roads and highways

roads and highways


      traveled way on which people, animals, or wheeled vehicles move. In modern usage the term road describes a rural, lesser traveled way, while the word street denotes an urban roadway. Highway refers to a major rural traveled way; more recently it has been used for a road, in either a rural or urban area, where points of entrance and exit for traffic are limited and controlled.

      The most ancient name for these arteries of travel seems to be the antecedent of the modern way. Way stems from the Middle English wey, which in turn branches from the Latin veho (“I carry”), derived from the Sanskrit vah (“carry,” “go,” or “move”). The word highway goes back to the elevated Roman roads that had a mound or hill formed by earth from the side ditches thrown toward the centre, thus high way. The word street originates with the Latin strata (initially, “paved”) and later strata via (“a way paved with stones”). Street was used by the Anglo-Saxons for all the roads that they inherited from the Romans. By the Middle Ages, constructed roads were to be found only in the towns, and so street took on its modern limited application to town roads. The more recent word road, derived from the Old English word rád (“to ride”) and the Middle English rode or rade (“a mounted journey”), is now used to indicate all vehicular ways.

      Modern roads can be classified by type or function. The basic type is the conventional undivided two-way road. Beyond this are divided roads, expressways (divided roads with most side access controlled and some minor at-grade intersections), and freeways (expressways with side access fully controlled and no at-grade intersections). An access-controlled road with direct user charges is known as a tollway. In the United Kingdom freeways and expressways are referred to as motorways.

      Functional road types are local streets, which serve only adjacent properties and do not carry through traffic; collector, distributor, and feeder roads, which carry only through traffic from their own area; arterial roads, which carry through traffic from adjacent areas and are the major roads within a region or population centre; and highways, which are the major roads between regions or population centres.

      The first half of this article traces the history of roads from earliest times to the present, exploring the factors that have influenced their development and suggesting that in many ways roads have directly reflected the conditions and attitudes of their times. The road is thus one of the oldest continuous and traceable metaphors for civilization and society. The second half of the article explains the factors behind the design, construction, and operation of a modern road. It is shown that a road must interact closely and carefully with the terrain and community through which it passes, with changing vehicle technology, with information technologies, and with the various abilities, deficiencies, and frailties of the individual driver.


Roads of antiquity
Ancient roads of the Mediterranean and Middle East
      The first roads were paths made by animals and later adapted by humans. The earliest records of such paths have been found around some springs near Jericho and date from about 6000 BC. The first indications of constructed roads date from about 4000 BC and consist of stone-paved streets at Ur in modern-day Iraq and timber roads preserved in a swamp in Glastonbury, England. During the Bronze Age, the availability of metal tools made the construction of stone paving more feasible; at the same time, demand for paved roads rose with the use of wheeled vehicles, which were well established by 2000 BC.

Cretan stone roads
      At about this time the Minoans on the island of Crete built a 30-mile (50-kilometre) road from Gortyna on the south coast over the mountains at an elevation of about 4,300 feet (1,300 metres) to Knossos on the north coast. Constructed of layers of stone, the roadway took account of the necessity of drainage by a crown throughout its length and even gutters along certain sections. The pavement, which was about 12 feet (360 centimetres) wide, consisted of sandstone bound by a clay-gypsum mortar. The surface of the central portion consisted of two rows of basalt slabs 2 inches (50 millimetres) thick. The centre of the roadway seems to have been used for foot traffic and the edges for animals and carts. It is the oldest existing paved road.

Roads of Persia (Iran, ancient) and Babylon
      The earliest long-distance road was a 1,500-mile route between the Persian Gulf and the Mediterranean Sea. It came into some use about 3500 BC, but it was operated in an organized way only from about 1200 BC by the Assyrians, who used it to join Susa, near the Persian Gulf, to the Mediterranean ports of Smyrna (İzmir) and Ephesus. More a track than a constructed road, the route was duplicated between 550 and 486 BC by the great Persian kings Cyrus II and Darius I in their famous Royal Road. Like its predecessor, the Persian Royal Road began at Susa, wound northwestward to Arbela, and thence proceeded westward through Nineveh to Harran, a major road junction and caravan centre. The main road then continued to twin termini at Smyrna and Ephesus. The Greek historian Herodotus, writing about 475 BC, put the time for the journey from Susa to Ephesus at 93 days, although royal riders traversed the route in 20 days.

      In Babylon (Babylonia) about 615 BC the Chaldeans (Chaldea) connected the city's temples to the royal palaces with the Processional Way, a major road in which burned bricks and carefully shaped stones were laid in bituminous mortar.

      Herodotus credits the Egyptians with building their first roads to provide a solid track upon which to haul the immense limestone blocks used in the pyramids, and archaeological evidence indicates that such road building took place southwest of Cairo between 2600 and 2200 BC. The wheel arrived in Egypt at the relatively late date of about 1600 BC. There is little evidence of street surfacing in ancient Egyptian towns, though there is evidence of the use of paved processional roads leading to the temples. The ancient travel routes of Egypt ran from Thebes and Coptos on the central Nile east to the Red Sea and from Memphis (Cairo) across the land bridge to Asia Minor.

      The early Greeks depended primarily on sea travel. There is evidence of the building of special roads for religious purposes and transport about 800 BC, but there is little evidence of substantial road building for travel and transport prior to the Roman system. The Greeks did build a few ceremonial, or “sacred,” roads, paved with shaped stone and containing wheel ruts about 55 inches (140 centimetres) apart.

Ancient roads of Europe

The Amber Routes
      During the 2nd millennium BC, trade ways developed in Europe. One route, for example, ran between Italy and Spain via Marseille and nearby Heraclea, close to present-day Avignon, France. Such ways were used for the movement of flints from Denmark, freestone from Belgium, salt from Austria, lead and tin from England, and amber from northern Europe. By about 1500 BC many of the ways in eastern and central Europe had linked together into an extensive trading network known as the Amber Routes. Four routes have been identified, the first from modern Hamburg, Germany, southwestward by dual routes through Cologne and Frankfurt to Lyon and Marseille. The second also passed from Hamburg south to Passau on the Danube and then through the Brenner Pass to Venice. The third began at Samland on the East Prussian coast (where amber is still found), crossed the Vistula River at Thorn, and thence continued southeastward through the Moravian Gate to Aquileia on the Adriatic. The fourth, the Baltic-Pontus road, followed the main eastern rivers, the Vistula, Saw, Sereth, Prut, Bug, and Dnieper.

      While the Amber Routes were not roads in the modern sense, they were improved at river crossings, over mountain passes, and across wet and swampy areas. A few remnants of these roads survive today. They were constructed by laying two or three strings of logs in the direction of the road on a bed of branches and boughs up to 20 feet (6 metres) wide. This layer was then covered with a layer of transverse logs 9 to 12 feet in length laid side by side. In the best log roads, every fifth or sixth log was fastened to the underlying subsoil with pegs. There is evidence that the older log roads were built prior to 1500 BC. They were maintained in a level state by being covered with sand and gravel or sod. In addition, the Romans used side ditches to reduce the moisture content and increase the carrying capacity.

