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The new generation of drivers are a key symbol of the 20th century

The modern road Road engineering Since the beginning of the 20th century, as the automobile and truck have offered ever higher levels of mobility, vehicle ownership per head of population has increased. Road needs have been strongly influenced by this popularity and also by the mass movement of people to cities and thence to suburban fringes—a trend that has led to increasing travel needs and road congestion and to low-density cities, which are difficult to service by public transport.

Often the building of new roads to alleviate such problems has encouraged further urban sprawl and yet more road travel. Long-term solutions require the provision of alternatives to car and truck transport, controls over land use, and the proper pricing of road travel. To this end, road managers must be concerned not merely with lines on maps but also with the number, type, speed, and loading of individual vehicles, the safety, comfort, and convenience of the traveling public, and the health and welfare of bystanders and adjoining property owners.

Ideally, the development of a major road system is an orderly, continuous process. The process follows several steps: Planning Road needs are closely associated with the relative location of centres of population, commerce, industry, and transportation.

Traffic between two centres is approximately proportional to their populations and inversely proportional to the distance between them. Estimating traffic on a route thus requires a prediction of future population growth and economic activity, an estimation of their effects on land use and travel needs, and a knowledge of any potential transport alternatives.

Once the traffic demand has been estimated, it is necessary to predict the extent of the road works needed to handle that traffic. A starting point in these calculations is offered by surveys of the origins, destinations, and route choices of present traffic; computer models are then used to estimate future traffic volumes on each proposed route. Estimates of route choice are based on the understanding that most drivers select their estimate of the quickest, shortest, or cheapest route.

Consideration in planning is also given to the effect of new traffic on existing streets, roads, and parking provisions. Where feasiblethe next step in planning a road system is to refine the selected route to a narrow corridor.

The various alignment options are drawn, considering the local terrain and conditions. The economic, social, and environmental benefits and costs of these options are discussed with relevant official and community groups until an acceptable specific route is determined.

Road design Alignment and profile After a route has been selected, a three-dimensional road alignment and its associated cross-sectional profiles are produced. In order to reduce the amount of earth to be moved, the alignment is adjusted where practical so that the earth to be excavated is in balance with the embankments to be built.

Computers allow many options to be explored and realistic views of the future road to be examined. In order to fully understand the design stage, a few standard terms must be defined see figure. A traffic lane is the portion of pavement allocated to a single line of vehicles; it is indicated on the pavement by painted longitudinal lines or embedded markers.

The shoulder is a strip of pavement outside an outer lane; it is provided for emergency use by traffic and to protect the pavement edges from traffic damage.

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A set of adjoining lanes and shoulders is called a roadway or carriageway, while the pavement, shoulders, and bordering roadside up to adjacent property lines are known as the right-of-way. Schematic cross section of a modern roadway. In order to maintain quality and uniformity, design standards are established for each functional road type. The number of traffic lanes is directly determined by the combination of traffic volume and speed, since practical limits on vehicle spacing means that there is a maximum number of vehicles per hour that pass through a traffic lane.

The width of lanes and shoulders, which must strike a balance between construction cost and driver comfort, allows the carriageway width to be determined. Standards also specify roadside barriers or give the clear transverse distances needed on either side of the carriageway in order to provide safety in the event that vehicles accidentally leave the carriageway. Thus it is possible to define the total right-of-way width needed for the entire road, although intersections will add further special demands.

Design standards also help to determine the actual alignment of the road by specifying, for each design speed, the minimum radius of horizontal curves, the maximum vertical gradient, the clearance under bridges, and the distance a driver must be able to see the pavement ahead in order to stop or turn aside. Pavement Road traffic is carried by the pavement, which in engineering terms is a horizontal structure supported by in situ natural material.

In order to design this structure, existing records must be examined and subsurface explorations conducted. The engineering properties of the local rock and soil are established, particularly with respect to strength, stiffness, durability, susceptibility to moisture, and propensity to shrink and swell over time.

The relevant properties are determined either by field tests typically by measuring deflection under a loaded plate or the penetration of a rodby empirical estimates based on the soil type, or by laboratory measurements.

The material is tested in its weakest expected condition, usually at its highest probable moisture content.

  • The number of traffic lanes is directly determined by the combination of traffic volume and speed, since practical limits on vehicle spacing means that there is a maximum number of vehicles per hour that pass through a traffic lane;
  • The engineering properties of the local rock and soil are established, particularly with respect to strength, stiffness, durability, susceptibility to moisture, and propensity to shrink and swell over time.

Probable performance under traffic is then determined. Soils unsuitable for the final pavement are identified for removal, suitable replacement materials are earmarked, the maximum slopes of embankments and cuttings are established, the degree of compaction to be achieved during construction is determined, and drainage needs are specified.

In a typical rural pavement as shown in the figurethe top layer of the pavement is the wearing course. Made of compacted stone, asphalt, or concrete, the wearing course directly supports the vehicle, provides a surface of sufficient smoothness and traction, and protects the base course and natural formation from excessive amounts of water.

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The base course provides the required supplement to the strength, stiffness, and durability of the natural formation. Its thickness ranges from 4 inches 10 centimetres for very light traffic and a good natural formation to more than 40 inches 100 centimetres for heavy traffic and a poor natural formation.

The subbase is a protective layer and temporary working platform sometimes placed between the base course and the natural formation. Pavements are called either flexible or rigidaccording to their relative flexural stiffness.

