history of world road and transportation

7/21/2019 History of World Road and Transportation

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Ancient Cretan stone road, (top) cross section and (bottom) surface view.

Adapted from Hermann Schreiber, The History of Roads (Sinfonie der Strasse),copyright 1959 Econ-Verlag GmbH, used by permission

roads and highwaysroads and highways,traveled way on which people, animals, or wheeled vehicles move. In modern usage the term roaddescribes a rural, lesser traveled way, while the word streetdenotes an urban roadway. Highwayrefers to a major rural traveledway; more recently it has been used for a road, in either a rural or urban area, where points of entrance and exit for traffic arelimited and controlled.

The most ancient name for these arteries of travel seems to be the antecedent of the modern way. Waystems from the MiddleEnglish wey, which in turn branches from the Latin veho(I carry), derived from the Sanskrit vah(carry, go, or move). Theword highwaygoes back to the elevated Roman roads that had a mound or hill formed by earth from the side ditches throwntoward the centre, thus high way. The word streetoriginates with the Latin strata(initially, paved) and later strata via(a waypaved with stones). Streetwas used by the Anglo-Saxons for all the roads that they inherited from the Romans. By the MiddleAges, constructed roads were to be found only in the towns, and so streettook on its modern limited application to town roads.The more recent word road, derived from the Old English word rd(to ride) and the Middle English rodeor 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 aredivided roads, expressways (divided roads with most side access controlled and some minor at-grade intersections), andfreeways (expressways with side access fully controlled and no at-grade intersections). An access-controlled road with direct usercharges 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 trafficfrom adjacent areas and are the major roads within a region or population centre; and highways, which are the major roadsbetween 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 haveinfluenced their development and suggesting that in many ways roads have directly reflected the conditions and attitudes oftheir times. The road is thus one of the oldest continuous and traceable metaphors for civilization and society. The second halfof the article explains the factors behind the design, construction, and operation of a modern road. It is shown that a road mustinteract closely and carefully with the terrain and community through which it passes, with changing vehicle technology, withinformation technologies, and with the various abilities, deficiencies, and frailties of the individual driver.

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 foundaround some springs near Jericho and date from about 6000 BC. The first indications of constructed roads date from about 4000BCand 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 themountains at an elevation of about 4,300 feet (1,300 metres) toKnossos on the north coast. Constructed of layers of stone, theroadway took account of the necessity of drainage by a crownthroughout its length and even gutters along certain sections. Thepavement, which was about 12 feet (360 centimetres) wide,consisted of sandstone bound by a clay-gypsum mortar. The surfaceof the central portion consisted of two rows of basalt slabs 2 inches(50 millimetres) thick. The centre of the roadway seems to have beenused for foot traffic and the edges for animals and carts. It is theoldest existing paved road.

ROADS OF PERSIA AND BABYLON

The earliest long-distance road was a 1,500-mile route between thePersian Gulf and the Mediterranean Sea. It came into some useabout 3500 BC, but it was operated in an organized way only from

History

Roads of antiquity

s and highways -- Britannica Online Encyclopedia http://www.britannica.com/print/article/505109

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Ancient Greek road, (top) cross section and (bottom) surface view.

Adapted from R.J. Forbes, Notes on the History of Roads, copyright 1934 North-Holland Publishing Company, used by permission

about 1200 BCby the Assyrians, who used it to join Susa, near the Persian Gulf, to the Mediterranean ports of Smyrna (zmir) andEphesus. More a track than a constructed road, the route was duplicated between 550 and 486 BCby the great Persian kingsCyrus II and Darius I in their famous Royal Road. Like its predecessor, the Persian Royal Road began at Susa, woundnorthwestward to Arbela, and thence proceeded westward through Nineveh to Harran, a major road junction and caravancentre. The main road then continued to twin termini at Smyrna and Ephesus. The Greek historian Herodotus, writing about 475BC, put the time for the journey from Susa to Ephesus at 93 days, although royal riders traversed the route in 20 days.

In Babylon about 615 BCthe Chaldeans connected the citys temples to the royal palaces with the Processional Way, a major roadin which burned bricks and carefully shaped stones were laid in bituminous mortar.

EGYPT

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

GREECE

The early Greeks depended primarily on sea travel. There isevidence of the building of special roads for religious purposes and

transport about 800 BC, but there is little evidence of substantialroad building for travel and transport prior to the Roman system.The Greeks did build a few ceremonial, or sacred, roads, pavedwith shaped stone and containing wheel ruts about 55 inches (140centimetres) 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 Marseilleand nearby Heraclea, close to present-day Avignon, France. Suchways were used for the movement of flints from Denmark,freestone from Belgium, salt from Austria, lead and tin fromEngland, and amber from northern Europe. By about 1500 BCmany

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 Cologneand Frankfurt to Lyon and Marseille. The second also passed from Hamburg south to Passau on the Danube and then throughthe Brenner Pass to Venice. The third began at Samland on the East Prussian coast (where amber is still found), crossed theVistula 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, andacross wet and swampy areas. A few remnants of these roads survive today. They were constructed by laying two or threestrings 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 thencovered 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 weremaintained in a level state by being covered with sand and gravel or sod. In addition, the Romans used side ditches to reduce themoisture content and increase the carrying capacity.