The Roman roads (Roman road system)
      The greatest systematic road builders of the ancient world were the Romans, who were very conscious of the military, economic, and administrative advantages of a good road system. The Romans drew their expertise mainly from the Etruscans (Etruscan)—particularly in cement technology and street paving—though they probably also learned skills from the Greeks (masonry), Cretans, Carthaginians (pavement structure), Phoenicians, and Egyptians (surveying). concrete made from cement was a major development that permitted many of Rome's construction advances.

      The Romans began their road-making task in 334 BC and by the peak of the empire had built nearly 53,000 miles of road connecting their capital with the frontiers of their far-flung empire. Twenty-nine great military roads, the viae militares, radiated from Rome. The most famous of these was the Appian Way. Begun in 312 BC, this road eventually followed the Mediterranean coast south to Capua and then turned eastward to Beneventum, where it divided into two branches, both reaching Brundisium (Brindisi). From Brundisium the Appian Way traversed the Adriatic coast to Hydruntum, a total of 410 miles from Rome.

 The typical Roman road was bold in conception and construction. Where possible, it was built in a straight line from one sighting point to the next, regardless of obstacles, and was carried over marshes, lakes, ravines, and mountains. In its highest stage of development, it was constructed by excavating parallel trenches about 40 feet apart to provide longitudinal drainage—a hallmark of Roman road engineering. The foundation was then raised about three feet above ground level, employing material taken from the drains and from the adjacent cleared ground. As the importance of the road increased, this embankment was progressively covered with a light bedding of sand or mortar on which four main courses were constructed: (1) the statumen layer 10 to 24 inches (250 to 600 millimetres) thick, composed of stones at least 2 inches in size, (2) the rudus, a 9-inch-thick layer of concrete made from stones under 2 inches in size, (3) the nucleus layer, about 12 inches thick, using concrete made from small gravel and coarse sand, and, for very important roads, (4) the summum dorsum, a wearing surface of large stone slabs at least 6 inches deep. The total thickness thus varied from 3 to 6 feet. The width of the Appian Way in its ultimate development was 35 feet. The two-way, heavily crowned central carriageway was 15 feet wide. On each side it was flanked by curbs 2 feet wide and 18 inches high and paralleled by one-way side lanes 7 feet wide. This massive Roman road section, adopted about 300 BC, set the standard of practice for the next 2,000 years.

      The public transport of the Roman Empire was divided into two classes: (1) cursus rapidi, the express service, and (2) agnarie, the freight service. In addition, there was an enormous amount of travel by private individuals. The two most widely used vehicles were the two-wheeled chariot drawn by two or four horses and its companion, the cart used in rural areas. A four-wheeled raeda in its passenger version corresponded to the stagecoaches of a later period and in its cargo version to the freight wagons. Fast freight raedae were drawn by 8 horses in summer and 10 in winter and, by law, could not haul in excess of 750 pounds (340 kilograms). Speed of travel ranged from a low of about 15 miles per day for freight vehicles to 75 miles per day by speedy post drivers.

Ancient roads of South and East Asia

      The Indus civilization in Sindh, Balochistān, and the Punjab probably flourished in the period 3250–2750 BC. Excavations indicate that the cities of this civilization paved their major streets with burned bricks cemented with bitumen. Great attention was devoted to drainage. The houses had drainpipes that carried the water to a street drain in the centre of the street, two to four feet deep and covered with slabs or bricks.

      Evidence from archaeological and historical sources indicates that by AD 75 several methods of road construction were known in India. These included the brick pavement, the stone slab pavement, a kind of concrete as a foundation course or as an actual road surface, and the principles of grouting (filling crevices) with gypsum, lime, or bituminous mortar. Street paving seems to have been common in the towns in India at the beginning of the Common Era, and the principles of drainage were well known. The crowning of the roadway and the use of ditches and gutters were common in the towns. Northern and western India in the period 300 to 150 BC had a network of well-built roads. The rulers of the Mauryan empire (4th century BC), which stretched from the Indus River to the Brahmaputra River and from the Himalayas to the Vindhya Range, generally recognized that the unity of a great empire depended on the quality of its roads. The Great Royal Road of the Mauryans began at the Himalayan border, ran through Taxila (near modern Rāwalpindi, Pakistan), crossed the five streams of the Punjab, proceeded by way of Jumna to Prayag (now Allahābād, India), and continued to the mouth of the Ganges River. A “Ministry of Public Works” was responsible for construction, marking, and maintenance of the roads and rest houses and for the smooth running of ferries.

China's (China) Imperial Highway
      China had a road system that paralleled the Persian Royal Road and the Roman road network in time and purpose. Its major development began under Emperor Shihuangdi about 220 BC. Many of the roads were wide, surfaced with stone, and lined with trees; steep mountains were traversed by stone-paved stairways with broad treads and low steps. By AD 700 the network had grown to some 25,000 miles (about 40,000 kilometres). Traces of a key route near Xi'an are still visible.

      The trade route from China to Asia Minor and India, known as the Silk Road, had been in existence for 1,400 years at the time of Marco Polo (Polo, Marco)'s travels (c. AD 1270–90). It came into partial existence about 300 BC, when it was used to bring jade from Khotan (modern Hotan, China) to China. By 200 BC it was linked to the West, and by 100 BC it was carrying active trade between the two civilizations. At its zenith in AD 200 this road and its western connections over the Roman system constituted the longest road on Earth. In Asia the road passed through Samarkand to the region of Fergana, where, near the city of Osh, a stone tower marked the symbolic watershed between East and West. From Fergana the road traversed the valley between the Tien Shan and Kunlun Mountains through Kashgar, where it divided and skirted both sides of the Takla Makan Desert to join again at Yuanquan. The road then wound eastward to Jiayuguan (Suzhou), where it passed through the westernmost gateway (the Jade Gate, or Yumen) of the Great Wall of China. It then went southeast on the Imperial Highway to Xi'an and eastward to Shanghai on the Pacific Ocean. From Kashgar, trade routes to the south passed over the mountains to the great trading centre of Bactria and to northern Kashmir.

Europe and Asia
      At the zenith of the Roman Empire, overland trade joined the cultures of Europe, North Africa, Asia Minor, China, and India. But the system of road transport was dependent on the Roman, Chinese, and Mauryan empires, and, as these great empires declined in the early Christian era, the trade routes became routes of invasion. Except in the Byzantine Empire, road networks fell into centuries of disrepair. Transport relied on pack trains, which could negotiate the badly maintained roads and sufficed to carry the reduced stream of commerce.

      The first signs of a road revival came during the reign of Charlemagne late in the 8th century. In the 9th century the Moors (Moor) established an extensive street network in Córdoba, Spain. The Vikings (Viking) operated the Varangian Road, a major trade route linking the Baltic and the Middle East via Russia. Further road revival was aided first by the need to service the regular round of trade fairs and then, in the 11th century, by a centralization of power and an increase in religious fervour.

      Eventually a commercial revival set in. By the 12th century old cities were reviving and new ones were being built, especially in western Europe. Street paving became a reputable artisan activity, and by the 15th century well-maintained roads bringing food to the cities from their hinterlands were of critical importance. At the same time, wheeled vehicles increased in number and quality. There was an awakened interest in better overland travel, better protection of merchants and other travelers, and the improvement of roads. Public funds, chiefly derived from tolls, were committed to road upkeep. The corvée, or road-labour tax, made an even more substantial contribution. Long-distance overland commerce increased rapidly and included a restoration of the trade (international trade) route between Europe and China through Central Asia that Marco Polo traveled in the late 13th century.

      Across the Atlantic, the period witnessed the rise of another notable road-building empire, that of the Incas. The Inca road system extended from Quito, Ecuador, through Cuzco, Peru, and as far south as Santiago, Chile. It included two parallel roadways, one along the coast about 2,250 miles in length, the other following the Andes about 3,400 miles in length with a number of cross connections. At its zenith, when the Spaniards arrived early in the 16th century, a network of some 14,000 miles of road served an area of about 750,000 square miles (1,940,000 square kilometres) in which lived nearly 10 million people. The network was praised by 16th-century explorers as superior to that in contemporary Europe.

      The Andes (Andes Mountains) route was remarkable. The roadway was 25 feet wide and traversed the loftiest ranges. It included galleries cut into solid rock and retaining walls built up for hundreds of feet to support the roadway. Ravines and chasms were filled with solid masonry, suspension bridges with wool or fibre cables crossed the wider mountain streams, and stone surfacing was used in difficult areas. The steeper gradients were surmounted by steps cut in the rocks. Traffic consisted entirely of pack animals (llamas) and people on foot; the Inca lacked the wheel. Yet they operated a swift foot courier system and a visual signaling system along the roadway from watchtower to watchtower.

The birth of the modern road
The master road builders
      In Europe, gradual technological improvements in the 17th and 18th centuries saw increased commercial travel, improved vehicles, and the breeding of better horses. These factors created an incessant demand for better roads, and supply and invention both rose to meet that demand. In 1585 the Italian engineer Guido Toglietta wrote a thoughtful treatise on a pavement system using broken stone that represented a marked advance on the heavy Roman style. In 1607 Thomas Procter published the first English-language book on roads. The first highway engineering (civil engineering) school in Europe, the School of Bridges and Highways, was founded in Paris in 1747. Late in the 18th century the Scottish political economist Adam Smith (Smith, Adam), in discussing conditions in England, wrote,

Good roads, canals, and navigable rivers, by diminishing the expense of carriage, put the remote parts of the country more nearly upon a level with those in the neighbourhood of a town. They are upon that account the greatest of all improvements.

      Up to this time roads had been built, with minor modifications, to the heavy Roman cross section, but in the last half of the 18th century the fathers of modern road building and road maintenance appeared in France and Britain.

 In France, Pierre-Marie-Jérôme Trésaguet (Trésaguet, Pierre-Marie-Jérôme), an engineer from an engineering family, became in 1764 engineer of bridges and roads at Limoges and in 1775 inspector general of roads and bridges for France. In that year he developed an entirely new type of relatively light road surface, based on the theory that the underlying natural formation, rather than the pavement, should support the load. His standard cross section was 18 feet wide and consisted of an eight-inch-thick course of uniform foundation stones laid edgewise on the natural formation and covered by a two-inch layer of walnut-sized broken stone. This second layer was topped with a one-inch layer of smaller gravel or broken stone. In order to maintain surface levels, Trésaguet's pavement was placed in an excavated trench—a technique that made drainage a difficult problem.

 Thomas Telford (Telford, Thomas), born of poor parents in Dumfriesshire, Scotland, in 1757, was apprenticed to a stone mason. Intelligent and ambitious, Telford progressed to designing bridges and building roads. He placed great emphasis on two features: (1) maintaining a level roadway with a maximum gradient of 1 in 30 and (2) building a stone surface capable of carrying the heaviest anticipated loads. His roadways were 18 feet wide and built in three courses: (1) a lower layer, seven inches thick, consisting of good-quality foundation stone carefully placed by hand (this was known as the Telford base), (2) a middle layer, also seven inches thick, consisting of broken stone of two-inch maximum size, and (3) a top layer of gravel or broken stone up to one inch thick.

      The greatest advance came from John Loudon McAdam (McAdam, John Loudon), born in 1756 at Ayr in Scotland. McAdam began his road-building career in 1787 but reached major heights after 1804, when he was appointed general surveyor for Bristol, then the most important port city in England (United Kingdom). The roads leading to Bristol were in poor condition, and in 1816 McAdam took control of the Bristol Turnpike. There he showed that traffic could be supported by a relatively thin layer of small, single-sized, angular pieces of broken stone placed and compacted on a well-drained natural formation and covered by an impermeable surface of smaller stones. He had no use for the masonry constructions of his predecessors and contemporaries.

 Drainage was essential to the success of McAdam's method, and he required the pavement to be elevated above the surrounding surface. The structural layer of broken stone was eight inches thick and used stone of two to three inches maximum size laid in layers and compacted by traffic—a process adequate for the traffic of the time. The top layer was two inches thick, using three-fourths- to one-inch stone to fill surface voids between the large stones. Continuing maintenance was essential.

      Although McAdam drew on the successes and failures of others, his total structural reliance on broken stone represented the largest paradigm shift in the history of road pavements. The principles of the “ macadam” road are still used today. McAdam's success was also due to his efficient administration and his strong view that road managers needed skill and motivation.

Early U.S. road systems

The Lancaster Turnpike
      The first engineered and planned road in the United States was the Lancaster Turnpike, a privately constructed toll road built between 1793 and 1795. Connecting Philadelphia and Lancaster in Pennsylvania, its 62-mile length had a maximum grade of 7 percent and was surfaced with broken stone and gravel in a manner initially uninfluenced by the work of Telford and McAdam. However, pavement failures in 1796 led to the introduction of some of the new European methods.

      The Cumberland Road, also known as the National Pike, was an even more notable road-building feat. It had been advocated by both George Washington and Thomas Jefferson to aid western expansion and national unity. Work commenced in 1811, and the road opened for traffic between Cumberland, Maryland, and Wheeling, West Virginia, in 1818. By 1838 it extended to Springfield, Ohio, and part of the way to Vandalia, Illinois. Specification requirements called for a 66-foot right-of-way completely cleared. The roadway was to be covered 20 feet in width with stone 18 inches deep at the centre and 12 inches deep at the edge. The upper six inches were to consist of broken stone of three-inch maximum size and the lower stratum of stone of seven-inch maximum size. The road was constructed by the federal government, much of the finance being raised by land sales. Although maintenance was funded by tolls and federal appropriations, the road surface began to deteriorate in the 1820s. Federal funding ceased in 1838, and in 1841 the project was abandoned at Vandalia for political and practical reasons.

