I.

Introduction

Road, path established over land for the passage of vehicles, people, and animals. Roads provide dependable pathways for moving people and goods from one place to another. They range in quality from dirt paths to concrete-paved multilane highways.

Roads are used by various forms of transportation, such as trucks, automobiles, buses, motorcycles, and bicycles. Roads allow trucks to move goods from points of production, such as fields and factories, directly to markets and shopping centers. Private individuals rely on roads for safe and efficient automobile, motorcycle, and bicycle travel. Fire departments, medical services, and other government agencies depend on an organized system of roads to provide emergency services to the public in times of need.

The earliest roads evolved from animal paths and served as trails for early hunters. Paths eventually grew around primitive settlements, and as trade grew, longer routes were developed to transport food and other important materials. The use of wheeled vehicles encouraged construction of better roads. The roads built by the ancient Romans were carefully planned and solidly constructed.

Modern methods of road construction were first developed in the 18th century. Innovations of the time included waterproof surfaces and better drainage systems. Modern engineers make use of a variety of materials and construction techniques to build roads that can handle the high volumes and stresses of modern automobile and truck traffic.

II.

Types of Roads

There are many different types of roads, from multilane freeways and expressways to two-way country roads. One important quality of a road is known as control of access. This term describes how vehicles are allowed to enter and exit a road. By controlling access to a road, the road can support more traffic at higher speeds. Roads can be classified into three broad categories: highways, urban or city streets, and rural roads. Each type of road controls access to different degrees. Each type also differs in location, the amount of traffic it can safely support, and the speed at which traffic can safely travel.

A.

Highways

Highways are high-speed roads designed to connect major cities. There are many different types of highways. Highways differ primarily in the amount of access control they have and, therefore, in the amount of traffic they are designed to carry.

Highways with fully controlled access can handle the most traffic and are built to the highest construction standards. Interstate highways, freeways, and expressways are examples of fully controlled-access highways. Vehicles that enter or exit these types of highways can do so only at certain points along the highway, generally by using special entrance and exit ramps. The ramps allow vehicles to access the road without disturbing the flow of traffic. Incoming vehicles must merge with flowing traffic, and vehicles leaving the highway use exit ramps that guide them off the highway without blocking the traffic behind. Intersections with other roads are avoided by using either bridges known as overpasses to carry one roadway over another or short tunnel-like structures called underpasses to carry one roadway under another. Finished strips called shoulders on the edges of highways allow drivers of disabled vehicles to make repairs or await assistance without blocking traffic.

Highways with fully controlled access generally have two or more lanes for each direction of travel and often include medians (dividers in the middle of the road) to separate traffic moving in opposite directions. In Europe, highways with fully controlled access are called motorways, motor-routes, or autobahns.

Some highways offer only partial control of access. These types of highways handle less traffic than do highways with fully controlled access. Highways with partially controlled access may intersect other roads at the same level (called at-grade), rather than using overpasses or underpasses. Vehicles can enter highways with partially controlled access at intersections rather than using ramps. However, the right-of-way is often given to one direction of travel, rather than requiring all traffic to stop at the intersection (see Traffic Control). Giving the right-of-way to one direction of traffic helps keep traffic moving at higher speeds, although typically not at speeds as high as those on a highway with fully controlled access. One benefit of highways with partially controlled access is that they are much cheaper to construct than highways with fully controlled access. Many U.S. and state highways are roads with partially controlled access.

B.

Urban Streets

Urban streets, which cover cities, towns, and most suburbs, allow vehicles to access properties such as homes and businesses. Urban streets are used by private motor vehicles, public transportation, bicycle traffic, and pedestrians. Urban streets also accommodate underground public-utility facilities, such as electrical wiring, water and sewage pipes, and telecommunications lines. In addition, these streets must often be built around existing buildings and other barriers, such as parks and rivers. Rather than shoulders, urban streets usually have raised edges called curbs, which provide a barrier between the street and the adjoining property or sidewalk.

Urban streets are generally two-way paved roads that intersect each other frequently, allowing a high degree of access but at slow speeds. Traffic lights and signs help regulate the movement of vehicles along these streets and control the access to some streets. Urban streets are usually classified as one of three types: local, collector, and arterial. These designations are based on the amount of traffic each type is designed to carry.