THE ROMAN ROADS

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 Etruscansparticularlyin cement technology and street pavingthough they probably also learned skills from the Greeks (masonry), Cretans,Carthaginians (pavement structure), Phoenicians, and Egyptians (surveying). Concrete made from cement was a majordevelopment that permitted many of Romes construction advances.

The Romans began their road-making task in 334 BCand by the peak of the empire had built nearly 53,000 miles of roadconnecting their capital with the frontiers of their far-flung empire. Twenty-nine great military roads, the viae militares, radiatedfrom Rome. The most famous of these was the Appian Way. Begun in 312 BC, this road eventually followed the Mediterraneancoast 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


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Ancient Roman road shown in cross section.

construction. Where possible, it was built in a straight line fromone sighting point to the next, regardless of obstacles, and wascarried over marshes, lakes, ravines, and mountains. In itshighest stage of development, it was constructed by excavatingparallel trenches about 40 feet apart to provide longitudinaldrainagea hallmark of Roman road engineering. Thefoundation was then raised about three feet above ground level,employing material taken from the drains and from the adjacentcleared 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 statumenlayer 10 to 24 inches (250 to 600 millimetres) thick, composed of stones at least 2 inches in size, (2) the rudus, a9-inch-thick layer of concrete made from stones under 2 inches in size, (3) the nucleuslayer, about 12 inches thick, using concretemade from small gravel and coarse sand, and, for very important roads, (4) the summum dorsum, a wearing surface of largestone slabs at least 6 inches deep. The total thickness thus varied from 3 to 6 feet. The width of the Appian Way in its ultimatedevelopment was 35 feet. The two-way, heavily crowned central carriageway was 15 feet wide. On each side it was flanked bycurbs 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, thefreight service. In addition, there was an enormous amount of travel by private individuals. The two most widely used vehicleswere the two-wheeled chariot drawn by two or four horses and its companion, the cart used in rural areas. A four-wheeled raedain its passenger version corresponded to the stagecoaches of a later period and in its cargo version to the freight wagons. Fastfreight raedaewere 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 postdrivers.

ANCIENT ROADS OF SOUTH AND EAST ASIA

INDIA

The Indus civilization in Sindh, Balochistn, and the Punjab probably flourished in the period 32502750 BC. Excavations indicatethat the cities of this civilization paved their major streets with burned bricks cemented with bitumen. Great attention wasdevoted to drainage. The houses had drainpipes that carried the water to a street drain in the centre of the street, two to fourfeet deep and covered with slabs or bricks.

Evidence from archaeological and historical sources indicates that by AD75 several methods of road construction were known inIndia. These included the brick pavement, the stone slab pavement, a kind of concrete as a foundation course or as an actualroad surface, and the principles of grouting (filling crevices) with gypsum, lime, or bituminous mortar. Street paving seems tohave 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 theperiod 300 to 150 BChad a network of well-built roads. The rulers of the Mauryan empire (4th century BC), which stretched fromthe Indus River to the Brahmaputra River and from the Himalayas to the Vindhya Range, generally recognized that the unity of agreat empire depended on the quality of its roads. The Great Royal Road of the Mauryans began at the Himalayan border, ranthrough Taxila (near modern Rwalpindi, Pakistan), crossed the five streams of the Punjab, proceeded by way of Jumna to Prayag(now Allahbd, India), and continued to the mouth of the Ganges River. A Ministry of Public Works was responsible forconstruction, marking, and maintenance of the roads and rest houses and for the smooth running of ferries.

CHINAS 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 withtrees; steep mountains were traversed by stone-paved stairways with broad treads and low steps. By AD700 the network hadgrown to some 25,000 miles (about 40,000 kilometres). Traces of a key route near Xian are still visible.

THE SILK ROAD

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 ofMarco Polos travels (c. AD127090). It came into partial existence about 300 BC, when it was used to bring jade from Khotan(modern Hotan, China) to China. By 200 BCit was linked to the West, and by 100 BCit was carrying active trade between the twocivilizations. At its zenith in AD200 this road and its western connections over the Roman system constituted the longest road onEarth. 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 KunlunMountains through Kashgar, where it divided and skirted both sides of the Takla Makan Desert to join again at Yuanquan. Theroad 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 Xian and eastward to Shanghai on the PacificOcean. From Kashgar, trade routes to the south passed over the mountains to the great trading centre of Bactria and tonorthern Kashmir.


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EUROPE AND ASIA

At the zenith of the Roman Empire, overland trade joined the cultures of Europe, North Africa, Asia Minor, China, and India. Butthe system of road transport was dependent on the Roman, Chinese, and Mauryan empires, and, as these great empiresdeclined in the early Christian era, the trade routes became routes of invasion. Except in the Byzantine Empire, road networksfell 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 Moorsestablished an extensive street network in Crdoba, Spain. The Vikings operated the Varangian Road, a major trade route linkingthe Baltic and the Middle East via Russia. Further road revival was aided first by the need to service the regular round of tradefairs 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 inwestern Europe. Street paving became a reputable artisan activity, and by the 15th century well-maintained roads bringing foodto the cities from their hinterlands were of critical importance. At the same time, wheeled vehicles increased in number andquality. There was an awakened interest in better overland travel, better protection of merchants and other travelers, and theimprovement of roads. Public funds, chiefly derived from tolls, were committed to road upkeep. The corve, or road-labour tax,made an even more substantial contribution. Long-distance overland commerce increased rapidly and included a restoration ofthe trade route between Europe and China through Central Asia that Marco Polo traveled in the late 13th century.

INCA ROADS OF SOUTH AMERICA

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

The Andes route was remarkable. The roadway was 25 feet wide and traversed the loftiest ranges. It included galleries cut intosolid rock and retaining walls built up for hundreds of feet to support the roadway. Ravines and chasms were filled with solidmasonry, suspension bridges with wool or fibre cables crossed the wider mountain streams, and stone surfacing was used indifficult 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 signalingsystem along the roadway from watchtower to watchtower.

THE MASTER ROAD BUILDERS

In Europe, gradual technological improvements in the 17th and 18th centuries saw increased commercial travel, improvedvehicles, and the breeding of better horses. These factors created an incessant demand for better roads, and supply andinvention 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 publishedthe first English-language book on roads. The first highway 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, in discussing conditions inEngland, wrote,

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

TRSAGUET

In France, Pierre-Marie-Jrme Trsaguet, an engineerfrom an engineering family, became in 1764 engineerof bridges and roads at Limoges and in 1775 inspector

The Middle Ages

The birth of the modern road

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.


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Cross sections of three 18th-century European roads, as designed by (top) Pierre Trsaguet,

Encyclopdia Britannica, Inc.





general of roads and bridges for France. In that yearhe developed an entirely new type of relatively lightroad surface, based on the theory that the underlyingnatural formation, rather than the pavement, shouldsupport the load. His standard cross section was 18feet wide and consisted of an eight-inch-thick courseof uniform foundation stones laid edgewise on thenatural formation and covered by a two-inch layer ofwalnut-sized broken stone. This second layer wastopped with a one-inch layer of smaller gravel or

broken stone. In order to maintain surface levels,Trsaguets pavement was placed in an excavatedtrencha technique that made drainage a difficultproblem.

TELFORD

Thomas Telford, born of poor parents inDumfriesshire, Scotland, in 1757, was apprenticed to astone mason. Intelligent and ambitious, Telfordprogressed 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 ofcarrying the heaviest anticipated loads. His roadwayswere 18 feet wide and built in three courses: (1) alower layer, seven inches thick, consisting ofgood-quality foundation stone carefully placed byhand (this was known as the Telford base), (2) a middlelayer, also seven inches thick, consisting of brokenstone of two-inch maximum size, and (3) a top layer ofgravel or broken stone up to one inch thick.

MCADAM

The greatest advance came from John LoudonMcAdam, born in 1756 at Ayr in Scotland. McAdam

began his road-building career in 1787 but reachedmajor heights after 1804, when he was appointed general surveyor for Bristol, then the most important port city in England. Theroads leading to Bristol were in poor condition, and in 1816 McAdam took control of the Bristol Turnpike. There he showed thattraffic could be supported by a relatively thin layer of small, single-sized, angular pieces of broken stone placed and compactedon a well-drained natural formation and covered by an impermeable surface of smaller stones. He had no use for the masonryconstructions of his predecessors and contemporaries.

Drainage was essential to the success of McAdamsmethod, and he required the pavement to be elevatedabove the surrounding surface. The structural layer ofbroken stone was eight inches thick and used stone oftwo to three inches maximum size laid in layers andcompacted by traffica process adequate for thetraffic of the time. The top layer was two inches thick,using three-fourths- to one-inch stone to fill surfacevoids between the large stones. Continuingmaintenance was essential.