Roads in the age of the automobile
      Beginning in the 1840s, the rapid development of railroads brought the construction of lightweight Trésaguet-McAdam roads to a virtual halt. For the next 60 years, road improvements were essentially confined to city streets or to feeder roads to railheads. Other rural roads became impassable in wet weather.

      The initial stimulus for a renewal of road building came not from the automobile, whose impact was scarcely felt before 1900, but from the bicycle, for whose benefit road improvement began in many countries during the 1880s and '90s. Nevertheless, while the requirements of the lightweight, low-speed bicycle were satisfied by the old “macadamized” surfaces, the automobile began to raise its own seemingly insatiable demands as the world entered the 20th century.

New paving materials
      When urban street paving became widespread in the latter half of the 19th century, the common paving materials were hoof-sized stone blocks, similarly sized wooden blocks, bricks, McAdam's broken stone, and occasionally asphalt and concrete. McAdam's broken stone provided the cheapest pavement, but its unbound surface was difficult to maintain and was usually either slimy or dusty as a consequence of water, weather, and copious amounts of horse excrement. Thus, roads at the turn of the 20th century were largely inadequate for the demands about to be placed on them by the automobile and truck. As vehicle speeds increased rapidly, the available friction between road and tire became critical for accelerating, braking, and cornering. In addition, numerous pavement failures made it obvious that much stronger and tougher materials were required. The result was an ongoing search for a better pavement. asphalt and concrete both offered promise.

      Asphalt is a mixture of bitumen and stone, and concrete is a mixture of cement and stone. Asphalt footpaths were first laid in Paris in 1810, but the method was not perfected until after 1835. The first road use of asphalt occurred in 1824, when asphalt blocks were placed on the Champs-Élysées in Paris, but the first successful major application was made in 1858 on the nearby rue Saint-Honoré. The first successful concrete pavement was built in Inverness, Scotland, in 1865. Neither technology, however, advanced far without the pressures of the car, and they both required the availability of powerful stone-crushing, mixing, and spreading equipment.

      The impetus for the development of modern road asphalt came from the United States, which had few deposits of natural bitumen to draw upon and where engineers were therefore forced to study the principles behind the behaviour of this material. The first steps came in the 1860s, with the work of Belgian immigrant Edward de Smedt at Columbia University in New York City. De Smedt conducted his first tests in New Jersey in 1870 and by 1872 was producing the equivalent of a modern “well-graded” maximum-density asphalt. The first applications were in Battery Park and on Fifth Avenue in New York City in 1872. De Smedt went to Washington, D.C., in 1876 as part of President Ulysses S. Grant (Grant, Ulysses S.)'s desire to make that town “a Capital City worthy of a great Nation.” Grant had appointed a commission to oversee road making, and it conducted its first trials on Pennsylvania Avenue in 1877. Sixty percent of the trials used de Smedt's new product and were great successes.

      In 1887 de Smedt was followed as inspector of asphalts and cements by Clifford Richardson, who set about the task of codifying the specifications for asphalt mixes. Richardson basically developed two forms of asphalt: asphaltic concrete, which was strong and stiff and thus provided structural strength; and hot-rolled asphalt, which contained more bitumen and thus produced a far smoother and better surface for the car and bicycle.

      One of the great convenient coincidences of asphalt development was that the automobile ran on gasoline, which at that time was simply a by-product of the distillation of kerosene from petroleum. Another by-product was bitumen. Until that time, most manufacturers had used coal tar (a by-product of the making of gas from coal) as the binder for road asphalt. As the demand for automobile fuel increased, however, so did the availability of bitumen and, hence, of good asphalt designed to the standards of de Smedt and Richardson. This gave American road builders a major advantage over their European counterparts, who were still wedded to the virtues of the various natural asphalts, such as those from Neuchâtel, Switzerland, and the island of Trinidad.

      Richardson published a standard textbook on asphalt paving in 1905, and the practice did not change greatly thereafter. The biggest change was in the machinery available to produce, place, and finish the material rather than in the product itself. Toward the end of the century, there were major movements toward the use of recycled asphalt, chemical modifiers for improving bitumen properties, and small fibres for improving crack resistance. In addition, developments in testing and structural analysis made it possible to design an asphalt pavement as a sophisticated structural composite.

      The first modern concrete roads were produced by Joseph Mitchell, a follower of Telford, who conducted three successful trials in England and Scotland in 1865–66. Like asphalt technology, concrete road building was largely developed by the turn of the 20th century and was restricted more by the available machinery than by the material. Problems were also encountered in producing a surface that could match the performance of the surface produced almost accidentally by hot-rolled asphalt. For the following century the two materials remained in intense competition, both offering a similar product at a similar cost, and there was little evidence that one would move far ahead of the other as they continued on their paths of gradual improvement. (The principles of modern pavement design are described below in Pavement (roads and highways).)

Changes in finance

From corvée to toll
      Through the millennia, responsibility for financing and building roads and highways has been both a local and a national responsibility in the nations of the world. It is notable that this responsibility has changed along with political attitudes toward road building and has not rested easily with any party. Many roads initially were built to provide rulers with a means of conquest, control, and taxation; in periods of peace, the same rulers usually tried to pass the maintenance responsibilities on to local authorities, adjoining landowners, or the travelers who used the road. Local authorities and landowners usually fulfilled their responsibilities via the corvée (statute labour), in which people were required to donate their labour to road work. Corvée was always unpopular and unproductive, but it was nevertheless more effective than attempts at direct taxation.

      The last option, charging the traveler, gave rise to the toll road, a system that blossomed with the Industrial Revolution. Private turnpike trusts dominated British road building and maintenance throughout the 19th century, eventually covering 15 percent of the entire network. In the United States many toll roads were constructed in the first half of the 19th century under charters granted by the states.

From local to national funding
      Thus, through the 19th century most road building was administered and financed on a local basis. British (United Kingdom) road building remained entirely local despite clear evidence that local responsibility was not providing adequate roads. The national government edged into the picture only through increased pressure from the cyclists, climaxed by the establishment in 1909 of a national Road Board authorized to construct and maintain new roads and to make advances to highway authorities to build new or improve old roads.

      Except for the National Pike, early highway building in the United States was also carried on by local government. Congress made a number of land grants for the opening of wagon roads but exercised no control over the expenditure of funds—with the result that, as in Britain, little road building was accomplished.

      In 1891 New Jersey enacted a law providing for state aid to the counties and established procedures for raising money at the township and county levels for road building. In 1893 Massachusetts established the first state highway commission. By 1913 most of the states had adopted similar legislation, and by 1920 all states had their own road organization. However, there was little coordination among the states. National funding began in 1912 with the Post Office Appropriation Act, and the Federal Aid Road Act of 1916 established federal aid for highways as a national policy. The Bureau of Public Roads, established in the Department of Agriculture in 1893 to make “inquiries with regard to road management,” was given responsibility for the program, and an apportionment formula based on area, population, and mileage of post roads in each state was adopted. Funds were allocated for construction costs, with the states being required to bear all maintenance costs. The location and selection of roads to be improved was left to the states, an arrangement that had some shortcomings.

