Freeway

Freeways, also called superhighways, 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.

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 system—an important part of any roadway—are 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, so as 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, then covered with gravel or stones that 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.