Although McAdam drew on the successes and failuresof others, his total structural reliance on broken stonerepresented the largest paradigm shift in the history ofroad pavements. The principles of the macadamroad are still used today. McAdams success was alsodue to his efficient administration and his strong viewthat 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 builtbetween 1793 and 1795. Connecting Philadelphia and Lancaster in Pennsylvania, its 62-mile length had a maximum grade of 7percent and was surfaced with broken stone and gravel in a manner initially uninfluenced by the work of Telford and McAdam.


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Photo-Auto Guide, Chicago to Rockford(1905) by H. Sargent Michaels is

The Newberry Library (A Britannica Publishing Partner)

However, pavement failures in 1796 led to the introduction of some of the new European methods.

THE CUMBERLAND ROAD

The Cumberland Road, also known as the National Pike, was an even more notable road-building feat. It had been advocated byboth George Washington and Thomas Jefferson to aid western expansion and national unity. Work commenced in 1811, and theroad 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. Theupper six inches were to consist of broken stone of three-inch maximum size and the lower stratum of stone of seven-inchmaximum size. The road was constructed by the federal government, much of the finance being raised by land sales. Althoughmaintenance was funded by tolls and federal appropriations, the road surface began to deteriorate in the 1820s. Federal fundingceased in 1838, and in 1841 the project was abandoned at Vandalia for political and practical reasons.

Beginning in the 1840s, the rapid development ofrailroads brought the construction of lightweightTrsaguet-McAdam roads to a virtual halt. For the next

60 years, road improvements were essentiallyconfined 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 buildingcame not from the automobile, whose impact wasscarcely felt before 1900, but from the bicycle, forwhose benefit road improvement began in manycountries during the 1880s and 90s. Nevertheless,while the requirements of the lightweight, low-speedbicycle were satisfied by the old macadamizedsurfaces, the automobile began to raise its ownseemingly insatiable demands as the world entered

the 20th century.

NEW PAVING MATERIALS

When urban street paving became widespread in thelatter half of the 19th century, the common pavingmaterials were hoof-sized stone blocks, similarly s izedwooden blocks, bricks, McAdams broken stone, and

occasionally asphalt and concrete. McAdams broken stone provided the cheapest pavement, but its unbound surface wasdifficult to maintain and was usually either slimy or dusty as a consequence of water, weather, and copious amounts of horseexcrement. Thus, roads at the turn of the 20th century were largely inadequate for the demands about to be placed on them bythe automobile and truck. As vehicle speeds increased rapidly, the available friction between road and tire became critical foraccelerating, braking, and cornering. In addition, numerous pavement failures made it obvious that much stronger and toughermaterials 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 inParis in 1810, but the method was not perfected until after 1835. The first road use of asphalt occurred in 1824, when asphaltblocks were placed on the Champs-lyses in Paris, but the first successful major application was made in 1858 on the nearbyrue Saint-Honor. 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, andspreading equipment.

The impetus for the development of modern road asphalt came from the United States, which had few deposits of naturalbitumen 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-gradedmaximum-density asphalt. The first applications were in Battery Park and on Fifth Avenue in New York City in 1872. De Smedtwent to Washington, D.C., in 1876 as part of President Ulysses S. Grants 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 PennsylvaniaAvenue in 1877. Sixty percent of the trials used de Smedts 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 codifyingthe specifications for asphalt mixes. Richardson basically developed two forms of asphalt: asphaltic concrete, which was strongand stiff and thus provided structural strength; and hot-rolled asphalt, which contained more bitumen and thus produced a far

Roads in the age of the automobile


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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 timewas simply a by-product of the distillation of kerosene from petroleum. Another by-product was bitumen. Until that time, mostmanufacturers had used coal tar (a by-product of the making of gas from coal) as the binder for road asphalt. As the demand forautomobile fuel increased, however, so did the availability of bitumen and, hence, of good asphalt designed to the standards ofde Smedt and Richardson. This gave American road builders a major advantage over their European counterparts, who were stillwedded to the virtues of the various natural asphalts, such as those from Neuchtel, 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. Towardthe end of the century, there were major movements toward the use of recycled asphalt, chemical modifiers for improvingbitumen properties, and small fibres for improving crack resistance. In addition, developments in testing and structural analysismade 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 trialsin England and Scotland in 186566. Like asphalt technology, concrete road building was largely developed by the turn of the20th century and was restricted more by the available machinery than by the material. Problems were also encountered inproducing a surface that could match the performance of the surface produced almost accidentally by hot-rolled asphalt. For thefollowing century the two materials remained in intense competition, both offering a similar product at a similar cost, and therewas little evidence that one would move far ahead of the other as they continued on their paths of gradual improvement. (Theprinciples of modern pavement design are described below in Pavement.)

