Road

• 1. Introduction

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  A road is a thoroughfare from one place to another, primarily used for the movement of traffic. That plain definition hides a question that engineers, lawmakers, and lexicographers still argue over. What exactly counts as a road? In urban design, the word "road" is not interchangeable with "street", even though most people treat the two as the same. A street exists for local access. A road exists to move you somewhere else. There is even a hybrid that planners call a stroad, which combines the features of both. The oldest known paved road was built in Egypt between 2600 and 2200 BC. The newest are still being cut into mountainsides today. Between those two points lies a story of timber causeways, Persian highways, Roman stone, and a fourth-power law that explains why a single truck can outweigh thousands of cars. This is the story of how humans decided where to put the ground beneath their wheels, and what it takes to keep it there.

• 2. Defining The Line

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  The Organization for Economic Co-operation and Development defines a road as "a line of communication (travelled way) using a stabilized base other than rails or air strips open to public traffic, primarily for the use of road motor vehicles running on their own wheels". That definition deliberately includes bridges, tunnels, junctions, interchanges, and toll roads, but excludes cycle paths. The Eurostat, ITF and UNECE Glossary for Transport Statistics echoes this, folding in gravel roads and other stabilized surfaces while again leaving out dedicated cycle lanes. The 1968 Vienna Convention on Road Traffic takes the broadest view of all, calling a road the entire surface of any way or street open to public traffic. National law multiplies the disagreement. In Australia, the National Transport Commission Regulations 2006 define a road as an area open to the public and developed mainly for driving or riding motor vehicles. New Zealand stretches the term further still, where common law sweeps in beaches, publicly accessible car parks, river beds, wharves, and bridges, regardless of whether the public has a legal right of access. The United Kingdom hinges its definition on the word "highway", which the relevant Act never formally defines. A 1984 ruling held that the land over which a public right of way exists is a highway, and that the presence or absence of a made road has nothing to do with the distinction. The United States keeps its split simpler, dividing public roads open to general use from private roads under private control.

• 3. From Animal Trails To Tar

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  The Icknield Way may be one of the oldest kinds of road of all, a natural line that human and animal both chose to follow. The claim that the first paths were simply animal trails has never been universally accepted, since animals do not always keep to constant routes. By about 10,000 BC, human travelers were using rough pathways. Corduroy roads, built from logs, date to 4000 BC in Glastonbury, England. The Sweet Track, a timber causeway in England, is one of the oldest engineered roads ever found. Tree-ring dating placed its construction in the winter of 3807 BC or the spring of 3806 BC. It held the title of oldest road in the world until a 6,000-year-old trackway turned up in Plumstead, London, in 2009. In 500 BC, Darius the Great launched an extensive road system for the Achaemenid Empire, including the Royal Road that connected Sardis to Susa and remained in use long after Roman times. From about 312 BC, the Roman Empire laid straight stone roads across Europe and North Africa to support its military campaigns. At its peak, Rome was linked by 29 major roads covering 78,000 kilometers of paved surface. In the 8th century AD, the Arab Empire built many roads of its own. The most sophisticated were in Baghdad, paved with tar drawn from regional oil fields through the chemical process of destructive distillation.

• 4. Tolls, Tourism, And Motorways

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  The Highways Act 1555 in Britain shifted responsibility for road upkeep from government to local parishes, which produced a poor and uneven state of roads. The fix arrived around 1706 with the first turnpike trust, built to make good roads and collect tolls from passing vehicles. Eventually Britain held roughly 1,100 trusts running some 36,800 kilometers of engineered road. The Rebecca Riots in Carmarthenshire and Rhayader, from 1839 to 1844, fed into a Royal Commission that ended the turnpike system in 1844, just as the railway network was rising. In the late 19th century, roading engineers began carving separate lanes alongside roadways for cyclists. From the start of the 20th century, roads were increasingly built for tourism and to create jobs. The Great Dolomite Road drew tourists, while the coastal Strada Costiera between Duino and Barcola, Italy, finished in 1928, was built mainly to put people to work. The Autostrada dei Laghi in Italy was the first controlled-access highway in the world, linking Milan to Lake Como and Lake Maggiore. Devised by Piero Puricelli and inaugurated in 1924, it carried only one lane in each direction and no interchanges. Parts of it now form the A8 and A9 motorways.

