The modern road

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. 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.

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 rod), by 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. 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 figure), the 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. The base course provides the required supplement to the strength, stiffness, and durability of the natural formation. Its thickness ranges from 4 inches (10 cm) for very light traffic and a good natural formation to more than 40 inches (100 cm) 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 rigid, according 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 1.5 inches in size and often less than 1 inch. Initially the bitumen must be heated to temperatures of 300–400 °F (150–200 °C) in order to make it fluid enough to mix with the stone. 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.

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. 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.

In places where the local natural material is substandard for use as a base course, it can be “stabilized” with relatively small quantities of lime, portland cement, pozzolana, or bitumen. The strength and stiffness of the mix are increased by the surface reactivity of the additive, which also reduces the material’s permeability and hence its susceptibility to water. 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.

The properties of the base course material are usually determined by laboratory tests, although field tests are sometimes conducted to check that the construction process has achieved the designer’s intent. 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 watercourses, removing water from the pavement surface, and controlling underground water in the pavement structure. In designing the system, the engineer first selects the “design storm”—that is, the most severe flood that can be expected in a nominated period of time (as much as 100 years for a major road or as little as 5 years for a minor street carrying local traffic). 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. Underground drains can also be used to lower the groundwater level by both preventing water entry and removing water that does enter the pavement structure.

Traffic control

The marking of roadway surfaces with painted lines and raised permanent markers is commonplace and effective, despite high maintenance costs and visibility problems at night, in heavy traffic, and in rain or snow. A solid line is a warning or instruction not to cross, and a broken line is for guidance. Thus, solid lines indicate dangerous conditions (such as restricted sight distance where overtaking would be dangerous), pavement edges, stop lines, and turning lanes at intersections; broken lines indicate interior lane lines and centre lines on two-way roads where the sight distance is good. Lines are usually white, but yellow is used for centre lines in North America.

Signs advise the driver of special regulations and provide information about hazards and navigation. They are classified as regulatory signs, which provide notice of traffic laws and regulations (e.g., signs for speed limits and for stop, yield or give-way, and no entry); warning signs, which call attention to hazardous conditions (e.g., sharp curves, steep grades, low vertical clearances, and slippery surfaces); and guide signs, which give route information (e.g., numbers or designations, distances, directions, and points of interest).

Signs have standard shapes and colours—for instance, the red octagon used for the stop sign, the triangle for warning signs, the green rectangle with white lettering for freeway directional signs (commonly mounted over the roadway and of large size for easy reading at high speeds). Tourist signs are brown rectangles, and special shapes and colours are used for route markers. Many signs, such as the stop sign, are universally used, but there are some differences between the two common international systems based on either the American or the European practice. Basically, these differences are derived from a complete reliance on symbolic signs and a greater range of blue guidance signs in multilingual Europe.

Traffic signals are primarily used to control traffic in urban street systems—particularly at conventional intersections accommodating large traffic volumes, where they allocate right-of-way to the various traffic streams. They can also meter traffic entering access lanes onto busy freeways or to indicate the lanes to use on two-way roads. Simple traffic signals work on preset timing plans that vary with the time of day. More advanced traffic-actuated signals automatically monitor the traffic streams and allocate right-of-way accordingly. Signals can also be linked to a computer so that traffic traveling along a major route can receive a continuous wave of green signals, obtaining maximum traffic output from the system.

Legal control

Legal rules governing the movement of traffic are an essential part of order on the road. The rules may be divided into three categories. First are those applying to the vehicle and the driver, such as vehicle and driver registration, vehicle safety equipment and roadworthiness, accident reporting, financial liability, and truck weights and axle loads (to protect pavements and bridges from damage). Second are the movement rules for drivers and pedestrians, known as the rules of the road; these dictate which side of the road to use, maximum speeds, right-of-way, and turning requirements. Third are those regulations that apply to limited road sections, indicating speed limits, one-way operations, and turning controls.

The important rules of the road are reasonably uniform throughout the world. For instance, in most countries drivers must give right-of-way to vehicles on their right. However, in practice the stop and yield (or give-way) signs have commonly supplanted the right-of-way rule. Speed limits vary greatly with jurisdiction, ranging from walking pace in a Dutch woonerf, or “shared” street, to unrestricted on a German autobahn. Speed limits are commonly reduced on roads approaching residential, shopping, or school areas and on dangerous road sections and sharp curves.

Special regulations are important for the efficient movement of traffic in specific segments of a street and road system. For instance, one-way streets in congested urban areas may provide safer driving conditions and increase the traffic-carrying capacity of the system. The provision of special turn arrows in traffic signals or the prohibition of turns at intersections contribute to safety, increase traffic throughput, and reduce conflict.