This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (September 2009)

A high-capacity interchange: the Judge Harry Pregerson Interchange in Los Angeles, California, United States.

The High Five in Dallas, Texas, USA: an extreme example of interchange design. This is a complicated five-level stack interchange due to the proximity of frontage roads.

In the field of road transport, an interchange is a road junction that typically uses grade separation, and one or more ramps, to permit traffic on at least one highway to pass through the junction without directly crossing any other traffic stream. It differs from a standard intersection, at which roads cross at grade. Interchanges are almost always used when at least one of the roads is a limited-access divided highway (expressway or freeway), though they may occasionally be used at junctions between two surface streets.

Terminology[link]

Note: The descriptions of road junctions are for countries where vehicles drive on the right side of the road. For countries where driving is on the left the layout of the junctions is the same, only left/right is reversed.

• A freeway junction or highway interchange (in the U.S.) or motorway junction (in the UK) is a type of road junction, linking one motorway to another; to other roads; or sometimes to just a motorway service station. On the UK motorway network, most (but not all) junctions with other roads are numbered sequentially. In the U.S., interchanges are either numbered according to cardinal interchange number, or by mileage (typically the latter in most states).
• A highway ramp (as in exit ramp and entrance ramp) or slip road is a short section of road which allows vehicles to enter or exit a freeway (motorway).
• A directional ramp always tends toward the desired direction of travel. This means that a ramp that makes a left turn exits from the left side of the roadway (a left exit). Left directional ramps are relatively uncommon as the left lane is usually reserved for high-speed through traffic. Ramps for a right turn are almost always right directional ramps. Where traffic drives on the left, these cases are reversed.^[1]
• A non-directional ramp goes in a direction opposite to the desired direction of travel. Many loop ramps (as in a cloverleaf) are non-directional.^[1]
• A semi-directional ramp exits a road in a direction opposite from the desired direction of travel, but then turns toward the desired direction of travel. Many flyover ramps (as in a stack) are semi-directional.^[1]
• A U-turn ramp leaves the road in one driving direction, turns over or under it and rejoins in the opposite direction. The use of u-turn ramps is common in less wealthy countries.

Off-ramp accessed from collector lanes along Highway 401 in Toronto, Ontario, Canada.

Weaving is an undesirable situation in which traffic veering right and traffic veering left must cross paths within a limited distance, to merge with traffic on the through lane.

The German Autobahn system splits Autobahn-to-Autobahn interchanges into two types: a four-way interchange, the Autobahnkreuz (AK), where 2 motorways cross, and a three-way interchange, the Autobahndreieck (AD) where two motorways merge.

Complete interchanges[link]

A complete interchange has enough ramps to provide access from any direction of any road in the junction to any direction of any other road in the junction. Barring u-turns, a complete interchange between two freeways requires eight ramps, while a complete interchange between a freeway and another road (not a freeway) requires at least four ramps. Using u-turns these numbers can be halved to four and two respectively, by making cars that want to turn left pass by the other road first, then make a u-turn and turn right. Depending on the interchange type and the connectivity offered other numbers of ramps may be used. For example, if a highway interchanged with a highway containing a collector/express system, additional ramps could be used to strictly link the interchanging highway with the collector and express lanes respectively. For highways with high occupancy vehicle/HOV lanes, ramps can be used to service these carriageways directly, thereby increasing the number of ramps used.

Between two motorways[link]

Four-way interchanges[link]

Cloverleaf interchange[link]

A typical cloverleaf interchange in Ohio, United States.

Cloverleaf

A cloverleaf interchange is typically a two-level, four-way interchange where all left turns are handled by loop ramps (right turns if traveling on the left). To go left, vehicles first cross over or under the targeted route, then bear right onto a sharply curved ramp that loops roughly 270 degrees, merging onto the interchanging road from the right, and crossing the route just departed.

The major advantage of cloverleafs is that they require only one bridge, which makes such junctions inexpensive as long as land is plentiful. A major shortcoming of cloverleafs, however, is weaving (see definition above), and the consequent low capacity of this design.

Cloverleafs also use a considerable area of land, and are suitable for countries such as the United States and Canada (mainly parclos, or partial cloverleaf interchanges). In Germany, the standard design is to separate all turning traffic into a parallel carriageway to minimize the problem of weaving. Collector and distributor roads are similar, but are usually separated from the main carriageway by a divider, such as a guard rail or Jersey barrier.

Cloverleafs are more often found along older highways, in rural areas, and within cities with low population densities.

Stack interchange[link]

A multi-level stack interchange in Shanghai, China.

Four-level stack

A stack interchange is a four-way interchange whereby left turns are handled by semi-directional flyover/under ramps. To go left (right in countries with left-hand drive), vehicles first turn slightly right (on a right-turn off-ramp) to exit, then complete the turn via a ramp which crosses both highways, eventually merging with the right-turn on-ramp traffic from the opposite quadrant of the interchange. A stack interchange, then, has two pairs of left-turning ramps, of which can be stacked in various configurations above or below the two interchanging highways.