      Since 1892 a national Good Roads movement had lobbied for a system of national roads joining the major population centres and contributing to the national economy. This point of view was recognized by the Federal Aid Highway Act of 1921, which required each state to designate a system of state highways not to exceed 7 percent of the total highway mileage in each state. Federal-aid funding was limited to this system, which was not to exceed three-sevenths of total highway mileage. Bureau of Public Roads approval of the system was required, and federal aid was limited to 50 percent of the estimated cost.

New highways

The parkway (expressway)
      The achievement of such a system in the automobile age required a new form of road. This grew from the parkway, which had many historical precedents but was introduced in its modern form in 1858 with the work of the landscape architects Frederick Law Olmsted and Calvert Vaux for Central Park in New York City. The concept was given further prominence by William Niles White of New York as a part of the Bronx River protection program of New York City and Westchester County. The 15-mile, four-lane single carriageway known as the Bronx River Parkway was built between 1916 and 1925. Protected on both sides by broad bands of parkland that limited access, the highway was located and designed so as to cause minimum disturbance to the landscape. Its use was restricted to passenger cars, and at-grade intersections were avoided. The success of the concept led to the creation of the Westchester County parkway system and the Long Island State Park Commission. More parkways were built in the New York area, including the Merritt Parkway (1934–40), which continued the Westchester Parkway System across Connecticut as a toll road providing divided roadways and limited access.

The freeway
      The success of the parkway system led to the introduction of the freeway, which is a divided highway with no conflicting traffic movements and no access from adjoining properties. In Germany between 1913 and 1921 a group called AVUS had built 10 kilometres (6 miles) of parkway through the Grunewald park in Berlin. Their successful experience led to the world's first full freeway being built from Cologne to Bonn between 1929 and 1932. In 1933 Adolf Hitler (Hitler, Adolf) began construction of an integrated freeway network known as the Reichsautobahnen, or “national motor roads,” beginning with the Frankfurt-Darmstadt-Mannheim-Heidelberg Autobahn. One purpose of the program was to alleviate unemployment, but the roads also appealed to German nationalism and had a strong militaristic intent. The entire system included three north-south routes and three east-west routes. The highway provided separate 7.5-metre (25-foot) carriageways divided by a median strip of 5 metres (16 feet). The roads were designed for large traffic volumes and speeds in excess of 150 kilometres (90 miles) per hour, bypassing cities and providing limited access. About 1,000 kilometres (600 miles) were completed by 1936, and 6,500 kilometres (4,000 miles) were in use when construction ceased in 1942.

      The viability of the freeway concept in the United States was demonstrated by the Pennsylvania Turnpike. The Pennsylvania Turnpike Commission, established in 1937 to raise funds and build a toll road across the Appalachian Mountains, found an unusually favourable situation in the form of an abandoned railroad right-of-way, with many tunnels and excellent grades over much of the route that allowed the tollway to be completed in 1940 to freeway standards. The turnpike provided two 24-foot carriageways and a 10-foot median with no cross traffic at grade and with complete control of access and egress at 11 traffic interchanges. Its alignment and grades were designed for high volumes of high-speed traffic and its pavement to accommodate the heaviest trucks. The favourable public reaction to this new type of highway provided the impetus for the post-World War II toll-road boom in the United States, advanced the start of a major interstate highway program, and influenced highway developments elsewhere. The Pennsylvania Turnpike, originally running from Harrisburg to Pittsburgh, was later extended 100 miles east to Philadelphia and 67 miles west to the Ohio border, making it 327 miles long. An original feature of the turnpike, later widely copied, was the provision of restaurant and fueling facilities.

National and international highway systems
      The Romans had realized that a coordinated system of roadways connecting the major areas of their empire would be of prime significance for both commercial and military purposes. In the modern era, the nations of Europe first introduced the concept of highway systems. In France, for example, the State Department of Roads and Bridges was organized in 1716, and by the middle of the 18th century the country was covered by an extensive network of roads built and maintained primarily by the national government. In 1797 the road system was divided into three classes of descending importance: (1) roads leading from Paris to the frontiers, (2) roads leading from frontier to frontier but not passing through Paris, and (3) roads connecting towns. By the early 1920s this general plan remained essentially the same except that a gradual change in class and responsibility had taken place. At that time the road system was divided into four classes: (1) national highways, improved and maintained by the national government, (2) regional highways, improved and maintained by the department under a road service bureau appointed by the Department Commission, (3) main local roads, connecting smaller cities and villages, built and maintained from funds of the communes supplemented by grants from the department, and (4) township roads, built and maintained by the communities alone.

      While the British recognized the necessity for national support of highways and a national system as early as 1878, it was the Ministry of Transport Act of 1919 that first classified the roadway system into 23,230 miles of Class I roads and 14,737 miles of Class II roads. Fifty percent of the cost of Class I roads and 25 percent of the cost of Class II roads were to be borne by the national government. In the mid-1930s the need for a national through-traffic system was recognized, and the Trunk Roads Act of 1939, followed by the Trunk Roads Act of 1944, created a system of roadways for through traffic. The Special Roads Act of 1949 authorized existing or new roads to be classified as “motorways” that could be reserved for special classes of traffic. The Highways Act of 1959 swept away all previous highway legislation in England and Wales and replaced it with a comprehensive set of new laws.

      The mammoth U.S. Interstate Highway System (formally, the National System of Interstate and Defense Highways) developed in response to strong public pressures in the 1950s for a better road system. These pressures culminated in the establishment by President Dwight Eisenhower (Eisenhower, Dwight D.) of the Clay Committee in 1954. Following this committee's recommendations, the Federal Aid Highway Act and the Highway Revenue Act of 1956 provided funding for an accelerated program of construction. A federal gasoline tax was established, the funds from which, with other highway-user payments, were placed in a Highway Trust Fund. The federal-state ratio for funding construction of the Interstate System was changed to 90 percent federal and 10 percent state. It was expected that the system would be completed no later than 1971, but cost increases and planning delays extended this time by some 25 years. The system grew to a total length of more than 45,000 miles, connecting nearly all the major cities in the United States and carrying more than 20 percent of the nation's traffic on slightly more than 1 percent of the total road and street system.

      The Canadian Highway Act of 1919 provided for a system of 40,000 kilometres (25,000 miles) of highways and provided for a federal allotment for construction not to exceed 40 percent of the cost. By the end of the century, more than 134,000 kilometres (83,000 miles) of highway had been built, of which approximately 16,000 kilometres (9,900 miles) were freeway.

