Road

A road is a thoroughfare from one place to another, primarily used for movement of traffic. Many roads are paved.
The words "road" and "street" are commonly considered to be interchangeable, but the distinction is important in urban design. Roads differ from streets, whose primary use is local access. Roads also differ from stroads, which combine the features of streets and roads.
There are many types of roads, including parkways, avenues, controlled-access highways, tollways, highways, local roads, public roads and private roads.
The primary features of roads include lanes, sidewalks, roadways, medians, shoulders, verges, bike paths, and shared-use paths.

Definitions

Historically, many roads were simply recognizable routes without any formal construction or some maintenance.
The Organization for Economic Co-operation and Development defines a road as "a line of communication 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".
The Eurostat, ITF and UNECE Glossary for Transport Statistics Illustrated defines a road as a "Line of communication 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."
The 1968 Vienna Convention on Road Traffic defines a road as the entire surface of any way or street open to public traffic.
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.

Australia

Part 2, Division 1, clauses 11–13 of the National Transport Commission Regulations 2006 defines a road in Australia as 'an area that is open to or used by the public and is developed for, or has as one of its main uses, the driving or riding of motor vehicles.'
Further, it defines a shoulder and a road-related area which includes green areas separating roads, areas designated for cyclists and areas generally accessible to the public for driving, riding or parking vehicles.

New Zealand

In New Zealand, the definition of a road is broad in common law where the statutory definition includes areas the public has access to, by right or not. Beaches, publicly accessible car parks and yards, river beds, road shoulders, wharves and bridges are included. However, the definition of a road for insurance purposes may be restricted to reduce risk.

United Kingdom

In the United Kingdom The Highway Code details rules for "road users", but there is some ambiguity between the terms highway and road. For the purposes of the English law, Highways Act 1980, which covers England and Wales but not Scotland or Northern Ireland, road is "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 public road.
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.

United States

In the United States, laws distinguish between public roads, which are open to public use, and private roads, which are privately controlled.

History

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

The world's oldest known paved road was constructed in Egypt some time between 2600 and 2200 BC.
Corduroy roads are found dating to 4000 BC in Glastonbury, England.
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,. It was claimed to be the oldest road in the world until the 2009 discovery of a 6,000-year-old trackway in Plumstead, London.
In 500 BC, Darius I the Great started an extensive road system for the Achaemenid Empire, including the Royal Road, which was one of the finest highways of its time, connecting Sardis to Susa. The road remained in use after Roman times. These road systems reached as far east as 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 distillation.
The 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 of engineered roads. 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, 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, Italy, in 1928 was very much focused on creating jobs.
The Autostrada dei Laghi in Italy, the first controlled-access highway built in the world, connecting Milan to Lake Como and Lake Maggiore, and now parts of the A8 and A9 motorways, was devised by Piero Puricelli and was inaugurated in 1924. This motorway, called autostrada, contained only one lane in each direction and no interchanges.

File:MortarlessPavement.jpg|thumb|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.File:Motorway construction in Ireland.JPG|thumb|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, 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 and followed by the laying of pavement material. A variety of road building equipment is employed in road building.
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. Great care is taken to preserve reference benchmarks.
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.
A borrow pit 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 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 are not steeper than one vertical to two horizontal for safety reasons.
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.
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. 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.
General fill material should be free of organics, meet minimum California bearing ratio 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 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.
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.
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 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, 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.