A road is a thoroughfare, route, or way on land between two places
that has been paved or otherwise improved to allow travel by foot or
some form of conveyance, including a motor vehicle, cart, bicycle, or
Roads consist of one or two roadways (British English: carriageways),
each with one or more lanes and any associated sidewalks (British
English: pavement) and road verges. In countries like the Netherlands
Denmark there is often a bike path provided for cycling.
Roads available for use by the public may be referred to as parkways,
avenues, freeways, interstates, highways, or primary, secondary, and
tertiary local roads.
1.1 United Kingdom
1.2 United States
6.1 Slab stabilization
6.3 Joint sealing
7 Safety considerations
8 Environmental performance
9.1 Right- and left-hand traffic
12 Global connectivity
13 See also
Organisation for Economic Co-operation and Development
Organisation for Economic Co-operation and Development (OECD)
defines a road as "a line of communication (travelled way) using a
stabilized base other than rails or air strips open to public traffic,
primarily for the use of road motor vehicles running on their own
wheels, "which includes" bridges, tunnels, supporting structures,
junctions, crossings, interchanges, and toll roads, but not cycle
In urban areas roads may diverge through a city or village and be
named as streets, serving a dual function as urban space easement and
route. Modern roads are normally smoothed, paved, or otherwise
prepared to allow easy travel. Historically many roads were simply
recognizable routes without any formal construction or maintenance.
In the United Kingdom there is some ambiguity between the terms
highway and road. The
Highway code details rules for "road users".
For the purposes of the English law, Highways Act 1980, which covers
England and Wales
England and Wales but not
Scotland or Northern Ireland, the term road
is defined to be "any length of highway or of any other road to which
the public has access, and includes bridges over which a road
passes." This includes footpaths, bridleways and cycle tracks, and
also road and driveways on private land and many car parks. Vehicle
Excise Duty, a road use tax, is payable on some vehicles used on the
The definition of a road depends on the definition of a highway; there
is no formal definition for a highway in the relevant Act. A 1984
ruling said "the land over which a public right of way exists is known
as a highway; and although most highways have been made up into roads,
and most easements of way exist over footpaths, the presence or
absence of a made road has nothing to do with the distinction.
Another legal view is that while a highway historically included
footpaths, bridleways, driftways, etc., it can now be used to mean
those ways that allow the movement of motor-vehicles, and the term
rights of way can be used to cover the wider usage.
In the United States, laws distinguish between public roads, which are
open to public use, and private roads, which are privately
Main article: History of road transport
Transfăgărășan, named "the best road in the world"
The Porta Rosa, a Greek street dating from the 3rd to 4th century BC
in Velia, with a paved surface and gutters
Roman road in Pompeii
Old tractor road over farmland, Ystad, Sweden
The assertion that the first pathways were the trails made by animals
has not been universally accepted; in many cases animals do not follow
constant paths. Others believe that some roads originated from
following animal trails. The
Icknield Way is given as an
example of this type of road origination, where man and animal both
selected the same natural line. By about 10,000 BC, rough
roads/pathways were used by human travelers.
The world's oldest known paved road was constructed in Egypt some time
between 2600 and 2200 BC.
Stone-paved streets are found in the city of Ur in the Middle East
dating back to 4000 BC.
Corduroy roads (log roads) are found dating to 4000 BC in Glastonbury,
The Sweet Track, a timber track causeway in England, is one of the
oldest engineered roads discovered and the oldest timber trackway
discovered in Northern Europe. Built in winter 3807 BC or spring 3806
BC, tree-ring dating (Dendrochronology) enabled very precise dating.
It was claimed to be the oldest road in the world until the
2009 discovery of a 6,000-year-old trackway in Plumstead,
Brick-paved streets were used in
India as early as 3000 BC .
In 500 BC,
Darius I the Great
Darius I the Great started an extensive road system for the
Achaemenid Empire (Persia), including the Royal Road, which was one of
the finest highways of its time, connecting
westernmost major city of the empire) to Susa. The road remained in
use after Roman times. The easternmost destinations of these road
systems were in
Bactria and India.
In ancient times, transport by river was far easier and faster than
transport by road, especially considering the cost of road
construction and the difference in carrying capacity between carts and
river barges. A hybrid of road transport and ship transport beginning
in about 1740 is the horse-drawn boat in which the horse follows a
cleared path along the river bank.
From about 312 BC, the
Roman Empire built straight strong stone
Roman roads throughout Europe and North Africa, in support of its
military campaigns. At its peak the
Roman Empire was connected by 29
major roads moving out from Rome and covering 78,000 kilometers or
52,964 Roman miles of paved roads.