The modern road

Road engineering
      Since the beginning of the 20th century, as the automobile and truck have offered ever higher levels of mobility, vehicle ownership per head of population has increased. Road needs have been strongly influenced by this popularity and also by the mass movement of people to cities and thence to suburban fringes—a trend that has led to increasing travel needs and road congestion and to low-density cities, which are difficult to service by public transport. Often the building of new roads to alleviate such problems has encouraged further urban sprawl and yet more road travel. Long-term solutions require the provision of alternatives to car and truck transport, controls over land use, and the proper pricing of road travel. To this end, road managers must be concerned not merely with lines on maps but also with the number, type, speed, and loading of individual vehicles, the safety, comfort, and convenience of the traveling public, and the health and welfare of bystanders and adjoining property owners.

      Ideally, the development of a major road system is an orderly, continuous process. The process follows several steps: assessing road needs and transport options; planning a system to meet those needs; designing an economically, socially, and environmentally acceptable set of roads; obtaining the required approval and financing; building, operating, and maintaining the system; and providing for future extensions and reconstruction.

      Road needs are closely associated with the relative location of centres of population, commerce, industry, and transportation. Traffic between two centres is approximately proportional to their populations and inversely proportional to the distance between them. Estimating traffic on a route thus requires a prediction of future population growth and economic activity, an estimation of their effects on land use and travel needs, and a knowledge of any potential transport alternatives. The key variables defining road needs are the traffic volumes, tonnages, and speeds to be expected throughout the road's life.

      Once the traffic demand has been estimated, it is necessary to predict the extent of the road works needed to handle that traffic. A starting point in these calculations is offered by surveys of the origins, destinations, and route choices of present traffic; computer models are then used to estimate future traffic volumes on each proposed route. Estimates of route choice are based on the understanding that most drivers select their estimate of the quickest, shortest, or cheapest route. Consideration in planning is also given to the effect of new traffic on existing streets, roads, and parking provisions.

      Where feasible, the next step in planning a road system is to refine the selected route to a narrow corridor. The various alignment options are drawn, considering the local terrain and conditions. The economic, social, and environmental benefits and costs of these options are discussed with relevant official and community groups until an acceptable specific route is determined.

Road design

Alignment and profile
      After a route has been selected, a three-dimensional road alignment and its associated cross-sectional profiles are produced. In order to reduce the amount of earth to be moved, the alignment is adjusted where practical so that the earth to be excavated is in balance with the embankments to be built. Computers allow many options to be explored and realistic views of the future road to be examined.

 In order to fully understand the design stage, a few standard terms must be defined (see figure). A traffic lane is the portion of pavement allocated to a single line of vehicles; it is indicated on the pavement by painted longitudinal lines or embedded markers. The shoulder is a strip of pavement outside an outer lane; it is provided for emergency use by traffic and to protect the pavement edges from traffic damage. A set of adjoining lanes and shoulders is called a roadway or carriageway, while the pavement, shoulders, and bordering roadside up to adjacent property lines are known as the right-of-way.

      In order to maintain quality and uniformity, design standards are established for each functional road type. The number of traffic lanes is directly determined by the combination of traffic volume and speed, since practical limits on vehicle spacing means that there is a maximum number of vehicles per hour that pass through a traffic lane. The width of lanes and shoulders, which must strike a balance between construction cost and driver comfort, allows the carriageway width to be determined. Standards also specify roadside barriers or give the clear transverse distances needed on either side of the carriageway in order to provide safety in the event that vehicles accidentally leave the carriageway. Thus it is possible to define the total right-of-way width needed for the entire road, although intersections will add further special demands.

      Design standards also help to determine the actual alignment of the road by specifying, for each design speed, the minimum radius of horizontal curves, the maximum vertical gradient, the clearance under bridges, and the distance a driver must be able to see the pavement ahead in order to stop or turn aside.

      Road traffic is carried by the pavement, which in engineering terms is a horizontal structure supported by in situ natural material. In order to design this structure, existing records must be examined and subsurface explorations conducted. The engineering properties of the local rock and soil are established, particularly with respect to strength, stiffness, durability, susceptibility to moisture, and propensity to shrink and swell over time. The relevant properties are determined either by field tests (typically by measuring deflection under a loaded plate or the penetration of a rod), by empirical estimates based on the soil type, or by laboratory measurements. The material is tested in its weakest expected condition, usually at its highest probable moisture content. Probable performance under traffic is then determined. Soils unsuitable for the final pavement are identified for removal, suitable replacement materials are earmarked, the maximum slopes of embankments and cuttings are established, the degree of compaction to be achieved during construction is determined, and drainage needs are specified.

 In a typical rural (rural society) pavement (as shown in the figure), the top layer of the pavement is the wearing course. Made of compacted stone, asphalt, or concrete, the wearing course directly supports the vehicle, provides a surface of sufficient smoothness and traction, and protects the base course and natural formation from excessive amounts of water. The base course provides the required supplement to the strength, stiffness, and durability of the natural formation. Its thickness ranges from 4 inches (10 centimetres) for very light traffic and a good natural formation to more than 40 inches (100 centimetres) for heavy traffic and a poor natural formation. The subbase is a protective layer and temporary working platform sometimes placed between the base course and the natural formation.

 Pavements are called either flexible or rigid, according to their relative flexural stiffness. Flexible pavements (see figure,—> left) have base courses of broken stone pieces either compacted into place in the style of McAdam or glued together with bitumen to form asphalt. In order to maintain workability, the stones are usually less than 1.5 inches in size and often less than 1 inch. Initially the bitumen must be heated to temperatures of 300°–400° F (150°–200° C) in order to make it fluid enough to mix with the stone. At the road site a paving machine places the hot mix in layers about twice the thickness of the stone size. The layers are then thoroughly rolled before the mix cools and solidifies. In order to avoid the expense of heating, increasing use has been made of bitumen emulsions or cutbacks, in which the bitumen binder is either treated with an emulsifier or thinned with a lighter petroleum fraction that evaporates after rolling. These treatments allow asphalts to be mixed and placed at ambient temperatures.

      The surface course of a flexible pavement protects the underlying base course from traffic and water while also providing adequate tire friction, generating minimal noise in urban areas, and giving suitable light reflectance for night-time driving. Such surfaces are provided either by a bituminous film coated with stone (called a spray-and-chip seal) or by a thin asphalt layer. The spray-and-chip seal is used over McAdam-style base courses for light to moderate traffic volumes or to rehabilitate existing asphalt surfaces. It is relatively cheap, effective, and impermeable and lasts about 10 years. Its main disadvantage is its high noise generation. Maintenance usually involves further spray coating with a surface dressing of bitumen. Asphalt surfacing is used with higher traffic volumes or in urban areas. Surfacing asphalt commonly contains smaller and more wear-resistant stones than the base course and employs relatively more bitumen. It is better able to resist horizontal forces and produces less noise than a spray-and-chip seal.