Stacks do not suffer from the problem of weaving but require massive construction work for their flyovers. A standard stack interchange includes roads on four levels. This is not only expensive but also creates an eyesore among local residents, leading to considerable NIMBY (Not In My Back Yard) opposition. Large stacks with multiple levels are often colloquially described as Mixmasters or spaghetti bowls due to their complex appearance, being compared to boiled spaghetti.

Cloverstack interchange[link]

Three-level cloverstack

Two-level cloverstack

In the late 1960s, partial cloverleaf interchange (parclo) designs modified for freeway traffic emerged, eventually leading to the cloverstack interchange. Its ramps are longer to allow for higher ramp speeds, and loop ramp radii are made larger as well. The large loop ramps eliminate the need for a fourth, and sometimes a third level in a typical stack interchange, as only two directions of travel use flyover/under ramps.

Cloverstacks are cheaper to build than stack interchanges and are less of an eyesore for local residents. By using the loop ramps in opposite quadrants, weaving is also eliminated. However, cloverstacks require a lot of land to construct and the loop ramps are not as efficient as flyover/under ramps in terms of traffic flow. The cloverstack design is becoming more and more popular, and is commonly used to upgrade cloverleaf interchanges to increase their capacity and eliminate weaving.

Turbine interchange[link]

Two-level Turbine

The Circle Interchange in Chicago, a notable turbine interchange

Three-level Turbine

Another alternative to the four-level stack interchange is the turbine interchange (also known as a whirlpool). The turbine/whirlpool interchange requires fewer levels (usually two or three) while retaining semi-directional ramps throughout, and has its left-turning ramps sweep around the center of the interchange in a spiral pattern in right-hand driving.

Turbine interchanges offer slightly less vehicle capacity because the ramps typically turn more often and change height quicker. They also require more land to construct than the typical four-level stack interchange.

In areas with rolling or mountainous terrain, turbine interchanges can take advantage of the natural topography of the land due to the constant change in the height of their ramps, and hence these are commonly used in these areas where conditions apply, reducing construction costs compared to turbine interchanges built on level ground.

Roundabout interchange[link]

Roundabout interchange

A further alternative found often in the United Kingdom is called a roundabout interchange. This is a normal roundabout except one (two-level) or both (three-level) mainlines pass under or over the whole interchange. The ramps of the interchanging highways meet at a roundabout or rotary on a separated level above, below, or in the middle of the two highways.

Roundabout interchanges are much more economical in use of materials and land than other interchange designs, as the junction does not normally require more than three bridges to be constructed. However, their capacity is limited when compared to other interchanges and can become congested easily with high traffic volumes.

A variation of this interchange has been proposed by a Ukrainian engineer Viktor Petruk.^{[citation needed]} The claimed advantages of this type of interchange are: low number of conflict points, short car paths on the circle, and relatively small overall size that allows it to be used in dense urban environment. Unfamiliarity of clockwise traffic flows on roundabouts is its most frequently cited drawback.

Other/hybrid interchanges[link]

Hybrid interchange near Rotterdam, Netherlands.

Hybrid interchange near Cross-Harbour Tunnel, Hong Kong.

Hybrid interchanges use a mixture of interchange types and are not uncommon. Their construction can consist of multiple interchange designs such as loop ramps, flyovers and roundabouts.

A windmill interchange is similar to a turbine interchange, but it has much sharper turns, reducing its size and capacity. A variation of the windmill, called the diverging windmill, increases capacity by altering the direction of traffic flow of the interchanging highways, making the connecting ramps much more direct. The interchange is named for its similar overhead appearance to the blades of a windmill.

Divided volleyball interchanges create a wide median between the carriageways of the two interchanging highways, using this space for connecting ramps.

Full diamond interchanges are large, multi-level interchanges that use flyover/under ramps to handle both right and left ramps.

• 

  Windmill

• 

  Diverging windmill

• 

  Divided volleyball

• 

  Full diamond

Three-way interchanges[link]

[edit] Trumpet interchange

A trumpet interchange on the Ottawa River Parkway.

Trumpet

Trumpet interchanges have been used where one highway terminates at another highway. These involve at least one loop ramp connecting traffic either entering or leaving the terminating expressway with the far lanes of the continuous highway.

These interchanges are useful for highways as well as toll roads, as they concentrate all entering and exiting traffic into a single stretch of roadway, where toll booths can be installed. A double-trumpet interchange version can be found where a toll road meets another toll road or a free highway.

Trumpet interchanges are named as such due to their resemblance to trumpets. The bell of a trumpet can be seen where the terminating highway begins to interchange with the continuous highway, and the resemblance to the tubing is seen along the connecting loop ramps.

Directional T interchange[link]

Semi-directional-T interchange in Lausanne, Switzerland.

Semi-directional T

A directional T interchange uses flyover/under ramps in all directions at a three-way interchange. A semi-directional T does the same, but some of the splits and merges are switched to avoid ramps to and from the passing lane. Directional T interchanges are very efficient, but are expensive to build compared to other three-way interchanges. They also require three levels, which can be an eyesore for local residents. However, the T-interchange is perferred to a trumpet interchange because a trumpet requires a loop ramp which speeds can be reduced to as much as 25 MPH (US), but flyover ramps can handle much faster speeds.