CHANGES IN FINANCE

FROM CORVE TO TOLL

Through the millennia, responsibility for financing and building roads and highways has been both a local and a nationalresponsibility in the nations of the world. It is notable that this responsibility has changed along with political attitudes towardroad 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 localauthorities, adjoining landowners, or the travelers who used the road. Local authorities and landowners usually fulfilled theirresponsibilities via the corve, in which people were required to donate their labour to road work. Corve was always unpopularand 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. Privateturnpike trusts dominated British road building and maintenance throughout the 19th century, eventually covering 15 percent ofthe entire network. In the United States many toll roads were constructed in the first half of the 19th century under chartersgranted 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 road buildingremained entirely local despite clear evidence that local responsibility was not providing adequate roads. The nationalgovernment edged into the picture only through increased pressure from the cyclists, climaxed by the establishment in 1909 of anational Road Board authorized to construct and maintain new roads and to make advances to highway authorities to build newor improve old roads.

Except for the National Pike, early highway building in the United States was also carried on by local government. Congress madea number of land grants for the opening of wagon roads but exercised no control over the expenditure of fundswith 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 thetownship and county levels for road building. In 1893 Massachusetts established the first state highway commission. By 1913most of the states had adopted similar legislation, and by 1920 all states had their own road organization. However, there waslittle coordination among the states. National funding began in 1912 with the Post Office Appropriation Act, and the Federal AidRoad Act of 1916 established federal aid for highways as a national policy. The Bureau of Public Roads, established in theDepartment of Agriculture in 1893 to make inquiries with regard to road management, was given responsibility for theprogram, and an apportionment formula based on area, population, and mileage of post roads in each state was adopted. Fundswere allocated for construction costs, with the states being required to bear all maintenance costs. The location and selection ofroads 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 centresand 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 ofPublic Roads approval of the system was required, and federal aid was limited to 50 percent of the estimated cost.

NEW HIGHWAYS


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U.S. Sen. Robert Bulkley of Ohio, with a map of a proposed federal network that was aprecursor to

Harris & Ewing Collection/Library of Congress, Washington, D.C. (LC-DIG-hec-24067)

THE PARKWAY

The achievement of such a system in the automobile age required a new form of road. This grew from the parkway, which hadmany historical precedents but was introduced in its modern form in 1858 with the work of the landscape architects FrederickLaw Olmsted and Calvert Vaux for Central Park in New York City. The concept was given further prominence by William NilesWhite 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 bybroad bands of parkland that limited access, the highway was located and designed so as to cause minimum disturbance to thelandscape. Its use was restricted to passenger cars, and at-grade intersections were avoided. The success of the concept led tothe 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 (193440), which continued the Westchester Parkway System acrossConnecticut 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 trafficmovements and no access from adjoining properties. In Germany between 1913 and 1921 a group called AVUS had built 10kilometres (6 miles) of parkway through the Grunewald park in Berlin. Their successful experience led to the worlds first fullfreeway being built from Cologne to Bonn between 1929 and 1932. In 1933 Adolf Hitler began construction of an integratedfreeway 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 Germannationalism 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 providinglimited access. About 1,000 kilometres (600 miles) were completed by 1936, and 6,500 kilometres (4,000 miles) were in use whenconstruction ceased in 1942.

The viability of the freeway concept in the United States was demonstrated by the Pennsylvania Turnpike. The PennsylvaniaTurnpike Commission, established in 1937 to raise funds and build a toll road across the Appalachian Mountains, found anunusually favourable situation in the form of an abandoned railroad right-of-way, with many tunnels and excellent grades overmuch of the route that allowed the tollway to be completed in 1940 to freeway standards. The turnpike provided two 24-footcarriageways and a 10-foot median with no cross traffic at grade and with complete control of access and egress at 11 trafficinterchanges. Its alignment and grades were designed for high volumes of high-speed traffic and its pavement to accommodatethe heaviest trucks. The favourable public reaction to this new type of highway provided the impetus for the post-World War IItoll-road boom in the United States, advanced the start of a major interstate highway program, and influenced highwaydevelopments elsewhere. The Pennsylvania Turnpike, originally running from Harrisburg to Pittsburgh, was later extended 100miles 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 ofroadways connecting the major areas of their empirewould be of prime significance for both commercialand military purposes. In the modern era, the nationsof Europe first introduced the concept of highwaysystems. In France, for example, the State Departmentof Roads and Bridges was organized in 1716, and bythe middle of the 18th century the country wascovered by an extensive network of roads built andmaintained primarily by the national government. In

1797 the road system was divided into three classes ofdescending importance: (1) roads leading from Paris tothe frontiers, (2) roads leading from frontier to frontierbut not passing through Paris, and (3) roadsconnecting towns. By the early 1920s this general planremained essentially the same except that a gradualchange in class and responsibility had taken place. Atthat time the road system was divided into fourclasses: (1) national highways, improved andmaintained by the national government, (2) regionalhighways, improved and maintained by thedepartment under a road service bureau appointed bythe Department Commission, (3) main local roads,connecting smaller cities and villages, built andmaintained from funds of the communessupplemented by grants from the department, and (4)township roads, built and maintained by the

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THE UNITED KINGDOM

While the British recognized the necessity for national support of highways and a national system as early as 1878, it was theMinistry of Transport Act of 1919 that first classified the roadway system into 23,230 miles of Class I roads and 14,737 miles ofClass 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 thenational government. In the mid-1930s the need for a national through-traffic system was recognized, and the Trunk Roads Actof 1939, followed by the Trunk Roads Act of 1944, created a system of roadways for through traffic. The Special Roads Act of1949 authorized existing or new roads to be classified as motorways that could be reserved for special classes of traffic. TheHighways Act of 1959 swept away all previous highway legislation in England and Wales and replaced it with a comprehensive setof new laws.

THE UNITED STATES AND CANADA

The mammoth U.S. Interstate Highway System (formally, the National System of Interstate and Defense Highways) developed inresponse to strong public pressures in the 1950s for a better road system. These pressures culminated in the establishment byPresident Dwight Eisenhower of the Clay Committee in 1954. Following this committees recommendations, the Federal AidHighway Act and the Highway Revenue Act of 1956 provided funding for an accelerated program of construction. A federalgasoline 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 thistime by some 25 years. The system grew to a total length of more than 45,000 miles, connecting nearly all the major cities in theUnited States and carrying more than 20 percent of the nations traffic on slightly more than 1 percent of the total road andstreet system.

The Canadian Highway Act of 1919 provided for a system of 40,000 kilometres (25,000 miles) of highways and provided for afederal 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.

Since the beginning of the 20th century, as the automobile and truck have offered ever higher levels of mobility, vehicleownership per head of population has increased. Road needs have been strongly influenced by this popularity and also by themass movement of people to cities and thence to suburban fringesa trend that has led to increasing travel needs and roadcongestion and to low-density cities, which are difficult to service by public transport. Often the building of new roads to alleviatesuch problems has encouraged further urban sprawl and yet more road travel. Long-term solutions require the provision ofalternatives to car and truck transport, controls over land use, and the proper pricing of road travel. To this end, road managersmust be concerned not merely with lines on maps but also with the number, type, speed, and loading of individual vehicles, thesafety, 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: assessingroad needs and transport options; planning a system to meet those needs; designing an economically, socially, andenvironmentally acceptable set of roads; obtaining the required approval and financing; building, operating, and maintaining thesystem; and providing for future extensions and reconstruction.

PLANNING

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 distancebetween them. Estimating traffic on a route thus requires a prediction of future population growth and economic activity, anestimation of their effects on land use and travel needs, and a knowledge of any potential transport alternatives. The keyvariables defining road needs are the traffic volumes, tonnages, and speeds to be expected throughout the roads 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 basedon the understanding that most drivers select their estimate of the quickest, shortest, or cheapest route. Consideration inplanning is also given to the effect of new traffic on existing streets, roads, and parking provisions.

The modern road

Road engineering


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Schematic cross section of a modern roadway.


Schematic cross section of a modern roadway.


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

ROAD DESIGN

ALIGNMENT AND PROFILE

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

In order to fully understand the design stage, a few standardterms must be defined (see figure). A traffic lane is the portion ofpavement allocated to a single line of vehicles; it is indicated onthe pavement by painted longitudinal lines or embedded markers.The shoulder is a strip of pavement outside an outer lane; it isprovided for emergency use by traffic and to protect thepavement edges from traffic damage. A set of adjoining lanes andshoulders 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 areestablished for each functional road type. The number of trafficlanes 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 passthrough a traffic lane. The width of lanes and shoulders, which must strike a balance between construction cost and drivercomfort, allows the carriageway width to be determined. Standards also specify roadside barriers or give the clear transversedistances needed on either side of the carriageway in order to provide safety in the event that vehicles accidentally leave thecarriageway. Thus it is possible to define the total right-of-way width needed for the entire road, although intersections will addfurther special demands.

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

PAVEMENT

Road traffic is carried by the pavement, which in engineering terms is a horizontal structure supported by in situ naturalmaterial. In order to design this structure, existing records must be examined and subsurface explorations conducted. Theengineering 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 fieldtests (typically by measuring deflection under a loaded plate or the penetration of a rod), by empirical estimates based on thesoil type, or by laboratory measurements. The material is tested in its weakest expected condition, usually at its highest probablemoisture content. Probable performance under traffic is then determined. Soils unsuitable for the final pavement are identifiedfor 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 pavement (as shown in the figure), the top layerof the pavement is the wearing course. Made of compacted stone,asphalt, or concrete, the wearing course directly supports thevehicle, provides a surface of sufficient smoothness and traction,and protects the base course and natural formation fromexcessive amounts of water. The base course provides therequired supplement to the strength, stiffness, and durability ofthe natural formation. Its thickness ranges from 4 inches (10centimetres) for very light traffic and a good natural formation tomore than 40 inches (100 centimetres) for heavy traffic and a poornatural formation. The subbase is a protective layer andtemporary working platform sometimes placed between the basecourse and the natural formation.