 Rigid pavements (see figure,—> right) are made of portland cement concrete. The concrete slab ranges in thickness from 6 to 14 inches. It is laid by a paving machine, often on a supporting layer that prevents the pressure caused by traffic from pumping water and natural formation material to the surface through joints and cracks. Concrete shrinks as it hardens, and this shrinkage is resisted by friction from the underlying layer, causing cracks to appear in the concrete. Cracking is usually controlled by adding steel reinforcement in order to enhance the tensile strength of the pavement and ensure that any cracking is fine and uniformly distributed. Transverse joints are sometimes also used for this purpose. Longitudinal joints are used at the edge of the construction run when the whole carriageway cannot be cast in one pass of the paving machine.

      In places where the local natural material is substandard for use as a base course, it can be “stabilized” with relatively small quantities of lime, portland cement, pozzolana, or bitumen. The strength and stiffness of the mix are increased by the surface reactivity of the additive, which also reduces the material's permeability and hence its susceptibility to water. Special machines distribute the stabilizer into the upper 8 to 20 inches of soil.

      In deciding whether to use a flexible, rigid, or stabilized pavement, engineers take into account lifetime cost, riding characteristics, traffic disruptions due to maintenance, ease and cost of repair, and the effect of climatic conditions. Often there is little to choose between rigid and flexible pavements.

      The properties of the base course material are usually determined by laboratory tests, although field tests are sometimes conducted to check that the construction process has achieved the designer's intent. Designers typically consider the possibility of structural failure resulting from a single overload and also from damage accumulating under the passage of many routine loads. Both of these types of failure are almost entirely caused by trucks.

      Adequate drainage is the single most important element in pavement performance, and drainage systems can be extensive and expensive. Drainage involves handling existing watercourses (water), removing water from the pavement surface, and controlling underground water in the pavement structure. In designing the system, the engineer first selects the “design storm”—that is, the most severe flood that can be expected in a nominated period of time (as much as 100 years for a major road or as little as 5 years for a minor street carrying local traffic). The drainage system must be able to carry the storm water produced by this design storm without flooding the roadway or adjacent property. In areas where land use is changing from agricultural to residential or commercial, peak flows will increase notably as the surrounding area is covered with roofs and paving.

      Safety requires that water be rapidly removed from the pavement surface. In urban areas, the water runs into shallow gutters and thence into the inlets of underground drains. In rural areas, surface water flows beyond the shoulders to longitudinal drainage ditches, which have flat side slopes to enable vehicles leaving the pavement to recover without serious incident. Cut-off surface drains are used to prevent water from flowing without restriction down the slopes of cuttings and embankments.

      Vertical drainage layers, formed from single-sized aggregate or special sheets called geofabrics and geomembranes, are used to prevent groundwater from seeping laterally into the pavement structure. In addition, a horizontal drainage layer is often inserted between base course and natural ground in order to remove water from the pavement structure and stop upward capillary movement of any natural groundwater. Underground drains can also be used to lower the groundwater level by both preventing water entry and removing water that does enter the pavement structure.

      The full design of a proposed road is analyzed with respect to its costs and its economic, social, and environmental effects. It may also be subjected to public review. This step can be lengthy, as new roads are usually popular with the traveling public but sometimes cause distress in the communities through which they pass.

      Local streets and collector roads are usually administered by local governments and financed by local taxes (taxation). Arterial roads and highways, however, need a wider administrative and financial input in order to guarantee route continuity and uniformity. Since the 1920s the financing of roads has been largely transferred to the road user. A variety of taxes is employed: on fuel and oil, on road usage, on vehicle purchase and ownership, on driver licensing, on truck mass and mass times distance traveled, on tire and accessory purchases, and on the economic benefits provided by roads (e.g., higher property values or increased productivity). Fuel taxes usually provide the simplest source of revenue, but they are not necessarily intended solely for expenditure on roads. Many local roads are funded by property taxes.

      After the road has been approved and financing found, surveyors define its three-dimensional location on the ground. Forming of the in-situ material to its required shape and installation of the underground drainage system can then begin. Imported pavement material is placed on the natural formation and may have water added; rollers are then used to compact the material to the required density. If possible, some traffic is permitted to operate over the completed earthwork in order to detect weak spots.

      In countries where labour is inexpensive and less skilled, traditional manual methods of road construction are still commonplace. However, the developed world relies heavily on purpose-built construction plant. This can be divided into equipment for six major construction purposes: clearing, earthmoving, shaping, and compacting the natural formation; installing underground drainage; producing and handling the road-making aggregate; manufacturing asphalt and concrete; placing and compacting the pavement layers; and constructing bridges and culverts.

      For clearing vegetation and undesirable materials from the roadway, the bulldozer is often employed. The construction of rock cuts is commonly done with shovels, draglines, and mobile drills. Shaping the formation and moving earth from cuttings to embankments is accomplished with bulldozers, graders, hauling scrapers, elevating graders, loaders, and large dump trucks. The material is placed in layers, brought to the proper moisture content, and compacted to the required density. Compaction is accomplished with tamping, sheeps-foot, grid, steel-wheeled, vibrating, and pneumatic-tired rollers. Backhoes, back actors, and trenchers are used for drainage work.

      In order to avoid high haulage costs, the materials used for base course construction are preferably located near the construction site; it is economically impossible to use expensive materials for long lengths of road construction. The excavation process is the same as for rock cuts, although rippers may be used for obtaining lower-grade material. Crushers, screens, and washers produce stone of the right size, shape, and cleanliness.

      The placement of paving material increasingly involves a paving machine for distributing the aggregate, asphalt, or concrete uniformly and to the required thickness, shape, and width (typically, one or two traffic lanes). The paving machine can slipform the edges of the course, thus avoiding the need for fixed side-forms. As it progresses down the road, it applies some preliminary compaction and also screeds and finishes the pavement surface. In modern machines, level control is by laser sighting.

      In producing a spray-and-chip seal surface (or a bituminous surface treatment), a porous existing surface is covered with a film of hot, fluid bitumen that is sprayed in sufficient quantity to fill voids, cracks, and crevices without leaving excess bitumen on the surface. The surface is then sprayed with a more viscous hot bitumen, which is immediately covered with a layer of uniform-size stone chips spread from a dump truck. The roadway is then rolled to seat the stone in the sticky bitumen, and excess stone is later cleared by a rotary broom.

      The life of a road structure depends on the quality of its maintenance and minor renovation. Maintenance keeps the roadway safe, provides good driving conditions, and prolongs the life of the pavement, thus protecting the road investment. Maintenance consists of activities concerned with the condition of the pavement, shoulders, drainage, traffic facilities, and right-of-way. It includes the prompt sealing of cracks and filling of potholes to prevent water entering through the surface, the removal of trash thrown on the wayside by the traveling public, and the care of pavement markings, signs, and signals. In rigorous winter climates, substantial effort is required to remove snow and ice from the pavement, to scatter salt for snow and ice removal, and to spread sand for better traction.

Road operation
Traffic management
      Road users are subject to traffic control via instructions and information provided by roadway markings, signs, and signals, and they are subject to legal control via the rules of the road (particularly those concerned with vehicular priority).