• 5. Building The Roadbed

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  Subgrade is the native material lying beneath a constructed road, and everything above it is a fight against geography. Road construction demands an engineered continuous right-of-way, with grades low enough for travel and obstacles overcome by digging, blasting, embankments, bridges, and tunnels. After design and approval, a surveyor sets out the alignment, staking the radii and gradient to suit the natural ground and minimize cut and fill. Topsoil and vegetation are stripped and stockpiled for later rehabilitation, while protected trees keep the area inside their drip line clear of equipment. Earthwork proceeds through excavation, filling, compacting, and trimming, with fill generally compacted to 90 to 95 percent relative compaction. Engineers favor the intact rock structure as an ideal road base, so blasting is used sparingly. Depressions are filled by the compacted layer method, spreading a layer of fill and compacting it under saturated conditions until the grade is reached. General fill must be free of organics, meet minimum California bearing ratio results, and carry a low plasticity index, while each compacted layer is proof rolled to check for deformation or spring. Geosynthetics such as geotextiles, geogrids, and geocells serve four functions in a road: separation, reinforcement, filtration, and drainage. The finished surface may be paved or left as gravel, then fitted with traffic signs, crash barriers, and surface markings.

• 6. The Fourth-Power Law

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  A typical tractor-trailer weighing 80,000 pounds does about 7,800 times more damage to a road than a 2,000-pound-per-axle passenger vehicle. That startling ratio comes from the AASHO Road Test, a series of experiments in the late 1950s. It found that the effective damage to a road is roughly proportional to the fourth power of axle weight. So a few heavy axles, not the daily flood of cars, drive most of the wear. Roads still deteriorate on their own through frost heaves, thermal cracking, and oxidation, and potholes form from rain damage and braking. Pavements are designed for a set service life. In parts of the United Kingdom the standard design life for new bitumen and concrete is 40 years, with maintenance planned at 10, 20, and 30-year milestones. Some asphalt is built as perpetual pavement, meant to last beyond 50 years. Many concrete pavements laid since the 1950s have outlived their design lives, and Chicago's Wacker Drive, a two-level downtown roadway, is being rebuilt for a 100-year service life. Maintenance crews fight back with thin asphalt overlays, crack sealing, fog sealing, diamond grinding, and waterblasting, which lifts the top bitumen layer to raise surface friction. Faulted concrete can be repaired with a dowel bar retrofit, cutting slots at each joint and filling them with dowel bars and patching material, extending the pavement's life by 15 years.

• 7. Voids, Grout, And Sealant

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  Distress on a concrete road often begins out of sight, where voids open beneath the slabs near cracks or joints. Surface water infiltration causes them, through pumping, consolidation, subgrade failure, and bridge approach failure. Slab stabilization is a non-destructive cure, pumping a cementitious grout or polyurethane mixture through holes drilled in the slab to fill those voids and displace free water. It restores support and reduces deflection under load, but it cannot correct depressions, increase structural capacity, or stop erosion. Ground penetrating radar pulses electromagnetic waves into the pavement and graphically displays the reflected signal, revealing voids and defects. The epoxy/core test drills a 25 to 50 millimeter hole, pours in dyed two-part epoxy, and lets technicians see whether it sticks to the core. Deflection testing is best done at night, since cooler temperatures open the joints, diminish aggregate interlock, and push load deflections to their highest. Joint sealing tackles a related leak, minimizing infiltration of surface water and incompressible material into the joint system. Resealing runs through old sealant removal, shaping and cleaning the reservoir, installing the backer rod, and placing the sealant. The materials on offer include hot pour bituminous liquid, silicone, and preformed compression seals.

• 8. The World's Largest Network

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  The United States holds the largest road network of any country, with 4,050,717 miles as of 2009. India runs second with 4,689,842 kilometers as of 2013, China third with 3,583,715 kilometers as of 2007, and Brazil fourth with 1,751,868 kilometers as of 2002. Looking only at expressways, China's National Trunk Highway System grew from 45,000 kilometers at the end of 2006 to 130,000 kilometers by 2017, while the United States held 90,000 kilometers in 2005. These networks knit whole continents together, yet they do not quite meet. The Darién Gap is the only break in the Pan-American Highway, separating the road systems of North and South America. Eurasia and Africa join across the Sinai Peninsula, and Bahrain is the only island country tied to a continental network by road, through the King Fahd Causeway to Saudi Arabia. Connection is far from universal. As of 2014, only 43 percent of rural Africans had access to an all-season road, and all 25 communities of Nunavut sit disconnected from each other and from the main North American network. Roads also carry costs beyond pavement. Their runoff is a major source of nickel, copper, zinc, cadmium, lead, and polycyclic aromatic hydrocarbons, while on-road transportation greenhouse gas emissions are, scientists say, the largest single cause of climate change. Even so, construction pushes outward, reaching Mêdog County by a road completed in 2013, with proposals on the table for a Bering Strait crossing to join Eurasia-Africa and North America directly.