Pavements are called either flexible or rigid, according to their relative flexural stiffness. Flexible pavements (see left) have basecourses of broken stone pieces either compacted intoplace in the style of McAdam or glued together withbitumen to form asphalt. In order to maintainworkability, the stones are usually less than 1.5 inches


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Cross sections of modern pavements


Cross sections of modern pavements


in size and often less than 1 inch. Initially the bitumenmust be heated to temperatures of 300400 F(150200 C) in order to make it fluid enough to mixwith the stone. At the road site a paving machineplaces the hot mix in layers about twice the thicknessof the stone size. The layers are then thoroughly rolledbefore the mix cools and solidifies. In order to avoidthe expense of heating, increasing use has been madeof 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 providingadequate tire friction, generating minimal noise in urban areas, and giving suitable light reflectance for night-time driving. Suchsurfaces are provided either by a bituminous film coated with stone (called a spray-and-chip seal) or by a thin asphalt layer. Thespray-and-chip seal is used over McAdam-style base courses for light to moderate traffic volumes or to rehabilitate existingasphalt surfaces. It is relatively cheap, effective, and impermeable and lasts about 10 years. Its main disadvantage is its highnoise generation. Maintenance usually involves further spray coating with a surface dressing of bitumen. Asphalt surfacing isused with higher traffic volumes or in urban areas. Surfacing asphalt commonly contains smaller and more wear-resistant stonesthan the base course and employs relatively more bitumen. It is better able to resist horizontal forces and produces less noisethan a spray-and-chip seal.

Rigid pavements (see right) are made of portland

cement concrete. The concrete slab ranges inthickness from 6 to 14 inches. It is laid by a pavingmachine, often on a supporting layer that prevents thepressure caused by traffic from pumping water andnatural formation material to the surface through

joints and cracks. Concrete shrinks as it hardens, andthis shrinkage is resisted by friction from theunderlying layer, causing cracks to appear in theconcrete. Cracking is usually controlled by adding steelreinforcement in order to enhance the tensile strengthof the pavement and ensure that any cracking is fineand uniformly distributed. Transverse joints are

sometimes also used for this purpose. Longitudinal joints are used at the edge of the construction run when the wholecarriageway 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 smallquantities of lime, portland cement, pozzolana, or bitumen. The strength and stiffness of the mix are increased by the surfacereactivity of the additive, which also reduces the materials permeability and hence its susceptibility to water. Special machinesdistribute 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, ridingcharacteristics, traffic disruptions due to maintenance, ease and cost of repair, and the effect of climatic conditions. Often thereis 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 sometimesconducted to check that the construction process has achieved the designers intent. Designers typically consider the possibilityof structural failure resulting from a single overload and also from damage accumulating under the passage of many routineloads. Both of these types of failure are almost entirely caused by trucks.

DRAINAGE

Adequate drainage is the single most important element in pavement performance, and drainage systems can be extensive andexpensive. Drainage involves handling existing watercourses, removing water from the pavement surface, and controllingunderground water in the pavement structure. In designing the system, the engineer first selects the design stormthat is, themost 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 5years for a minor street carrying local traffic). The drainage system must be able to carry the storm water produced by thisdesign storm without flooding the roadway or adjacent property. In areas where land use is changing from agricultural toresidential 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 guttersand 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-offsurface 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 toprevent groundwater from seeping laterally into the pavement structure. In addition, a horizontal drainage layer is often insertedbetween base course and natural ground in order to remove water from the pavement structure and stop upward capillary


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movement of any natural groundwater. Underground drains can also be used to lower the groundwater level by both preventingwater entry and removing water that does enter the pavement structure.

FINANCING

The full design of a proposed road is analyzed with respect to its costs and its economic, social, and environmental effects. It mayalso be subjected to public review. This step can be lengthy, as new roads are usually popular with the traveling public butsometimes 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. Arterial roads and

highways, however, need a wider administrative and financial input in order to guarantee route continuity and uniformity. Sincethe 1920s the financing of roads has been largely transferred to the road user. A variety of taxes is employed: on fuel and oil, onroad usage, on vehicle purchase and ownership, on driver licensing, on truck mass and mass times distance traveled, on tire andaccessory 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.

