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: assessing road
needs and transport options; planning a system to meet those needs; designing
an economically, socially, and environmentally acceptable set of roads;
obtaining the required approval and financing; building, operating, and
maintaining the system; and providing for future extensions and reconstruction.
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. The key variables defining
road needs are the traffic volumes, tonnages, and speeds to be expected
throughout the road’s life.
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 feasible, the 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. 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.
Schematic cross section of a modern roadway.
Encyclopædia Britannica, Inc.
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 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 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.
Cross sections of modern pavements(Left) Flexible
asphalt-based pavement. (Right) Rigid portland-cement concrete pavement.
Cross sections of modern pavements(Left) Flexible
asphalt-based pavement. (Right) Rigid portland-cement concrete pavement.
Encyclopædia Britannica, Inc.
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.
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: on
fuel and oil, on road usage, on vehicle purchase and ownership, on driver
licensing, on truck mass and mass times distance traveled, on tire and
accessory purchases, and on the economic benefits provided by roads (e.g.,
higher property values or increased productivity). 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. However,
the developed world relies heavily on purpose-built construction plant. This
can be divided into equipment for six major construction purposes: clearing,
earthmoving, shaping, and compacting the natural formation; installing
underground drainage; producing and handling the road-making aggregate;
manufacturing asphalt and concrete; placing and compacting the pavement layers;
and constructing bridges and culverts.
For clearing vegetation and undesirable materials from the
roadway, the bulldozer is often employed. The construction of rock cuts is
commonly done with shovels, draglines, and mobile drills. Shaping the formation
and moving earth from cuttings to embankments is accomplished with bulldozers,
graders, hauling scrapers, elevating graders, loaders, and large dump trucks.
The material is placed in layers, brought to the proper moisture content, and
compacted to the required density. Compaction is accomplished with tamping,
sheeps-foot, grid, steel-wheeled, vibrating, and pneumatic-tired rollers.
Backhoes, back actors, and trenchers are used for drainage work.
In order to avoid high haulage costs, the materials used for
base course construction are preferably located near the construction site; it
is economically impossible to use expensive materials for long lengths of road
construction. The excavation process is the same as for rock cuts, although
rippers may be used for obtaining lower-grade material. Crushers, screens, and
washers produce stone of the right size, shape, and cleanliness.
The placement of paving material increasingly involves a
paving machine for distributing the aggregate, asphalt, or concrete uniformly
and to the required thickness, shape, and width (typically, one or two traffic
lanes). The paving machine can slipform the edges of the course, thus avoiding
the need for fixed side-forms. As it progresses down the road, it applies some
preliminary compaction and also screeds and finishes the pavement surface. In
modern machines, level control is by laser sighting.
In producing a spray-and-chip seal surface (or a bituminous
surface treatment), a porous existing surface is covered with a film of hot, fluid
bitumen that is sprayed in sufficient quantity to fill voids, cracks, and
crevices without leaving excess bitumen on the surface. 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. The roadway is then
rolled to seat the stone in the sticky bitumen, and excess stone is later
cleared by a rotary broom.
Maintenance
The life of a road structure depends on the quality of its
maintenance and minor renovation. Maintenance keeps the roadway safe, provides
good driving conditions, and prolongs the life of the pavement, thus protecting
the road investment. Maintenance consists of activities concerned with the
condition of the pavement, shoulders, drainage, traffic facilities, and
right-of-way. It includes the prompt sealing of cracks and filling of potholes
to prevent water entering through the surface, the removal of trash thrown on
the wayside by the traveling public, and the care of pavement markings, signs,
and signals. In rigorous winter climates, substantial effort is required to
remove snow and ice from the pavement, to scatter salt for snow and ice
removal, and to spread sand for better traction.
Road operation
Traffic management
Road users are subject to traffic control via instructions
and information provided by roadway markings, signs, and signals, and they are
subject to legal control via the rules of the road (particularly those
concerned with vehicular priority).
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.
Safety
Traffic police (or road patrols or highway police) help
improve road safety and traffic flow by enforcing driving regulations. They
also regulate traffic at the scene of an accident and investigate accidents.
Traffic enforcement has been aided by the use of technology—cameras, radar,
video, and inductance loops—to detect and record traffic offenders
automatically.
An important aspect of traffic regulation and accident
prevention is the control of excessive speed, which contributes significantly
to the number and severity of road crashes. Speed is commonly measured by radar
devices or by pacing with a patrol car. In crash investigations, the speed of
the cars is determined by the length of skid marks. Another key factor in road
accidents is the influence of alcohol and drugs. Tests for intoxication are now
widely conducted; the most common is the breath test, in which the driver blows
into a device that analyzes the alcohol content of the breath and indicates the
approximate blood alcohol level. Many authorities believe that 0.50 gram of
alcohol per litre of blood is a realistic limit for ordinary motorists, but
that zero levels should be demanded for critical operators such as drivers of
public transport vehicles.
Road safety can also be built into the road. Divided roads
are many times safer than two-way roads. Crash severity can be reduced by the
use of “soft” signs and light poles and by guardrails and impact attenuators in
front of fixed roadside objects such as bridge piers and the noses at the exit
ramps of a freeway. Better road surfaces, alignments, signing, and marking
improve driving conditions and increase road safety.
Nevertheless, about 90 percent of crashes are primarily due
to human error. Many crashes have been attributed to simple inattention or
failure to see warnings. Alcohol, fatigue, inexperience, aggression, and
excessive risk taking are the most common crash causes involving behavioral
changes in drivers. Lack of driving skills is rarely an issue; most drivers do
not need training as much as they need education and experience. Meanwhile,
road engineers must design road systems that attempt to reduce the frequency
and impact of human error.
Fred J. Benson Maxwell Gordon Lay
https://www.britannica.com