Most urban streets are residential, or local, streets that allow vehicles to access public and private property. These streets make up 70 percent of total urban-road mileage but handle only 14 percent of all urban traffic. Collector streets convey traffic from residential streets to larger roads called arterials. Arterials are used to get quickly from one point to another and can accommodate high volumes of traffic. In large cities, arterials are often similar to highways in construction even though they are located within city limits. Although arterials account for only 17 percent of the urban-road mileage, they account for most of the traffic in cities.

C.

Rural Roads

Rural roads are found in areas of the country outside cities, towns, and suburbs. Rural roads are also classified as local, collector, or arterial roads. Because of the light traffic and extensive mileage on these roads, design standards are relatively low. Rural roads may or may not have shoulders.

Local rural roads, which make up 69 percent of all rural roads, provide access to individual properties and farms. In many areas, local rural roads may be unpaved gravel roads. Two travel lanes, one for each direction of travel, can accommodate normal traffic on local rural roads. Rural collector roads, which make up 23 percent of all rural roads, carry moderate traffic coming in from a number of local roads. Collector roads have more and faster traffic than local roads and are constructed to a higher standard of quality.

Rural arterial roads provide a high-speed network for heavier traffic between major towns in rural areas. They range from two-lane roadways to multiple-lane highways. These roadways are designed to carry more traffic than collector roads can and are usually constructed to even higher standards than rural collector roads are. Rural arterial roads often have finished shoulders and may have more than one lane in each direction. These roads make up 8 percent of total rural-road mileage.

III.

Roadway Engineering

To support heavy vehicles moving at high speeds, a modern road is made up of several layers. Each layer helps the layers above it support the weight and pressure of moving traffic. Roads that carry more traffic at higher speeds, like highways, are built to stronger standards than roads that carry less traffic, such as rural collector roads. The number of layers in a road often depends on the intended use of the road, but generally roads have three distinct layers. From bottom to top, the layers are the roadbed, the base course, and the wearing course.

A.

Roadbed

The roadbed is the very bottom layer of a road. Natural soil is the most common roadbed material. The roadbed is shaped to make a smooth, level surface that will support the layers built over it. Engineers use bulldozers and other construction equipment to distribute soil evenly along the roadbed. If a road is planned through an area where the natural landscape is uneven, soil can be removed or filled in as needed to obtain a level surface.

B.

Base Course

The base course rests directly on top of the roadbed and is often made up of compacted gravel. If the roadbed material itself is suitable, it may be treated, or stabilized, and used as the base. Soil can be stabilized by adding or mixing materials such as calcium chloride, bituminous material, lime, or portland cement to the soil. For very inexpensive, light-traffic roads, stabilized soil alone can suffice as the finished road surface. Drainpipes are usually installed within the base course to control rain and moisture drainage. Without adequate drainage, roads may buckle or collapse as water swells the ground underneath. Some roads include a second base layer, called the top course, for extra support.

C.

Wearing Course

A road’s top layer, which directly supports moving vehicles, is called the wearing course. It is made of a solid layer of pavement and is designed to be smooth and to withstand erosion from traffic and weather. Two main types of pavements are used—bituminous, or flexible, pavement and concrete, or rigid, pavement. Bituminous pavement is cheaper and easier to construct, but it requires more maintenance. Concrete pavement lasts for a very long time with minimal upkeep but is much more expensive and time-consuming to build.

C.1.

Bituminous Pavement

Bituminous pavements are made with by-products of petroleum, such as asphalt. Weather and seasonal changes can cause roadways and the earth below them to rise or fall slightly. As these natural shifts occur, bituminous pavements allow the road surface to bend or flex slightly without breaking.

Bituminous material softens when heated and can be prepared and applied in a wide range of concentrations. Thin layers of bituminous material are sometimes applied with a pressure sprayer to the base course or the top course. The bituminous material seeps into the crushed rock and penetrates the tiny spaces between the compacted rocks, binding the aggregate together. To provide traction for moving traffic, a thin layer of aggregate may then be spread over the bituminous material and compacted.

Asphalt, a thick bituminous material, can be used directly as a pavement. Asphalt can be applied in thin overlays less than 2 cm (0.75 in) deep or in layers several centimeters deep, depending on the type of surface and its purpose. Asphalt is commonly mixed with aggregates for added strength and traction. The asphalt and aggregates are usually mixed and heated at a central location. The material is then transported to the construction site, where it is spread directly over the base course or top course and compacted. Asphalt surfaces are fairly easy to construct and repair.

C.2.

Concrete Pavement

Concrete, or rigid, pavements are made from a mix of portland cement and aggregate. Concrete pavements have a long life and require little maintenance. Concrete is generally laid as a single thick layer directly over a base course. For heavily traveled roads, concrete layers can range in thickness from 20 to 36 cm (8 to 14 in). Concrete is usually laid in long sections or slabs of varying length. Metal bars or dowels inserted into the edges of the slabs help connect the joints where one slab ends and another begins.

Concrete is a strong material and can withstand compression, but it has poor tensile strength (resistance to being pulled). When the ground underneath expands and contracts from seasonal or weather changes, the concrete becomes prone to cracking. Cracks can occur at or near the joints where concrete slabs meet or on the slabs themselves. Deep cracks can allow the broken concrete slabs to move upward or downward, creating an uneven road surface. Metal bars or dowels inserted between the slabs help hold the slabs together. Reinforced concrete contains steel bars or mesh imbedded within the concrete layer. The steel helps hold concrete together over time, even if cracks occur. Unreinforced concrete may be used when cost is a factor, or where weather conditions are more mild. Unreinforced slabs have several shallow grooves cut into them, allowing the concrete to crack at defined points. The cracked slabs are kept in place by pressure and by the grainy texture of the concrete itself.

IV.

Road Planning and Administration

In the United States, the state governments are responsible for constructing and maintaining interstate highways, U.S. highways, and state highways. Local governments—counties, cities, and townships—are generally responsible for the rest of the road system. In Canada, the provincial governments share the obligation for road planning and construction with local cities and municipalities. Many groups, including road users, business owners, the general public, and environmental groups, have interests and concerns in regard to road construction. The various government agencies involved attempt to balance these concerns when planning a road system.

In the first quarter of the 20th century, roads in the United States and Canada were often little more than dirt paths. These primitive roads were impassable after heavy rains, and automobiles would become stranded in thick mud. At the time, highway agencies concentrated on little more than paving existing rural roads. A more organized program of planning in the United States began in earnest in 1934, when the U.S. Congress passed the Hayden-Cartwright Act. This act provided federal aid for state highway departments to plan in-depth studies. By 1940 most states were examining such factors as road conditions, volume and nature of highway traffic, highway lifespans, and future highway needs. Later highway acts expanded funding to include considerations of state policies and state, regional, and local issues.

Today, federal funding and planning guidelines in the United States are administered by the Federal Highway Administration (FHWA) of the U.S. Department of Transportation. The FHWA distributes funds according to formulas established by Congress and sets general policies for design, construction, operations, and maintenance. The agency does not, however, provide any of these functions. Its basic goal is to provide for a safe and consistent national system of highways. The state departments of transportation or local governments actually plan, construct, and maintain the highway systems.

Local highway agencies include those operated by counties, cities, and townships. Cities operate primarily with local funds, typically under a mayor or city manager. A city’s public works department may be responsible for most road functions, or there may be separate departments for design, construction, and maintenance. Counties and townships, as well as small towns and villages, typically have less complex systems.

In Canada, the majority of roads are constructed and funded by individual provinces and cities, with the federal government playing a minor role. Provincial and municipal governments responded to increases in motor vehicle use during the early part of the 20th century by expanding and improving the road network already in existence. At the time, individual cities were responsible for road construction and maintenance. As automobile and truck use expanded, the provincial governments began to establish highway departments and allocate funds for highway construction.

The Canadian federal government does not play as large a role in highway planning and funding as does the U.S. government. However, in the late 1980s the Canadian federal government instituted the National Highway System, a network of over 24,000 km (15,000 mi) of roads connecting major cities and ports. The federal government also supports highways located on federal property, such as national parks, and maintains the Canadian portion of the Alaska Highway.

In planning roadway improvements, engineers collect data about current roadway use and planned new uses of roads, as well as other information, such as planned construction or nearby development. They also examine the land and decide where bridges or viaducts may be needed to carry the roadway over obstacles like rivers or other roads. Engineers then use computers to simulate or model new roadway designs. Depending on available funding, projects are planned for construction based on the priorities of the highway department and of the state, provincial, or local government in charge.

V.

History of Road Construction

Roads originated for economic reasons, related to the need to move food and other goods from one point to another. Early transportation focused on moving food from a hunt or a harvest to the places where people lived. Trails evolved from prehistoric animal paths, and early humans carried or dragged their loads along these paths. As humans learned to domesticate animals, they transferred their loads to pack animals, such as horses, mules, camels, llamas, elephants, and dogs.

The discovery and use of the wheel was undoubtedly a driving force in building and improving roads. Crude roads were in use in Mesopotamia in about 3000 bc. Italy was connected to Denmark by a roadway as early as 2000 bc.

During the period from 1900 to 300 bc, four trade routes, known as amber roads, ran across central and eastern Europe. Amber was an important ingredient in primitive and medieval medicine and was also used for statuary and jewelry. One of the first preplanned roads was the Persian Royal Road, built by Darius I in 500 bc in what is now Iran. The Royal Road was about 2,400 km (about 1,500 mi) long and stretched throughout ancient Persia. The road was constructed for royal use, and it allowed Darius to keep informed, to convey orders, and to transport goods needed by the royal court. The road followed the shortest and most efficient route. It bypassed some of the largest towns—in part, for the sake of following the shortest route, but also because large towns had the potential to rebel. An uprising in a town near the road could have threatened the security of the route or prevented troops from being dispatched.

The Chinese Silk Road, established around 100 bc, was a series of land routes connecting ancient Rome and China. Caravans of traders traveled the Silk Road, exchanging goods and ideas. The road extended about 6000 km (about 4000 mi).

The ancient Romans built an extensive and durable system of roads to serve their needs. The Roman road-building era began in 312 bc. The roads provided economic and military access from Rome to distant parts of its far-flung empire. The first road constructed was the Appian Way, which led from Rome to Brundisium (now Brindisi), a port in what is now southern Italy. The Appian Way was the main route to Greece, and it ran over 560 km (350 mi). A second road, from Rome to Naples, provided the first stage of the route used by troops headed to Africa. Roman advances in road-building techniques included preparation of foundation soils and base courses, brick paving, and, most importantly, provision for adequate drainage.

After the fall of Rome in the 5th century ad, and throughout the Middle Ages, there was little government-planned road building in Europe. Regional rulers maintained local roads, but road conditions varied widely.

In the western hemisphere, road building evidently reached a high state of development in the 15th and 16th centuries under the Aztecs in Mexico, the Maya in Central America, and the Incas in South America. The Spanish conquistadors, in turn, constructed a series of caminos reales (royal roads) during the 16th and 17th centuries. These were the first American highways, and they stretched from Veracruz, Mexico, northward to San Francisco, California, and Santa Fe, New Mexico, and eastward to Saint Augustine, Florida.

In the middle of the 17th century, the French government instituted a system of enforced local labor on the roads. About 24,000 km (about 15,000 mi) of main roads were built by this method. At about the same time, the English Parliament began granting franchises to private companies for the maintenance of public roads, allowing the companies to charge tolls for the use of the roads. These toll roads were called turnpikes, named for the turning of a pole, or pike, to permit entry of a vehicle after the driver paid the toll. Over 1,000 turnpike companies maintained 32,000 km (20,000 mi) of roads in England in the 1830s. After this time, increased competition from railroads began to make the turnpike systems less profitable for the operators.

Important advances in road-building technology came in the 18th century. Pierre Trésaguet in France and Thomas Telford and John Loudon McAdam in Scotland introduced improved methods for creating roads made of stone. McAdam’s method stressed the need to keep the subsoil dry by including adequate drainage and waterproof covering. The term macadam originally referred to McAdam’s road-surface design, in which crushed or broken stone was mechanically locked together by rolling a large weight over the surface. Macadam was later used to describe other types of bound surfacing, including water-bound macadam, in which the crushed stone is cemented together with an application of water. In penetration macadam, bituminous material sprayed on the compacted rock binds the rock together.

In the American colonies and in the settlements of Canada, primitive roads following Native American and animal trails connected trading posts and forts. Fur trappers in Canada also blazed many trails inland. The inland posts, located away from direct waterways, depended on these roads to obtain supplies from larger communities. Seacoast communities were also linked for protection and communication. The first colonial road of importance in the United States was the Boston Post Road, connecting Boston, Massachusetts, with New York City. This road evolved gradually—it began as a rough path and then became a defined roadway paved first with wooden logs and then with flat wooden planks. Many other roads grew to connect cities along the coast. Inland trails were generally much more primitive. The Wilderness Road, blazed by Daniel Boone in 1775, was a crude pathway charted through the Cumberland Gap in Tennessee and Kentucky that became an important route across the Allegheny Mountains.