Flexible pavements see figure, left have base courses of broken stone pieces either compacted into place in the style of McAdam or glued together with bitumen to form asphalt. In order to maintain workability, the stones are usually less than 1. At the road site a paving machine places the hot mix in layers about twice the thickness of the stone size. The layers are then thoroughly rolled before the mix cools and solidifies. In order to avoid the expense of heating, increasing use has been made of bitumen emulsions or cutbacks, in which the bitumen binder is either treated with an emulsifier or thinned with a lighter petroleum fraction that evaporates after rolling.

These treatments allow asphalts to be mixed and placed at ambient temperatures. Cross sections of modern pavements Left Flexible asphalt-based pavement. Right Rigid portland-cement concrete pavement. The surface course of a flexible pavement protects the underlying base course from traffic and water while also providing adequate tire friction, generating minimal noise in urban areas, and giving suitable light reflectance for night-time driving.

Such surfaces are provided either by a bituminous film coated with stone called a spray-and-chip seal or by a thin asphalt layer. The spray-and-chip seal is used over McAdam-style base courses for light to moderate traffic volumes or to rehabilitate existing asphalt surfaces.

It is relatively cheap, effective, and impermeable and lasts about 10 years. Its main disadvantage is its high noise generation. Maintenance usually involves further spray coating with a surface dressing of bitumen.

Asphalt surfacing is used with higher traffic volumes or in urban areas. Surfacing asphalt commonly contains smaller and more wear-resistant stones than the base course and employs relatively more bitumen.

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It is better able to resist horizontal forces and produces less noise than a spray-and-chip seal. Rigid pavements see figure, right are made of portland cement concrete. The concrete slab ranges in thickness from 6 to 14 inches. It is laid by a paving machine, often on a supporting layer that prevents the pressure caused by traffic from pumping water and natural formation material to the surface through joints and cracks.

Concrete shrinks as it hardens, and this shrinkage is resisted by friction from the underlying layer, causing cracks to appear in the concrete. Cracking is usually controlled by adding steel reinforcement in order to enhance the tensile strength of the pavement and ensure that any cracking is fine and uniformly distributed. Transverse joints are sometimes also used for this purpose. Longitudinal joints are used at the edge of the construction run when the whole carriageway cannot be cast in one pass of the paving machine.

Special machines distribute the stabilizer into the upper 8 to 20 inches of soil. In deciding whether to use a flexible, rigid, or stabilized pavement, engineers take into account lifetime cost, riding characteristics, traffic disruptions due to maintenance, ease and cost of repair, and the effect of climatic conditions. Often there is little to choose between rigid and flexible pavements. Designers typically consider the possibility of structural failure resulting from a single overload and also from damage accumulating under the passage of many routine loads.

Both of these types of failure are almost entirely caused by trucks. Drainage Adequate drainage is the single most important element in pavement performance, and drainage systems can be extensive and expensive. Drainage involves handling existing watercoursesremoving water from the pavement surface, and controlling underground water in the pavement structure.

The drainage system must be able to carry the storm water produced by this design storm without flooding the roadway or adjacent property. In areas where land use is changing from agricultural to residential or commercial, peak flows will increase notably as the surrounding area is covered with roofs and paving.

Safety requires that water be rapidly removed from the pavement surface. In urban areas, the water runs into shallow gutters and thence into the inlets of underground drains.

In rural areas, surface water flows beyond the shoulders to longitudinal drainage ditches, which have flat side slopes to enable vehicles leaving the pavement to recover without serious incident. Cut-off surface drains are used to prevent water from flowing without restriction down the slopes of cuttings and embankments. Vertical drainage layers, formed from single-sized aggregate or special sheets called geofabrics and geomembranes, are used to prevent groundwater from seeping laterally into the pavement structure.

In addition, a horizontal drainage layer is often inserted between base course and natural ground in order to remove water from the pavement structure and stop upward capillary movement of any natural groundwater. Financing The full design of a proposed road is analyzed with respect to its costs and its economic, social, and environmental effects.

It may also be subjected to public review. This step can be lengthy, as new roads are usually popular with the traveling public but sometimes cause distress in the communities through which they pass. Local streets and collector roads are usually administered by local governments and financed by local taxes. Arterial roads and highways, however, need a wider administrative and financial input in order to guarantee route continuity and uniformity.

Since the 1920s the financing of roads has been largely transferred to the road user. A variety of taxes is employed: Fuel taxes usually provide the simplest source of revenue, but they are not necessarily intended solely for expenditure on roads.

Many local roads are funded by property taxes. Construction After the road has been approved and financing found, surveyors define its three-dimensional location on the ground. Forming of the in-situ material to its required shape and installation of the underground drainage system can then begin.

Imported pavement material is placed on the natural formation and may have water added; rollers are then used to compact the material to the required density. If possible, some traffic is permitted to operate over the completed earthwork in order to detect weak spots.

In countries where labour is inexpensive and less skilled, traditional manual methods of road construction are still commonplace.

  1. Simple traffic signals work on preset timing plans that vary with the time of day.
  2. Standards also specify roadside barriers or give the clear transverse distances needed on either side of the carriageway in order to provide safety in the event that vehicles accidentally leave the carriageway. The various alignment options are drawn, considering the local terrain and conditions.
  3. Drainage involves handling existing watercourses , removing water from the pavement surface, and controlling underground water in the pavement structure.
  4. The surface is then sprayed with a more viscous hot bitumen, which is immediately covered with a layer of uniform-size stone chips spread from a dump truck. Estimating traffic on a route thus requires a prediction of future population growth and economic activity, an estimation of their effects on land use and travel needs, and a knowledge of any potential transport alternatives.

However, the developed world relies heavily on purpose-built construction plant. This can be divided into equipment for six major construction purposes: