Road

A bricked road in Bangladesh

A gravel road in Namibia

Historically many roads were simply recognizable routes without any formal construction or some maintenance.[1]

The Organization 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 paths".[2]

The Eurostat, ITF and UNECE Glossary for Transport Statistics Illustrated defines a road as a "Line of communication (traveled way) open to public traffic, primarily for the use of road motor vehicles, using a stabilized base other than rails or air strips. [...] Included are paved roads and other roads with a stabilized base, e.g. gravel roads. Roads also cover streets, bridges, tunnels, supporting structures, junctions, crossings and interchanges. Toll roads are also included. Excluded are dedicated cycle lanes."[3]

The 1968 Vienna Convention on Road Traffic defines a road as the entire surface of any way or street open to public traffic.[4]

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.[5] Modern roads are normally smoothed, paved, or otherwise prepared to allow easy travel.[6]

Transfăgărășan called "the best road in the world" by Top Gear[19]

The Porta Rosa, a Greek street dating from the 3rd to 4th century BC in Velia, with a paved surface and gutters

A paved Roman road in Pompeii

Old tractor road over farmland, Ystad, Sweden

Ancient Sassanid Era Pathway in Behbahan, Persia

Ancient Cobblestoned Road in Behbahan

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.[1] Some believe that some roads originated from following animal trails.[20][21] The Icknield Way may exemplify this type of road origination, where human and animal both selected the same natural line.[22] By about 10,000 BC human travelers used rough roads/pathways.[1]

• The world's oldest known paved road was constructed in Egypt some time between 2600 and 2200 BC.[23]
• Stone-paved streets appear in the city of Ur in the Middle East dating back to 4000 BC.[1]
• Corduroy roads (log roads) are found dating to 4000 BC in Glastonbury, England.[1]
• 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[24][25] until the 2009 discovery of a 6,000-year-old trackway in Plumstead, London.[26][27]
• Brick-paved streets appeared in India as early as 3000 BC.[1]
• c. 1995 BC: an early subdividing of roadways evidenced with sidewalks built in Anatolia.[28]
• In 500 BC, 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,[29] connecting Sardis (the westernmost major city of the empire) to Susa. The road remained in use after Roman times. These road systems reached as far east as Bactria and India.[30]
• In ancient times, transport by river was far easier and faster than transport by road,[25] 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.[31][32]
• From about 312 BC, the Roman Empire built straight[33] 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.[25]
• 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 distillation.[34]
• The Highways Act 1555 in Britain transferred responsibility for maintaining roads from government to local parishes.[25] 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.[25] The Rebecca Riots in Carmarthenshire and Rhayader from 1839 to 1844 contributed to a Royal Commission that led to the demise of the system in 1844,[35] which coincided with the development of the UK railway system.
• In the late-19th century roading engineers began to cater for cyclists by building separate lanes alongside roadways.
• From the beginning of the 20th century, roads were increasingly built for tourism and also to create jobs. A typical example of the stimulation of tourism is the Great Dolomite Road, while the creation of the panoramic coastal road Strada Costiera between Duino and Barcola in 1928 was very much focused on creating jobs.[36][37]

A highway in Tampere (Finland)

Road design is part of highway engineering. Structural road design is designing a road for the environment in order to extend its longevity and reduce maintenance. The Shell pavement design method is used in many countries for the design of new asphalt roadsides.

Adverse camber

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.

Alignment

The route of the road, defined as a series of horizontal tangents and curves.

All-weather road

Unpaved road that is constructed of a material that does not create mud during rainfall.

Banked turn

Belisha Beacon

An orange globe, lit at night, used to highlight a pedestrian crossing

Bicycle boulevard

A street that allows local vehicle traffic, but is prioritized for bicycles and other non-motorized travel

A beach road (Newcastle, NSW, Australia)

Bollard

Rigid posts that can be arranged in a line to close a road or path to vehicles above a certain width

Byway

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

Bypass

Road that avoids or "bypasses" a built-up area, town, or village

Bottleneck

Section of a road with a carrying capacity substantially below that of other sections of the same road

Botts' dots

Non-reflective raised pavement marker used on roads

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.

Cant

Another name for cross slope or camber

Carriageway

Part of the road intended for the movement of road motor vehicles; the parts of the road which form a shoulder for the lower or upper layers of the road surface are not part of the roadway, nor are those parts of the road intended for the circulation of road vehicles which are not self-propelled or for the parking of vehicles.[3]

Cat's eye

Reflective raised pavement marker used on roads

Chicane

Sequence of tight serpentine curves (usually an S-shape curve or a bus stop)

Chipseal

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

Corniche

Road on the side of a cliff or mountain, with the ground rising on one side and falling away on the other

Cross slope

The slope of the pavement, expressed as units of rise per unit of run, or as a percentage

Curb (kerb)

A raised edge at the side of the roadway.

Curb extension

(also kerb extension, bulb-out, nib, elephant ear, curb bulge and blister) Traffic calming measure, intended to slow the speed of traffic and increase driver awareness, particularly in built-up and residential neighborhoods.

Cycle lane

Part of a carriageway designated for cycles and distinguished from the rest of the carriageway by longitudinal road markings. Mopeds may also be allowed to use a cycle lane.[3]

Cycle track

Independent road or part of a road designated for cycles and sign-posted as such. A cycle track is separated from other roads or other parts of the same road by structural means. Mopeds may also be allowed to use the cycle track.[3]

Cycling infrastructure

Cycling-friendly infrastructure integrated into the roadway or in its own right of way

Drainage gradient

Farm-to-market road

A state road or county road that connects rural or agricultural areas to market towns.

Fork

(literally "fork in the road") Type of intersection where a road splits

Grade

Longitudinal slope

Green lane

(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.

Guide rail

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.

Gutter

A drainage channel usually at the edge of the road or along a median.

Interstate Highway System (United States)

System of Interstate and Defense Highways

Lane

One of the longitudinal strips into which a carriageway is divisible, whether or not defined by longitudinal road markings, which is wide enough for one moving line of motor vehicles other than motorcycles.[3]

Layby (Pullout, pull-off)

A paved area beside a main road where cars can stop temporarily to let another car pass.

Loose chippings

The hazard of stone chippings that have come loose

Median

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 strip (North American English and Australian English), neutral ground [Louisiana English] or central nature strip (Australian English): Area that separates opposing lanes of traffic

Motorway

(Europe) (Freeways in the US, Special road in the UK) Road, specially designed and built for motor traffic, which does not serve properties bordering on it, with separate carriageways for traffic in two directions, with no crossings at the same level (road, railway, tramway track, or footpath) and sign-posted as a motorway and is reserved for specific categories of road motor vehicles.[3]

Mountain pass

A relatively low-level route through a range of mountains

Milestone

One of a series of numbered markers placed along a road, often at regular intervals, showing the distance to destinations.

National Highway

Road built and maintained by a national authority.

Pavement

The road regarded as a geoconstruction. In the UK the term is road surface and the pavement is a pedestrian walkway alongside the road.

Pedestrian crossing

Designated point on a road where road marking or other means helps pedestrians cross safely

Pelican crossing

(officially Pelicon crossing) (UK) A PEdestrian LIght CONtrolled crossing.

Private highway

Highway owned and operated for profit by private industry

Private road

Road owned and maintained by a private individual, organization, or company rather than by a government

Profile

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 stopping.[38]

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.[39]

Public space

Place where anyone has a right to come without being excluded because of economic or social conditions

Ranch road

U.S. road that connects rural and agricultural areas to market towns

Road number

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.

Road-traffic safety

Process to reduce the harm (deaths, injuries, and property damage) that result from vehicle crashes on public roads

Roadworks

Part or all of the road is occupied for work or maintenance

Roughness

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 78 mph (126 km/h), waves with up to 350 m will excite motion sickness.

Roundabout

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 traveled in a clockwise direction. Also known as an island in parts of the UK.

Segregated Bicycle Path

Cyclists ideally have a protected bike lane separated by a concrete median with splay kerbs if possible, and have a protected bike lane width of at least 2 meters if possible (one way). In the Netherlands, most one-way cycle paths are at least 2.5 metres wide.[39]

Shoulder (also hard shoulder)

A clear, level area to the side of the roadway available for stopping if needed.

State highway

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

Traffic

Pedestrians, ridden or herded animals, vehicles, bicycles, and other conveyances using any road for purposes of travel.[40]

Texture (roads)

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 500 mm.

Traffic calming

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

Traffic island

(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

Traffic light

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 intersection

Zebra crossing (UK)

A pedestrian crossing marked by black and white stripes on the carriageway

Layers in the construction of a mortarless pavement: A.) Subgrade B.) Subbase C.) Base course D.) Paver base E.) Pavers F.) Fine-grained sand

In transport engineering, subgrade is the native material underneath a constructed road.

Overpass under construction over Interstate 5 in Burbank, California[41][42]

Surveyor at work with a leveling instrument

Asphalt layer and Hamm road roller

Sub-base layer composed of cement-based material being applied during construction of the M8 motorway in Ireland

Road construction in Myanmar

Clearing rocks from mountain road in Spiti

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,[43]^: 15 and may be required to meet standards set by law[44] or official guidelines.[45] 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 building.[46][47]

After design, approval, planning, legal, and environmental considerations have been addressed alignment of the road is set out by a surveyor.[33] The radii and gradient are designed and staked out to best suit the natural ground levels and minimize the amount of cut and fill.[45]^: 34 Great care is taken to preserve reference Benchmarks[45]^: 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.[45]^: 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 topsoil 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.[45]^: 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.[45]^: 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.[45]^: 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.[45]^: 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.[48] Geosynthetics perform four main functions in roads: separation, reinforcement, filtration, and drainage; which increase the pavement performance, reduce construction costs and decrease maintenance.[49]

The completed roadway 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 conditions.

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,[50] 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 existing carriageway.

Roads that are intended for use by a particular mode of transport can be reallocated for another mode of transport,[51] i.e. by using traffic signs. For instance, in the ongoing road space reallocation effort, some roads (particularly in city centers) which are intended for use by cars are increasingly being repurposed for cycling and/or walking.[52][53][54]

"Road works ahead" sign, typically used in Europe

Like all structures, roads deteriorate over time. Deterioration is primarily due to environmental effects such as frost heaves, thermal cracking and oxidation often contribute, however accumulated damage from vehicles also contributes.[55] 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.[56] 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 construction works.

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.[57] 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.[58]

A white line used to designate a parking bay in Scotland. The line has deteriorated to the point that both an older yellow line and the underlying surface are visible beneath.

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.[59] Many concrete pavements built since the 1950s have significantly outlived their intended design lives.[60] Some roads like Chicago, Illinois's Wacker Drive, a major two-level (and at one point, three-level) roadway in the downtown area, are being rebuilt with a designed service life of 100 years.[61]

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. Software algorithms use this data to recommend maintenance or new construction.

Maintenance treatments for asphalt concrete generally include thin asphalt overlays, crack sealing, surface rejuvenating, fog sealingfo, 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.[62]

Older concrete pavements that develop faults can be repaired with a dowel bar retrofit, in which slots are cut in the pavement at each joint, and dowel bars are placed in the slots, which are then filled with concrete patching material. This can extend the life of the concrete pavement for 15 years.[63]

Failure to maintain roads properly can create significant costs to society, in a 2009 report released by the American Association of State 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).

Pedestrian crossing, line markings and street furniture

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 drivers.

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 the likelihood of head-on collisions.

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.[69]

Speed limits can improve road traffic safety and reduce the number of road traffic casualties from traffic collisions. In their World report on road traffic injury prevention report, the World Health Organization (WHO) identify speed control as one of various interventions likely to contribute to a reduction in road casualties.

Snow causing bad road conditions

Road conditions are the collection of factors describing the ease of driving on a particular stretch of road, or on the roads of a particular locality, including the quality of the pavement surface, potholes, road markings, and weather. It has been reported that "[p]roblems of transportation participants and road conditions are the main factors that lead to road traffic accidents".[70] It has further been specifically noted that "weather conditions and road conditions are interlinked as weather conditions affect the road conditions".[71] Specific aspects of road conditions can be of particular importance for particular purposes. For example, for autonomous vehicles such as self-driving cars, significant road conditions can include "shadowing and lighting changes, road surface texture changes, and road markings consisting of circular reflectors, dashed lines, and solid lines".[72]

Various government agencies and private entities, including local news services, track and report on road conditions to the public so that drivers going through a particular area can be aware of hazards that may exist in that area. News agencies, in turn, rely on tips from area residents with respect to certain aspects of road conditions in their coverage area.[73]

Air pollution along Pasadena Highway in Los Angeles

A dual carriageway section of National Highway 8 connecting Delhi to Gurgaon

Careful design and construction of a road can reduce any negative environmental impacts. Water management systems can be used to reduce the effect of pollutants from roads.[74][75] 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.[76]

De-icing chemicals and sand can run off into roadsides, contaminate groundwater and pollute surface waters;[77] and road salts can be toxic to sensitive plants and animals.[78] Sand applied to icy roads can be ground up by traffic into fine particulates and contribute to air pollution.

Roads are a chief source of noise pollution. In the early 1970s, it was recognized that design of roads can be conducted to influence and minimize noise generation.[79] Noise barriers can reduce noise pollution near 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.[80] Road dust kicked up by vehicles may trigger allergic reactions.[81] In addition, on-road transportation greenhouse gas emissions are the largest single cause of climate change, scientists say.[82]

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 to Bangalore

Ontario Highway 401, a route with a collector / express setup

The wide Coastal Road in the Philippines now called the Manila-Cavite Expressway

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).[87][88] or occasionally using road tolls.[89]

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:[90]

• design/build
• design/build/operate/maintain
• design/build/finance/operate
• build/own/operate

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.[91] The term was also commonly used to refer to roadsteads, waterways that lent themselves to use by shipping.

The United States has the largest network of roads of any country with 4,050,717 miles (6,518,997 km) as of 2009.[93] The Republic of India has the second-largest road system globally with 4,689,842 kilometres (2,914,133 mi) of road (2013).[94] 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. However, as of 2017, China has 130,000 km of Expressways.[95][96]

An ice road between the Hailuoto Island and the Mainland Finland at winter

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 Fahd 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 mi (182 km) Overseas Highway connecting many of the Florida Keys with the continental United States.

Even on mainlands, some settlements have no roads connecting with the primary continental network, due to natural obstacles like mountains or wetlands, or high cost compared to the population served. 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.[97] 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.

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.[98]

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 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.