In the 8th century AD, many roads were built throughout the Arab
Empire. The most sophisticated roads were those in Baghdad, which were
paved with tar.
Tar was derived from petroleum, accessed from oil
fields in the region, through the chemical process of destructive
Highways Act 1555 in Britain transferred responsibility for
maintaining roads from government to local parishes. This resulted
in a poor and variable state of roads. To remedy this, the first of
the "Turnpike trusts" was established around 1706, to build good roads
and collect tolls from passing vehicles. Eventually there were
approximately 1,100 trusts in Britain and some 36,800 km (22,870
miles) of engineered roads. The
Rebecca Riots in Carmarthenshire
Rhayader from 1839 to 1844 contributed to a
Royal Commission that
led to the demise of the system in 1844, which coincided with the
development of the UK railway system.
The subject of road design is considered part of highway engineering.
Structural road design is the science of designing a road for its
environment in order to extend its longevity and reduce maintenance.
Shell pavement design method is used in many countries for the
design of new asphalt roadsides.
the route of the road, defined as a series of horizontal tangents and
where a road slopes towards the outside of a bend, increasing the
likelihood that vehicles travelling at speed will skid or topple.
Usually only a temporary situation during road maintenance.
Unpaved road that is constructed of a material that does not create
mud during rainfall.
A motorway open to all traffic but optimized for cycling
cycling-friendly infrastructure integrated into the roadway or in its
own right of way
Camber (or crown)
the slope of the road surface downwards away from the centre of the
road, so that surface water can flow freely to the edge of the
carriageway, or on bends angling of the surface to lean traffic 'into
the bend' reducing the chance of a skid.
The slope of the pavement, expressed as units of rise per unit of run,
or as a percentage.
an orange globe, lit at night, used to highlight a pedestrian
A beach road (Newcastle NSW Australia)
Rigid posts that can be arranged in a line to close a road or path to
vehicles above a certain width
Highway over which the public have a right to travel for vehicular and
other kinds of traffic, but is used mainly as a footpath or bridleway
Road that avoids or "bypasses" a built-up area, town, or village
Section of a road with a carrying capacity substantially below that of
other sections of the same road
Non-reflective raised pavement marker used on roads
reflective raised pavement marker used on roads
Sequence of tight serpentine curves (usually an S-shape curve or a bus
Road surface composed of a thin layer of crushed stone 'chips' and
asphalt emulsion. It seals the surface and protects it from weather,
but provides no structural strength. It is cheaper than asphalt
concrete or concrete. In the United States it is usually only used on
low volume rural roads
Road on the side of a cliff or mountain, with the ground rising on one
side and falling away on the other
A raised edge at the side of the roadway.
(also kerb extension, bulb-out, nib, elephant ear, curb bulge and
Traffic calming measure, intended to slow the speed of
traffic and increase driver awareness, particularly in built-up and
a state road or county road that connects rural or agricultural areas
to market towns.
(literally "fork in the road") Type of intersection where a road
(UK) Unsurfaced road, may be so infrequently used that vegetation
colonises freely, hence 'green'. Many green lanes are ancient routes
that have existed for millennia.
Prevents vehicles from veering off the road into oncoming traffic,
crashing against solid objects or falling from a road. Also called a
guard rail or traffic barrier.
a drainage channel usually at the edge of the road or along a median.
Highway System (United States)
System of Interstate and Defense Highways
Layby (Pullout, pull-off)
A paved area beside a main road where cars can stop temporarily to let
another car pass.
the hazard of stone chippings that have come loose
On dual carriageway roads, including controlled-access highways,
divided highways and many limited-access roads, the central
reservation (British English), median (North American English), median
North American English
North American English and Australian English), neutral ground
[Louisiana English] or central nature strip (Australian English): Area
that separates opposing lanes of traffic
A relatively low level route through a range of mountains
One of a series of numbered markers placed along a road, often at
regular intervals, showing the distance to destinations.
Road built and maintained by a national authority.
The road regarded as a geoconstruction. In the UK the term is road
surface and the pavement is a pedestrian walkway alongside the road.
Designated point on a road where road marking or other means helps
pedestrians cross safely
(officially Pelicon crossing) (UK) a PEdestrian LIght CONtrolled
Highway owned and operated for profit by private industry
Road owned and maintained by a private individual, organization, or
company rather than by a government
the vertical alignment of a road, expressed as a series of grades,
connected by parabolic curves.
Protected Intersections for Bicycles
A much safer design with a corner refuge island, a setback crossing of
the pedestrians and cyclists, generally between 1.5–7 metres of
setback, a forward stop bar, which allows cyclists to stop for a
traffic light well ahead of motor traffic who must stop behind the
crosswalk. Separate signal staging or at least an advance green for
cyclists and pedestrians is used to give cyclists and pedestrians no
conflicts or a head start over traffic. The design makes a right turn
on red, and sometimes left on red depending on the geometry of the
intersection in question, possible in many cases, often without
Protected Bicycle Path
Cyclists ideally have a protected bike lane on separated by a concrete
median with splay kerbs if possible, and have a protected bike lane
width of at least 2 metres if possible (one way). In the Netherlands,
most one way cycle paths are at least 2.5 metres wide.
Place where anyone has a right to come without being excluded because
of economic or social conditions
U.S. road that connects rural and agricultural areas to market towns
Often assigned to identify a stretch of public roads – often
dependent on the type of road, with numbers differentiating between
interstates, motorways, arterial thoroughfares, etc.
Process to reduce the harm (deaths, injuries, and property damage)
that result from vehicle crashes on public roads
Part or all of the road is occupied for work or maintenance
Deviations from a true planar pavement surface, which affects vehicle
suspension deflection, dynamic loading, ride quality, surface drainage
and winter operations. Roughness have wavelengths ranging from
500 mm up to some 40 m. The upper limit may be as high as 350 m
when considering motion sickness aspects; motion sickness is generated
by motion with down to 0.1 Hz frequency; in an ambulance car
driving 35 m/s (126 km/h), waves with up to 350 m will
excite motion sickness.
a road junction where typically three or more roads are joined by a
circular section of road. Traffic 'on the roundabout' has priority
over traffic on approach roads, unless indicated otherwise. In
countries where traffic drives on the left the roundabout is travelled
in a clockwise direction. Also known as an island in parts of the UK.
Segregated Bicycle Path
Cyclists ideally have a protected bike lane on separated by a concrete
median with splay kerbs if possible, and have a protected bike lane
width of at least 2 metres if possible (one way). In the Netherlands,
most one way cycle paths are at least 2.5 metres wide.
Shoulder (also hard shoulder)
A clear, level area to the side of the roadway available for stopping
Road numbered by the state, falling below numbered national highways
(like U.S. Routes) in the hierarchy or a road maintained by the state,
including nationally numbered highways
Pedestrians, ridden or herded animals, vehicles, bicycles, and other
conveyances using any road for purposes of travel.
Deviations from a true planar pavement surface, which affects the
interaction between road and tire. Microtexture have wavelengths below
0.5 mm, Macrotexture below 50 mm and Megatexture below
Set of strategies used by urban planners and traffic engineers to slow
down or reduce motor vehicle traffic, thereby improving safety for
pedestrians and bicyclists and improving the environment for residents
(UK) a small raised area used to help define the traffic flow, which
may also act as a refuge for pedestrians crossing the carriageway or a
location for signs, barriers or lights – a synonym for roundabout in
some parts of the UK
Also known as a traffic signal, stop light, stop-and-go lights – a
signaling device at a road intersection, pedestrian crossing, or other
location that assigns right of way to different approaches to an
Zebra crossing (UK)
a pedestrian crossing marked by black and white stripes on the
Surveyor at work with a leveling instrument
Asphalt layer and Hamm road roller
"Roadbed" (from the 'related terms'
Bed (geology) & streambed), as
well as the term "
Road building" redirects here.
Sub-base layer composed of cement-based material being applied during
construction of the M8 motorway in Ireland
Road construction requires the creation of an engineered continuous
right-of-way or roadbed, overcoming geographic obstacles and having
grades low enough to permit vehicle or foot travel, (pg15) and may
be required to meet standards set by law or official
guidelines. The process is often begun with the removal of earth
and rock by digging or blasting, construction of embankments, bridges
and tunnels, and removal of vegetation (this may involve
deforestation) and followed by the laying of pavement material. A
variety of road building equipment is employed in road
After design, approval, planning, legal and environmental
considerations have been addressed alignment of the road is set out by
a surveyor. The radii and gradient are designed and staked out to
best suit the natural ground levels and minimize the amount of cut and
fill. (p. 34) Great care is taken to preserve reference
Benchmarks  (p. 59)
Roads are designed and built for primary use by vehicular and
pedestrian traffic. Storm drainage and environmental considerations
are a major concern.
Erosion and sediment controls are constructed to
prevent detrimental effects.
Drainage lines are laid with sealed
joints in the road easement with runoff coefficients and
characteristics adequate for the land zoning and storm water system.
Drainage systems must be capable of carrying the ultimate design flow
from the upstream catchment with approval for the outfall from the
appropriate authority to a watercourse, creek, river or the sea for
drainage discharge. (pp. 38–40)
A borrow pit (source for obtaining fill, gravel, and rock) and a water
source should be located near or in reasonable distance to the road
construction site. Approval from local authorities may be required to
draw water or for working (crushing and screening) of materials for
construction needs. The top soil and vegetation is removed from the
borrow pit and stockpiled for subsequent rehabilitation of the
extraction area. Side slopes in the excavation area not steeper than
one vertical to two horizontal for safety reasons.
(pp. 53–56 )
Old road surfaces, fences, and buildings may need to be removed before
construction can begin. Trees in the road construction area may be
marked for retention. These protected trees should not have the
topsoil within the area of the tree's drip line removed and the area
should be kept clear of construction material and equipment.
Compensation or replacement may be required if a protected tree is
damaged. Much of the vegetation may be mulched and put aside for use
during reinstatement. The topsoil is usually stripped and stockpiled
nearby for rehabilitation of newly constructed embankments along the
road. Stumps and roots are removed and holes filled as required before
the earthwork begins. Final rehabilitation after road construction is
completed will include seeding, planting, watering and other
activities to reinstate the area to be consistent with the untouched
surrounding areas. (pp. 66–67 )
Processes during earthwork include excavation, removal of material to
spoil, filling, compacting, construction and trimming. If rock or
other unsuitable material is discovered it is removed, moisture
content is managed and replaced with standard fill compacted to meet
the design requirements (generally 90-95% relative compaction).
Blasting is not frequently used to excavate the roadbed as the intact
rock structure forms an ideal road base. When a depression must be
filled to come up to the road grade the native bed is compacted after
the topsoil has been removed. The fill is made by the "compacted layer
method" where a layer of fill is spread then compacted to
specifications, under saturated conditions. The process is repeated
until the desired grade is reached. (pp. 68–69 ).
Typical pavement strata for a heavily traveled road
General fill material should be free of organics, meet minimum
California bearing ratio (CBR) results and have a low plasticity
index. The lower fill generally comprises sand or a sand-rich mixture
with fine gravel, which acts as an inhibitor to the growth of plants
or other vegetable matter. The compacted fill also serves as
lower-stratum drainage. Select second fill (sieved) should be composed
of gravel, decomposed rock or broken rock below a specified particle
size and be free of large lumps of clay.
Sand clay fill may also be
used. The roadbed must be "proof rolled" after each layer of fill is
compacted. If a roller passes over an area without creating visible
deformation or spring the section is deemed to comply.
(pp. 70–72 )
Geosynthetics such as geotextiles, geogrids and geocells are
frequently used in the various pavement layers to improve road
quality. These materials and methods are used in low-traffic private
roadways as well as public roads and highways. Geosynthetics
perform four main functions in roads: separation, reinforcement,
filtration and drainage; which increase the pavement performance,
reduce construction costs and decrease maintenance.[self-published
The completed road way is finished by paving or left with a gravel or
other natural surface. The type of road surface is dependent on
economic factors and expected usage. Safety improvements such as
traffic signs, crash barriers, raised pavement markers and other forms
of road surface marking are installed.
According to a May 2009 report by the American Association of State
Highway and Transportation Officials (AASHTO) and TRIP – a national
transportation research organization – driving on rough roads costs
the average American motorist approximately $400 a year in extra
vehicle operating costs. Drivers living in urban areas with
populations more than 250,000 are paying upwards of $750 more annually
because of accelerated vehicle deterioration, increased maintenance,
additional fuel consumption, and tire wear caused by poor road
When a single carriageway road is converted into dual carriageway by
building a second separate carriageway alongside the first, it is
usually referred to as duplication, twinning or doubling. The
original carriageway is changed from two-way to become one-way, while
the new carriageway is one-way in the opposite direction. In the same
way as converting railway lines from single track to double track, the
new carriageway is not always constructed directly alongside the
Roadworks and Road_surface § Surface_deterioration
Road works ahead" sign, typically used in Europe
Like all structures, roads deteriorate over time. Deterioration is
primarily due to accumulated damage from vehicles, however
environmental effects such as frost heaves, thermal cracking and
oxidation often contribute. According to a series of experiments
carried out in the late 1950s, called the AASHO
Road Test, it was
empirically determined that the effective damage done to the road is
roughly proportional to the
Fourth power of axle weight. A typical
tractor-trailer weighing 80,000 pounds (36.287 t) with 8,000 pounds
(3.629 t) on the steer axle and 36,000 pounds (16.329 t) on both of
the tandem axle groups is expected to do 7,800 times more damage than
a passenger vehicle with 2,000 pounds (0.907 t) on each axle. Potholes
on roads are caused by rain damage and vehicle braking or related
Manual road repair taking place in Howrah, India
Line marking in rural India
Pavements are designed for an expected service life or design life. In
some parts of the United Kingdom the standard design life is 40 years
for new bitumen and concrete pavement. Maintenance is considered in
the whole life cost of the road with service at 10, 20 and 30 year
milestones. Roads can be and are designed for a variety of lives
(8-, 15-, 30-, and 60-year designs). When pavement lasts longer than
its intended life, it may have been overbuilt, and the original costs
may have been too high. When a pavement fails before its intended
design life, the owner may have excessive repair and rehabilitation
costs. Some asphalt pavements are designed as perpetual pavements with
an expected structural life in excess of 50 years.
Many asphalt pavements built over 35 years ago, despite not being
specifically designed as a perpetual pavement, have remained in good
condition long past their design life. Many concrete pavements
built since the 1950s have significantly outlived their intended
design lives. Some roads like Chicago, Illinois's "Wacker Drive",
a major two-level viaduct in the downtown area, are being rebuilt with
a designed service life of 100 years.
Virtually all roads require some form of maintenance before they come
to the end of their service life. Pro-active agencies use pavement
management techniques to continually monitor road conditions and
schedule preventive maintenance treatments as needed to prolong the
lifespan of their roads. Technically advanced agencies monitor the
road network surface condition with sophisticated equipment such as
laser/inertial Profilometers. These measurements include road
curvature, cross slope, asperity, roughness, rutting and texture. This
data is fed into a pavement management system, which recommends the
best maintenance or construction treatment to correct the damage that
Maintenance treatments for asphalt concrete generally include thin
asphalt overlays, crack sealing, surface rejuvenating, fog sealing,
micro milling or diamond grinding and surface treatments. Thin
surfacing preserves, protects and improves the functional condition of
the road while reducing the need for routing maintenance, leading to
extended service life without increasing structural capacity.
Maintenance for the older concrete pavements that develop faults
includes the technique called dowel bar retrofit. This involves
cutting slots in the pavement at each joint, placing dowel bars in the
slots, then filling them with concrete patching material. This method
can extend the life of the concrete pavement for another 15 years.
Failure to maintain roads properly can create significant costs to
society, in a 2009 report released by the American Association of
Highway and Transportation Officials (US) about 50% of the roads
in the US are in bad condition with urban areas worse. The report
estimates that urban drivers pay an average of $746/year on vehicle
repairs while the average US motorist pays about $335/year. In
contrast, the average motorist pays about $171/year in road
maintenance taxes (based on 600 gallons/year and $0.285/gallon tax).
Distress and serviceability loss on concrete roads can be caused by
loss of support due to voids beneath the concrete pavement slabs. The
voids usually occur near cracks or joints due to surface water
infiltration. The most common causes of voids are pumping,
consolidation, subgrade failure and bridge approach failure. Slab
stabilization is a non-destructive method of solving this problem and
is usually employed with other
Concrete Pavement Restoration (CPR)
methods including patching and diamond grinding. The technique
restores support to concrete slabs by filing small voids that develop
underneath the concrete slab at joints, cracks or the pavement edge.
The process consists of pumping a cementitous grout or polyurethane
mixture through holes drilled through the slab. The grout can fill
small voids beneath the slab and/or sub-base. The grout also displaces
free water and helps keep water from saturating and weakening support
under the joints and slab edge after stabilization is complete. The
three steps for this method after finding the voids are locating and
drilling holes, grout injection and post-testing the stabilized slabs.
Slab stabilization does not correct depressions, increase the design
structural capacity, stop erosion or eliminate faulting. It does,
however, restore the slab support, therefore, decreasing deflections
under the load. Stabilization should only be performed at joints and
cracks where loss of support exists. Visual inspection is the simplest
manner to find voids. Signs that repair is needed are transverse joint
faulting, corner breaks and shoulder drop off and lines at or near
joints and cracks. Deflection testing is another common procedure
utilized to locate voids. It is recommended to do this testing at
night as during cooler temperatures, joints open, aggregate interlock
diminishes and load deflections are at their highest.
