until it is hard and smooth. Sudden bumps or dips in the road will make the freeway more dangerous to drive on, especially with the high speeds allowed on the freeway. The thickness of the road surface will depend on the type of traffic expected, that is, whether it is expected to be high volume, or whether it is expected to carry many heavy trucks as well as automobiles. The pavement must be watertight, because moisture can destroy the surface as it expands or contracts with temperature changes. The addition of stones or gravel in the blacktop, or surface layer, allows tires to grip the surface more easily.

Safety features

Keeping the driver alert to the road is important for preventing accidents. Lighting by overhead lamps allows the driver to see the road ahead at night, even at great distances. Guardrails may be placed alongside

KEY TERMS

Asphalt A substance composed primarily of hydrocarbons found in nature or produced as a by-product in petroleum refining; also refers to a road surface.

Grade A slope or inclination in a road.

Grade separation A crossing over or under a highway.

Interchange An intersection of two or more highways that allows the flow of traffic to occur without stopping or crossing the other traffic streams.

Ramp A section of roadway raising or lowering traffic from one level to second level used to allow the entrance or exiting of traffic to or from a freeway.

Right-of-way The width and length of land through which all structures of a freeway pass.

Tar A viscous liquid obtained by burning substances such as wood and coal that is used in the surfacing of roads and other structures requiring a waterproof seal.

the roadway at curves and where the land drops away suddenly beyond the shoulder. Reflectors are often placed on guardrails alongside the roadway and in the lines between lanes. Landscaping along the road and in the median helps to reduce the monotony of a long drive.

Speed limits

Freeways have higher speed limits than other local roads. Higher speeds are normally allowed in the sparsely populated western states while lower speeds are found in the more populated eastern states. Limits range from 65 to 80 mph (100 to 130 km/h) in rural areas, while urban areas generally have rates from 50 to 65 mph (80 to 100 km/h).

Resources

BOOKS

Baxter, John M. Great Highways of the World. Rand McNally, 1995.

Borth, Christy. Mankind on the Move: The Story of Highways. Automotive Safety Federation, 1969.

Davies, Richard O. The Age of Asphalt. J. B. Lippincott Company, 1975.

Kilareski, Walter P. Principles of Highway Engineering and Traffic Analysis. John Wiley δ Sons, 1990.

McNichol, Dan. The Roads that Built America: The Incredible Story of the U.S. Interstate System. New York: Sterling Publishing, 2006.

Williams, Owen. How Roads Are Made. Facts on File Publications, 1989.

M. L. Cohen

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