Financing of roads in North America has switched between private ventures and government control. In the late 1700s, turnpikes were introduced in the United States as profit-making ventures. Early U.S. turnpikes were local government projects, but later turnpikes were privately operated business undertakings. Many turnpikes failed to make a profit, and most returned to county control by 1850. Federal financing of road construction began in 1806 when the U.S. Congress authorized construction of the National Road, also called the Cumberland Road. It stretched west from Cumberland, Maryland, and eventually reached Vandalia, Illinois. The National Road helped expand trade and settlement west of the Appalachian Mountains.

At the time of the National Road, road quality was poor and construction proceeded slowly. Improvements in materials came with the introduction of asphalt paving in 1870, but the biggest motivation for road improvement came with the popularity of the automobile at the start of the 20th century. A government survey of public roads done in 1904 showed that about 3 million km (about 2 million mi) of all roads in the United States were unpaved. Only 248,000 km (154,000 mi) of these roads were surfaced. Automobile use increased the wear and tear on these roads, and efforts increased nationwide to improve road conditions.

Local governments administered most roads at the time. State governments soon formed highway agencies to take over and organize improvement efforts. In 1912 the federal government began funding one-third of the cost of roads on which mail was carried. The Federal Road Act of 1916 provided even more federal participation in state road improvements.

World War I (1914-1918) temporarily halted highway construction, but the rapidly expanding use of the automobile, along with increased truck traffic, brought on a growing demand for even better roads. In 1919 a tax was added to gasoline to help provide federal financing. By 1930 the principal population centers of the United States were connected by a system of all-weather, two-lane roads. However, much of the road construction was not adequate for the growing volume of traffic, especially truck traffic with its increasingly heavy loads and high speeds.

The early ideas for a network of federal interstate highways in the United States took form in the 1930s, during Franklin D. Roosevelt's presidency. Roosevelt’s New Deal programs helped pay for the Pennsylvania Turnpike, a toll road that opened in 1940. Roosevelt envisioned three east-west and three north-south transcontinental toll highways, but debate over whether the roads should be free or toll delayed the establishment of a national system.

By the end of World War II (1939-1945), many roads were congested and deteriorating. After the war, modern highway construction resumed nationwide with the construction of numerous four-lane and six-lane roadways with limited access. Many of these roadways were built as toll roads, patterned after the Pennsylvania Turnpike.

The passage of the Federal-Aid Highway Act of 1956 was perhaps the most significant event in U.S. road-building history. This act authorized and funded the construction of almost 70,000 km (more than 43,000 mi) of interstate highways. President Dwight D. Eisenhower, a big supporter of the program, believed that government spending on public works and defense was the way to prevent another depression. Eisenhower had personally experienced a transcontinental trip by car and truck while in the U.S. Army in 1919. The slow, 62-day crossing made a lasting impression on the young officer. Another positive influence on Eisenhower’s desire to improve road systems was the efficient German network of superhighways called autobahns that he encountered during World War II.

Construction of the interstates began in the 1950s and continued through the 1990s. In 1991 the U.S. Congress named the interstate highway system the Dwight D. Eisenhower System of Interstate and Defense Highways. Private, commercial, and government vehicles use the interstate highways. This network of roads saves the United States billions of dollars each year in time and transportation costs.

In the 1950s and 1960s, Canada also began major roadway construction to serve the rise in vehicle ownership that occurred after the end of World War II. Work began on construction of what became known as the MacDonald-Cartier Freeway, serving southern Ontario. The Trans-Canada Highway stretches across Canada, linking St. John’s, Newfoundland and Labrador, on the Atlantic coast with Victoria, British Columbia, on the Pacific coast, with ferries plying parts of the route. The highway was formally opened in 1962. It passes through every provincial capital and also through several other major cities in the southern part of Canada.

Civil engineers continue to research ways of designing and building the most efficient and cost-effective roads. New types of road surfacing make roads more durable and easier to construct. Computers play a large role in helping engineers experiment with different road designs and in anticipating how factors like population growth affect road and highway transportation.

See Alaska Highway; Inter-American Highway; Pan-American Highway.