Road

A road ascends a mountainside using hairpin bends in the French Alps.

A road is an identifiable route or path between two or more places. Roads are typically smoothed, paved, or otherwise prepared to allow easy travel; though they need not be, and historically many roads were simply recognisable routes without any formal construction or maintenance. In urban areas roads may pass along and be named as streets, serving a dual function as urban space and route.

Usage and etymology

Misty road in Australia, entering into a township.

In original usage, a "road" was simply any pathway fit for riding ("road" is cognate with "ride", e.g.: ships ride at anchor in roads). The word "street," whose origin is the Latin strata, was kept for paved pathways that had been prepared to ease travel in some way. Thus, many "Roman Roads" have the word "street" as part of their name.

However, modern usage does not usually make this distinction and it is only important since place names often hold the earlier usage in them; these days, roads are also prepared in some way. This includes, at the least, the removal of trees and smoothing of the ground. In some dialects, lower grade roads are called trails and tracks and it is uncertain where "road" begins and trail ends. Roads are a prerequisite for road transport of goods on wheeled vehicles.

The word "road" emphasizes its function of transportation along its length, while a "street" may be considered to have activity and commerce taking place on it (see street life).

History

A cobblestone Roman road in Pompeii.

The first pathways were the trails made by migrating animals. By about 10,000 BC, these rough pathways were used by human hunter nomads following these herds.

Street paving has been found from the first human settlements around 4000 BC.

The oldest engineered road discovered is the Sweet Track causeway in England, dating from around 3800 BC.

The ancient Egyptians constructed a stone paved road to help move materials for the building of the Great Pyramid in about 3000 BC.

The ancient Chinese constructed an extensive system of roads, some paved, from about 1100 BC onwards. By 20 AD, the Chinese road network extended over 40,000 kilometres.

The Incas built fine highways, the Inca road system, for couriers through the Andes, and the Mayans built an extensive network of paved roads in Mexico before the European discovery of the New World.

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 is the horse-drawn boat in which the horse follows a cleared path along the river bank.

From about 300 BC, the Roman Empire built straight strong stone Roman roads throughout Europe and North Africa, in support of its military campaigns.

Road construction and maintenance in Britain was traditionally done on a local parish basis. This resulted in a poor and variable state of roads. To remedy this, the first of the 'Turnpike Trusts' around 1706, to build good roads and collect tolls from passing vehicles. Eventually there were approximately 1,100 Trusts in Britain and some 38,000 km of engineered roads.

Engineered roads in the age of horse-drawn transport aimed for a maximum gradient of 1 in 30 on a macadamized surface, since this was the steepest a horse could exert to pull a load up hill, which it could manage easily on the flat. Notable road engineers from this period are Pierre Marie Jérôme Trésaguet (1716-1796) in France and John Loudon McAdam (1756-1836) in England.

During the industrial revolution, the railway developed as a solution to the problem of rutting of the road surface by heavy carts. Instead of trying to build a strong surface across the whole road, the cart was constrained to run either on rails or grooves which could be made of much stronger, wear resistant material.

Today, roads are almost exclusively built to enable travel by automobile and other wheeled vehicles. In most countries, road transport is the most utilized way to move goods. Also, in most developed countries, roads are formally divided into lanes to ensure the safe and smooth movement of traffic.

Funding

Road building and maintenance is an area of economic activity (compare military spending) that remains dominated by the public sector (though often through private contractors). Roads (except those on private property not accessible to the general public) are typically paid for by taxes (often raised through levies on fuel), though some public roads, especially highways are funded by tolls.

Driving on the right or the left

Main article: Driving on the left or right.

In India, driving is on the left side of the road.

Traffic drives on the right or on the left side of the road depending on the country. In countries where traffic drives on the right, traffic signs are mostly on the right side of the road, roundabouts and traffic circles go counter-clockwise, and pedestrians crossing a two-way road should watch out for traffic from the left first. In countries where traffic drives on the left, the reverse is true.

Traffic flow and road design in both cases are each other's mirror image.

Design

Road design consists of two important technical aspects:
*geometrical road design
*structural road design Besides these two technical sides of the design, environmental issues, planning issues and juridical issues are important.

Construction

A major road near Sibiu, Romania.

Road construction requires the creation of a continuous right-of-way, overcoming geographic obstacles and having grades low enough to permit vehicle or foot travel. Removal of earth and rock by digging or blasting, construction of embankments, bridges and tunnels, and removal of vegetation (this may involve deforestation) are often needed. A variety of road building equipment is employed in road building.

Once these activities are completed, construction of the pavement can begin. First the native soil, known as the subgrade, is compacted. Weak soils may also be stabilized with additives such as portland cement and quicklime, or dug out and replaced with imported soils.

Then a base course consisting of gravel or crushed stone is usually placed on the subgrade and compacted. On top of the base course is placed a surface course which typically consists of asphalt concrete or portland cement concrete. This surface course strengthens the pavement structure by spreading out the vehicle loads applied to the subgrade. It also provides a smooth and high-friction surface for vehicles to drive on.

Modern roads, and indeed many ancient ones, such as those built by the Romans, feature a convex transverse profile known as superelevation or camber. This is designed to allow water to drain away from the road to its edges. Water is then carried away by gutters to drains placed at intervals. Some roads don't have gutters and water simply drains away to a naturally porous verge, or into ditches. Modern roads that carry motor traffic also employ camber in curves to aid traffic stability by allowing them to "bank into" the bend to some extent.

On the side of the road there may be retroreflectors on pegs, rocks or crash barriers, white toward the direction of the traffic on that side of the road, and red toward the other direction. In the road surface there may be cat's eyes: retroreflectors that protrude slightly, but which can be driven over without damage.

Road signs are often also made retroreflective or even illuminated in rare circumstances. For greater visibility of road signs at daytime, sometimes fluorescence is applied to get very bright colors.

Maintenance

London Circuit, Canberra, Australia

A typical rural county road in Indiana, USA, where traffic drives on the right. The yellow lines indicate that passing is allowed in the ongoing direction but not in the oncoming direction.

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 4th power of axle weight. A typical tractor-trailer weighing 80,000 pounds with 8,000 pounds on the steer axle and 36,000 pounds on both of the tandem axle groups is expected to do 7,800 times more damage than a passenger vehicle with 2,000 pounds on each axle. In most pavement design methodologies trucks are considered to be the sole cause of pavement deterioration.

Pavements are designed for an expected service life. Most European countries have strict standards for road construction that require that most roads should be able to go 30 years or longer between major resurfacings. In the United States new pavements are typically designed for a service life of between 15 and 25 years, depending on the importance of the road. Service life predictions are inherently unreliable due to the difficulty of predicting future traffic and environmental conditions.

Virtually all roads require some form of maintenance before they come to the end of their service life. Maintenance activities can be divided into structural maintenance and functional maintenance, although there is a great deal of overlap. Structural maintenance is maintenance intended to preserve the structural integrity of the pavement, and includes patching potholes, sealing cracks and overlays. Functional maintenance is maintenance to improve the roadway's function of providing a smooth and safe surface for vehicles to drive on, and includes surface grinding and thin overlays.

Terminology

A sand road in Namibia

A typical rural dirt road in Indiana, USA.

A brick main street in Lebanon, Illinois

An unusual road at London Heathrow Airport, England. A British Airways Boeing 777-200 is being towed across a public road on its way to the maintenance hangars.