Ground penetrating radar pulses electromagnetic waves into the
pavement and measures and graphically displays the reflected signal.
This can reveal voids and other defects.
The epoxy/core test, detects voids by visual and mechanical methods.
It consists of drilling a 25 to 50 millimeter hole through the
pavement into the sub-base with a dry-bit roto-hammer. Next, a
two-part epoxy is poured into the hole – dyed for visual clarity.
Once the epoxy hardens, technicians drill through the hole. If a void
is present, the epoxy will stick to the core and provide physical
Common stabilization materials include pozzolan-cement grout and
polyurethane. The requirements for slab stabilization are strength and
the ability to flow into or expand to fill small voids. Colloidal
mixing equipment is necessary to use the pozzolan-cement grouts. The
contractor must place the grout using a positive-displacement
injection pump or a non-pulsing progressive cavity pump. A drill is
also necessary but it must produce a clean hole with no surface
spalling or breakouts. The injection devices must include a grout
packer capable of sealing the hole. The injection device must also
have a return hose or a fast-control reverse switch, in case workers
detect slab movement on the uplift gauge. The uplift beam helps to
monitor the slab deflection and has to have sensitive dial
Also called joint and crack repair, this method's purpose is to
minimize infiltration of surface water and incompressible material
into the joint system. Joint sealants are also used to reduce dowel
bar corrosion in
Concrete Pavement Restoration (CPR) techniques.
Successful resealing consists of old sealant removal, shaping and
cleaning the reservoir, installing the backer rod and installing the
sealant. Sawing, manual removal, plowing and cutting are methods used
to remove the old sealant. Saws are used to shape the reservoir. When
cleaning the reservoir, no dust, dirt or traces of old sealant should
remain. Thus, it is recommended to water wash, sand-blast and then air
blow to remove any sand, dirt or dust. The backer rod installation
requires a double-wheeled, steel roller to insert the rod to the
desired depth. After inserting the backer rod, the sealant is placed
into the joint. There are various materials to choose for this method
including hot pour bituminous liquid, silicone and preformed
Pedestrian crossing, line markings and street furniture.
Road traffic safety
Careful design and construction of roads can increase road traffic
safety and reduce the harm (deaths, injuries, and property damage) on
the highway system from traffic collisions.
On neighborhood roads traffic calming, safety barriers, pedestrian
crossings and cycle lanes can help protect pedestrians, cyclists, and
Lane markers in some countries and states are marked with Cat's eyes
or Botts dots, bright reflectors that do not fade like paint. Botts
dots are not used where it is icy in the winter, because frost and
snowplows can break the glue that holds them to the road, although
they can be embedded in short, shallow trenches carved in the roadway,
as is done in the mountainous regions of California.
For major roads risk can be reduced by providing limited access from
properties and local roads, grade separated junctions and median
dividers between opposite-direction traffic to reduce likelihood of
The placement of energy attenuation devices (e.g. guardrails, wide
grassy areas, sand barrels) is also common. Some road fixtures such as
road signs and fire hydrants are designed to collapse on impact. Light
poles are designed to break at the base rather than violently stop a
car that hits them.
Highway authorities may also remove larger trees
from the immediate vicinity of the road. During heavy rains, if the
elevation of the road surface isn't higher than the surrounding
landscape, it may result in flooding.
Main article: Environmental impacts of roads
Air pollution along Pasadena
Highway in Los Angeles
A dual carriageway section of National
Highway 8 connecting
Careful design and construction of a road can reduce any negative
Road after rain
Water management systems can be used to reduce the effect of
pollutants from roads.
Rainwater and snowmelt running off of
roads tends to pick up gasoline, motor oil, heavy metals, trash and
other pollutants and result in Water pollution.
Road runoff is a major
source of nickel, copper, zinc, cadmium, lead and polycyclic aromatic
hydrocarbons (PAHs), which are created as combustion byproducts of
gasoline and other fossil fuels.
De-icing chemicals and sand can run off into roadsides, contaminate
groundwater and pollute surface waters; and road salts can be
toxic to sensitive plants and animals.
Sand applied to icy roads
can be ground up by traffic into fine particulates and contribute to
Sand can alter stream bed environments, causing stress
for the plants and animals that live there.
Roads are a chief source of environmental noise generation. In the
early 1970s it was recognized that design of roads can be conducted to
influence and minimize noise generation. Noise barriers are used
to reduce noise pollution, in particular where roads are located close
to built-up areas. Regulations can restrict the use of engine braking.
Motor vehicle emissions contribute air pollution. Concentrations of
air pollutants and adverse respiratory health effects are greater near
the road than at some distance away from the road.
kicked up by vehicles may trigger allergic reactions. In addition,
on-road transportation greenhouse gas emissions are the largest single
cause of climate change, scientists say.
The A22(T) with line markings near Summer Hill, East Sussex, England
Road with guard rails in Kaluga Oblast, Russia
Road with traffic signs in the outskirts of Bern, Switzerland
NH 73 going Bangalore
Highway 401, a route with a collector / express setup
Coastal Road in the Philippines now called the Manila-Cavite
Road in Jujuy Province, Argentina
Right- and left-hand traffic
Main article: Right- and left-hand traffic
Traffic flows on the right or on the left side of the road depending
on the country. In countries where traffic flows on the right,
traffic signs are mostly on the right side of the road, roundabouts
and traffic circles go counter-clockwise/anti-clockwise, and
pedestrians crossing a two-way road should watch out for traffic from
the left first. In countries where traffic flows on the left, the
reverse is true.
About 33% of the world by population drive on the left, and 67% keep
right. By road distances, about 28% drive on the left, and 72% on the
right, even though originally most traffic drove on the left
Main article: Transport economics
A city street in Mumbai,
India with left-hand traffic
Transport economics is used to understand both the relationship
between the transport system and the wider economy and the complex
network effects when there are multiple paths and competing modes for
both personal and freight (road/rail/air/ferry) and where Induced
demand can result in increased on decreased transport levels when road
provision is increased by building new roads or decreased (for example
California State Route 480). Roads are generally built and maintained
by the public sector using taxation although implementation may be
through private contractors). or occasionally using road
Public-private partnerships are a way for communities to address the
rising cost by injecting private funds into the infrastructure. There
are four main ones:
Society depends heavily on efficient roads. In the
European Union (EU)
44% of all goods are moved by trucks over roads and 85% of all people
are transported by cars, buses or coaches on roads. The term was
also commonly used to refer to roadsteads, waterways that lent
themselves to use by shipping.
According to the New York State Thruway Authority, some sample
per-mile costs to construct multi-lane roads in several US
northeastern states were:
Connecticut Turnpike – $3,449,000 per mile
New Jersey Turnpike – $2,200,000 per mile
Pennsylvania Turnpike (Delaware Extension) – $1,970,000 per mile
Northern Indiana Toll
Road – $1,790,000 per mile
Parkway – $1,720,000 per mile
Massachusetts Turnpike – $1,600,000 per mile
Thruway, New York to Pennsylvania Line – $1,547,000 per mile
Ohio Turnpike – $1,352,000 per mile
Pennsylvania Turnpike (early construction) – $736,000 per mile
The United States has the largest network of roads of any country with
4,050,717 miles (6,518,997 km) as of 2009. The Republic of
India has the second largest road system in the world with 4,689,842
kilometres (2,914,133 mi) of road (2013). The People's
Republic of China is third with 3,583,715 kilometres
(2,226,817 mi) of road (2007). The Federative Republic of Brazil
has the fourth largest road system in the world with 1,751,868
kilometres (1,088,560 mi) (2002). See List of countries by road
network size. When looking only at expressways the National Trunk
Highway System (NTHS) in China has a total length of 45,000 kilometres
(28,000 mi) at the end of 2006, and 60,300 km at the end of
2008, second only to the United States with 90,000 kilometres
(56,000 mi) in 2005.
This section may require cleanup to meet's quality
standards. The specific problem is: Cities list on the mainland not
connected is either incomplete, America centric, or not formulated has
examples. Please help improve this section if you can. (September
2017) (Learn how and when to remove this template message)
Eurasia, Africa, North America, South America, and Australia each have
an extensive road network that connects most cities. The North and
South American road networks are separated by the Darién Gap, the
only interruption in the Pan-American Highway.
Eurasia and Africa are
connected by roads on the Sinai Peninsula. The
European Peninsula is
connected to the
Scandinavian Peninsula by the Øresund Bridge, and
both have many connections to the mainland of Eurasia, including the
bridges over the Bosphorus.
Antarctica has very few roads and no
continent-bridging network, though there are a few ice roads between
bases, such as the South Pole Traverse.
Bahrain is the only island
country to be connected to a continental network by road (the King
Causeway to Saudi Arabia). Even well-connected road networks are
controlled by many different legal jurisdictions, and laws such as
which side of the road to drive on vary accordingly.
Many populated domestic islands are connected to the mainland by
bridges. A very long example is the 113-mile (181.9 km) Overseas
Highway connecting many of the
Florida Keys with the continental
Even on mainlands, some settlements have no roads connecting with the
primary continental network, due to natural obstacles like mountains
or wetlands, remoteness, or general expense. Unpaved roads or lack of
roads are more common in developing countries, and these can become
impassible in wet conditions. As of 2014, only 43% of rural Africans
have access to an all-season road. Due to steepness, mud, snow, or
fords, roads can sometimes be passable only to four-wheel drive
vehicles, those with snow chains or snow tires, or those capable of
deep wading or amphibious operation.
Cities on the mainland of continents which do not have road access
Iquitos, Peru, population 437,376 (2015) in the Amazon rainforest
Juneau, Alaska, population 32,406 (2014)
Nome, Alaska, population 3,788 (2014)
Rankin Inlet, Nunavut, population 2,577 (2011)
Supai, Arizona, population 208 (2010) in the Grand Canyon
Most disconnected settlements have local road networks connecting
ports, buildings, and other points of interest.
Where demand for travel by road vehicle to a disconnected island or
mainland settlement is high, roll-on/roll-off ferries are commonly
available if the journey is relatively short. For long-distance trips,
passengers usually travel by air and rent a car upon arrival. If
facilities are available, vehicles and cargo can also be shipped to
many disconnected settlements by boat, or air transport at much
greater expense. The island of Great Britain is connected to the
European road network by
Eurotunnel Shuttle - an example of a car
shuttle train which is a service used in other parts of Europe to
travel under mountains and over wetlands.
In polar areas, disconnected settlements are often more easily reached
by snowmobile or dogsled in cold weather, which can produce sea ice
that blocks ports, and bad weather that prevents flying. For example,
resupply aircraft are only flown to Amundsen–Scott South Pole
Station October to February, and many residents of coastal Alaska have
bulk cargo shipped in only during the warmer months. Permanent
darkness during the winter can also make long-distance travel more
dangerous in polar areas. Continental road networks do reach into
these areas, such as the Dalton
Highway to the North
Slope of Alaska,
the R21 highway to
Murmansk in Russia, and many roads in Scandinavia
(though due to fjords water transport is sometimes faster). Large
areas of Alaska, Canada, Greenland, and
Siberia are sparsely
connected. For example, all 25 communities of
Nunavut are disconnected
from each other and the main North American road network.
Road transport of people and cargo by may also be obstructed by border
controls and travel restrictions. For example, travel from other parts
of Asia to South Korea would require passage through the hostile
country of North Korea. Moving between most countries in Africa and
Eurasia would require passing through Egypt and Israel, which is a
politically sensitive area.
Some places are intentionally car-free, and roads (if present) might
be used by bicycles or pedestrians.
Roads are under construction to many remote places, such as the
villages of the Annapurna Circuit, and a road was completed in 2013 to
Mêdog County. Additional intercontinental and transoceanic fixed
links have been proposed, including a
Bering Strait crossing
Bering Strait crossing that
would connect Eurasia-Africa and North America, a Malacca Strait
Bridge to the largest island of
Indonesia from Asia, and a Strait of
Gibraltar crossing to connect Europe and Africa directly.
List of roads and highways
Pavement management system
Structural road design
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See also: Architecture
Streets and roadways
Types of road
Freeway / Motorway
Dual carriageway / Divided highway / Expressway
Highway systems by country
Hierarchy of roads
Single-point urban (SPUI)
Diamond grinding of pavement
Full depth recycling
Dead Man's Curve
Space and time allocation
Barrier transfer machine
Contraflow lane reversal
High-occupancy toll lane
High-occupancy vehicle lane
Median / Central reservation
Runaway truck ramp
Sidewalk / Pavement
Street running railway
Traffic signal preemption
Wide outside lane
Cat's eye (road)
Concrete step barrier
Raised pavement marker
Road surface marking
Overpass / Flyover
Underpass / Tunnel
Glossary of road transport terms
Road types by features
Cone penetration test
Standard penetration test
Crosshole sonic logging
Nuclear densometer test
Static load testing
California bearing ratio
Direct shear test
Proctor compaction test
Triaxial shear test
Hydraulic conductivity tests
Water content tests
Angle of repose
Pore water pressure
Lateral earth pressure
Dynamic load testing
Pile integrity test
Wave equation analysis
Statnamic load test
Mechanically stabilized earth
Geosynthetic clay liner