CONSTRUCTION

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

In countries where labour is inexpensive and less skilled, traditional manual methods of road construction are stillcommonplace. However, the developed world relies heavily on purpose-built construction plant. This can be divided intoequipment for six major construction purposes: clearing, earthmoving, shaping, and compacting the natural formation; installingunderground drainage; producing and handling the road-making aggregate; manufacturing asphalt and concrete; placing andcompacting 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 rockcuts is commonly done with shovels, draglines, and mobile drills. Shaping the formation and moving earth from cuttings toembankments is accomplished with bulldozers, graders, hauling scrapers, elevating graders, loaders, and large dump trucks. Thematerial is placed in layers, brought to the proper moisture content, and compacted to the required density. Compaction isaccomplished with tamping, sheeps-foot, grid, steel-wheeled, vibrating, and pneumatic-tired rollers. Backhoes, back actors, andtrenchers are used for drainage work.

In order to avoid high haulage costs, the materials used for base course construction are preferably located near theconstruction site; it is economically impossible to use expensive materials for long lengths of road construction. The excavationprocess is the same as for rock cuts, although rippers may be used for obtaining lower-grade material. Crushers, screens, andwashers 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 concreteuniformly and to the required thickness, shape, and width (typically, one or two traffic lanes). The paving machine can slipformthe edges of the course, thus avoiding the need for fixed side-forms. As it progresses down the road, it applies some preliminarycompaction 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 filmof hot, fluid bitumen that is sprayed in sufficient quantity to fill voids, cracks, and crevices without leaving excess bitumen on thesurface. The surface is then sprayed with a more viscous hot bitumen, which is immediately covered with a layer of uniform-sizestone chips spread from a dump truck. The roadway is then rolled to seat the stone in the sticky bitumen, and excess stone islater cleared by a rotary broom.

MAINTENANCE

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

TRAFFIC MANAGEMENT

Road operation


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

TRAFFIC CONTROL

The marking of roadway surfaces with painted l ines and raised permanent markers is commonplace and effective, despite highmaintenance costs and visibility problems at night, in heavy traffic, and in rain or snow. A solid line is a warning or instruction notto cross, and a broken line is for guidance. Thus, solid lines indicate dangerous conditions (such as restricted sight distancewhere overtaking would be dangerous), pavement edges, stop lines, and turning lanes at intersections; broken lines indicateinterior 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 asregulatory 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 verticalclearances, 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 coloursfor instance, the red octagon used for the stop sign, the triangle for warning signs, thegreen rectangle with white lettering for freeway directional signs (commonly mounted over the roadway and of large size foreasy 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 internationalsystems 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 systemsparticularly at conventional intersectionsaccommodating large traffic volumes, where they allocate right-of-way to the various traffic streams. They can also meter trafficentering access lanes onto busy freeways or to indicate the lanes to use on two-way roads. Simple traffic signals work on presettiming plans that vary with the time of day. More advanced traffic-actuated signals automatically monitor the traffic streams andallocate right-of-way accordingly. Signals can also be linked to a computer so that traffic traveling along a major route can receivea 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 threecategories. First are those applying to the vehicle and the driver, such as vehicle and driver registration, vehicle safety equipmentand 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 whichside of the road to use, maximum speeds, right-of-way, and turning requirements. Third are those regulations that apply tolimited 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 giveright-of-way to vehicles on their right. However, in practice the stop and yield (or give-way) signs have commonly supplanted theright-of-way rule. Speed limits vary greatly with jurisdiction, ranging from walking pace in a Dutch woonerf, or shared street, tounrestricted on a German autobahn. Speed limits are commonly reduced on roads approaching residential, shopping, or schoolareas 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. Forinstance, one-way streets in congested urban areas may provide safer driving conditions and increase the traffic-carryingcapacity 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.

SAFETY

Traffic police (or road patrols or highway police) help improve road safety and traffic flow by enforcing driving regulations. Theyalso regulate traffic at the scene of an accident and investigate accidents. Traffic enforcement has been aided by the use oftechnologycameras, radar, video, and inductance loopsto 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 apatrol car. In crash investigations, the speed of the cars is determined by the length of skid marks. Another key factor in roadaccidents is the influence of alcohol and drugs. Tests for intoxication are now widely conducted; the most common is the breathtest, in which the driver blows into a device that analyzes the alcohol content of the breath and indicates the approximate bloodalcohol level. Many authorities believe that 0.50 gram of alcohol per litre of blood is a realistic limit for ordinary motorists, butthat zero levels should be demanded for critical operators such as drivers of public transport vehicles.


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

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

Fred J. Benson

Maxwell Gordon Lay

"roads and highways". Encyclopdia Britannica. Encyclopdia Britannica Online.Encyclopdia Britannica Inc., 2016. Web. 18 Feb. 2016.


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