      The marking of roadway surfaces with painted lines and raised permanent markers is commonplace and effective, despite high maintenance costs and visibility problems at night, in heavy traffic, and in rain or snow. A solid line is a warning or instruction not to cross, and a broken line is for guidance. Thus, solid lines indicate dangerous conditions (such as restricted sight distance where overtaking would be dangerous), pavement edges, stop lines, and turning lanes at intersections; broken lines indicate interior lane lines and centre lines on two-way roads where the sight distance is good. Lines are usually white, but yellow is used for centre lines in North America.

      Signs advise the driver of special regulations and provide information about hazards and navigation. They are classified as regulatory signs, which provide notice of traffic laws and regulations (e.g., signs for speed limits and for stop, yield or give-way, and no entry); warning signs, which call attention to hazardous conditions (e.g., sharp curves, steep grades, low vertical clearances, and slippery surfaces); and guide signs, which give route information (e.g., numbers or designations, distances, directions, and points of interest).

      Signs have standard shapes and colours—for instance, the red octagon used for the stop sign, the triangle for warning signs, the green rectangle with white lettering for freeway directional signs (commonly mounted over the roadway and of large size for easy reading at high speeds). Tourist signs are brown rectangles, and special shapes and colours are used for route markers. Many signs, such as the stop sign, are universally used, but there are some differences between the two common international systems based on either the American or the European practice. Basically, these differences are derived from a complete reliance on symbolic signs and a greater range of blue guidance signs in multilingual Europe.

      Traffic signals are primarily used to control traffic in urban street systems—particularly at conventional intersections accommodating large traffic volumes, where they allocate right-of-way to the various traffic streams. They can also meter traffic entering access lanes onto busy freeways or to indicate the lanes to use on two-way roads. Simple traffic signals work on preset timing plans that vary with the time of day. More advanced traffic-actuated signals automatically monitor the traffic streams and allocate right-of-way accordingly. Signals can also be linked to a computer so that traffic traveling along a major route can receive a continuous wave of green signals, obtaining maximum traffic output from the system.

Legal control
      Legal rules governing the movement of traffic are an essential part of order on the road. The rules may be divided into three categories. First are those applying to the vehicle and the driver, such as vehicle and driver registration, vehicle safety equipment and roadworthiness, accident reporting, financial liability, and truck weights and axle loads (to protect pavements and bridges from damage). Second are the movement rules for drivers and pedestrians, known as the rules of the road; these dictate which side of the road to use, maximum speeds, right-of-way, and turning requirements. Third are those regulations that apply to limited road sections, indicating speed limits, one-way operations, and turning controls.

      The important rules of the road are reasonably uniform throughout the world. For instance, in most countries drivers must give right-of-way to vehicles on their right. However, in practice the stop and yield (or give-way) signs have commonly supplanted the right-of-way rule. Speed limits vary greatly with jurisdiction, ranging from walking pace in a Dutch woonerf, or “shared” street, to unrestricted on a German autobahn. Speed limits are commonly reduced on roads approaching residential, shopping, or school areas and on dangerous road sections and sharp curves.

      Special regulations are important for the efficient movement of traffic in specific segments of a street and road system. For instance, one-way streets in congested urban areas may provide safer driving conditions and increase the traffic-carrying capacity of the system. The provision of special turn arrows in traffic signals or the prohibition of turns at intersections contribute to safety, increase traffic throughput, and reduce conflict.

      Traffic police (or road patrols or highway police) help improve road safety and traffic flow by enforcing driving regulations. They also regulate traffic at the scene of an accident and investigate accidents. Traffic enforcement has been aided by the use of technology—cameras, radar, video, and inductance loops—to detect and record traffic offenders automatically.

      An important aspect of traffic regulation and accident prevention is the control of excessive speed, which contributes significantly to the number and severity of road crashes. Speed is commonly measured by radar devices or by pacing with a patrol car. In crash investigations, the speed of the cars is determined by the length of skid marks. Another key factor in road accidents is the influence of alcohol (alcohol consumption) and drugs (drug). Tests for intoxication are now widely conducted; the most common is the breath test (breath analyzer), in which the driver blows into a device that analyzes the alcohol content of the breath and indicates the approximate blood alcohol level. Many authorities believe that 0.50 gram of alcohol per litre of blood is a realistic limit for ordinary motorists, but that zero levels should be demanded for critical operators such as drivers of public transport vehicles.

      Road safety can also be built into the road. Divided roads are many times safer than two-way roads. Crash severity can be reduced by the use of “soft” signs and light poles and by guardrails and impact attenuators in front of fixed roadside objects such as bridge piers and the noses at the exit ramps of a freeway. Better road surfaces, alignments, signing, and marking improve driving conditions and increase road safety.

      Nevertheless, about 90 percent of crashes are primarily due to human error. Many crashes have been attributed to simple inattention or failure to see warnings. Alcohol, fatigue, inexperience, aggression, and excessive risk taking are the most common crash causes involving behavioral changes in drivers. Lack of driving skills is rarely an issue; most drivers do not need training as much as they need education and experience. Meanwhile, road engineers must design road systems that attempt to reduce the frequency and impact of human error.

Fred J. Benson Maxwell Gordon Lay

Additional Reading
M.G. Lay, Ways of the World: A History of the World's Roads and of the Vehicles That Used Them (1992), provides an overview, while his Handbook of Road Technology, 2nd ed., 2 vol. (1990), includes technical information on planning, pavements, traffic, and transport. Additional historical works are Hermann Schreiber, The History of Roads: From Amber Route to Motorway (1961; originally published in German, 1959), on the ancient roads of Asia, Europe, and Egypt and the Incan roads in South America; R.J. Forbes, “Roads to c. 1900,” in Charles Singer et al. (eds.), A History of Technology, vol. 4 (1958), pp. 520–547; W.J. Reader, Macadam: The McAdam Family and the Turnpike Roads, 1798–1861 (1980), treating the roads in 19th-century Great Britain; and United States, Federal Highway Administration, America's Highways, 1776–1976 (1977), chronicling federal aid to transportation in the United States.Aspects of road engineering and operation are detailed in Highway Capacity Manual, 2nd ed., rev. (1993), published by the Transportation Research Board (United States); American Association of State Highway and Transportation Officials, AASHTO Guide for Design of Pavement Structures, vol. 1, new ed. (1993) and vol. 2 (1986); and several publications of the Organisation for Economic Cooperation and Development, Road Research Programme: Proceedings of the International Symposium on the Effects of Speed Limits on Traffic Accidents and Fuel Consumption (1982), Traffic Capacity of Major Routes (1983), Impacts of Heavy Freight Vehicles (1983), and Road Surface Characteristics: Their Interaction and Their Optimisation (1984). International traffic regulations are presented in Peter Kincaid, The Rule of the Road (1986).Maxwell Gordon Lay

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Universalium. 2010.

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