Full Y interchange[link]

Full Y

A full Y-interchange is typically used when a three-way interchange is required for two or three highways interchanging in semi-parallel/perpendicular directions, but it can also be used in right-angle case as well. Their connecting ramps can spur from either the right or left side of the highway, depending on the direction of travel and the angle.

Full Y-interchanges use flyover/flyunder ramps for both connecting and mainline segments, and they require a moderate amount of land and moderate costs since only two levels or roadway are typically used. They get their name due to their resemblance to the capital letter "Y", depending upon the angle from which the interchange is seen. Two examples of this type of interchange can be found in the Province of Ontario: the Highway 403/Highway 6 interchange in Hamilton, and the Highway 417/Highway 416 interchange in Ottawa. Three other examples can be found in Nashville, TN: Interstate 24 and Interstate 65 (southern and northern interchange); interstate 24 and Interstate 40 (western interchange); and interstate 40 and interstate 65 (southern interchange) along the downtown loop.

Other/hybrid interchanges[link]

Hybrid interchanges use a mixture of interchange types and are not uncommon. Their construction can consist of multiple interchange designs such as loop ramps and flyovers. One place to find hybrids are near Santa Clarita, California, all along Interstate 5 and California State Route 14.

A half-clover interchange is essentially half a cloverleaf interchange, constructed to connect in just three directions instead of four. These are rarely used due to the traffic weaving that they cause and the large amount of land that they consume, but they can be built in areas where the connecting ramp along the loop of a trumpet interchange is not feasible due to building developments or physical limitations. Half-clovers are designed to be readily upgraded to full cloverleafs if the terminating highway is ever extended past the through highway. A notable example of a half-clover interchange is with M-47 and U.S. Route 10 near Midland, Michigan, since M-47 was the relic of a scrapped plan to extend a freeway further north. Interstate 75 to its east near Bay City, Michigan, a more important destination, made this highway unnecessary.

A 3/4 volleyball interchange is a divided-volleyball interchange designed to meet at three points instead of four. Like the half-clover, it can easily be upgraded to a fully divided volleyball interchange if the terminating highway is extended beyond the through highway.

A T-bone interchange is essentially a compacted directional T interchange with sharper turns and a lower capacity, built when there is insufficient land or funds to build a directional T. Its two flyover/flyunder ramps contract together after passing over the through highway, but then they split into two segments, resembling the bone in a T-bone steak.

• 

  Half-clover

• 

  3/4-volley

• 

  T-bone

Two-way interchanges[link]

A half trumpet interchange is essentially a trumpet interchange without its loop ramp and one of its directional ramps designed to meet the continuous highway in one direction, usually on a ninety degree or semi-perpendicular angle. Should the need arise, it can easily be constructed into a trumpet interchange, making it a three-way interchange.

A partial y interchange is used where one highway terminates at another highway with the same general directional alignment (usually a maximum of sixty degrees). The trunk of the terminating highway merges with the trunk of the continuous highway; vehicles traveling into the interchange may only exit traveling in the same direction. This type of interchange is often used for bypass routes, and is named for the shape the two highways' confluence makes when drawn on a map, creating a lower-case "y".

Hybrids, variations, and rare types also exist for two-way interchanges.

Basketweave

A basketweave interchange is commonly found on highways using a collector/express system or long collector/distributor lanes. In a basketweave, one highway is able to interchange with itself, allowing traffic traveling in the same direction to switch between carriageways through the use of flyover/under ramps created between two carriageways without causing weaving. These interchanges usually involve left exits and entry for the outer carriageway (right in left-hand drive) but can be configured to meet on the right.

A basketweave interchange in Toronto, Canada.

Between a motorway and a non-motorway road[link]

Diamond interchange[link]

Diamond

Diamond interchange in Ohio, United States.

A diamond interchange is an interchange involving four ramps where they enter and leave the freeway at a small angle and meet the non-freeway at almost right angles. These ramps at the non-freeway can be controlled through stop signs, traffic signals, or turn ramps. Diamond interchanges are inexpensive to build and require little land but are prone to congestion and accidents if there is high traffic.

Dumbbell

A dumbbell interchange is similar to the diamond interchange, but uses a pair of roundabouts to join the ramps with the non-highway. This typically increases the efficiency of the interchange when compared to a diamond. Such an interchange is found on the Pat Bay Highway in North Saanich, British Columbia near Victoria International Airport.

Parclo interchange/folded diamond[link]

Parclo A-4 interchange in Ontario, Canada.

A parclo/diamond hybrid in British Columbia, Canada.

A Parclo interchange, also known as a partial cloverleaf, is an interchange usually involving four to six ramps, two of which are loop ramps, which connect to the non-highway.

The parclo is a safer modification of the cloverleaf design. Depending on the number of ramps used, they take up a moderate to large amount of land and are typically inexpensive to build. Parclos with more ramps have a greater capacity and efficiently than parclos with fewer ramps. Parclos are sometimes called a folded diamond when only four ramps, in two quadrants, are used. If the loop ramps are constructed opposite or mirrored along the highway, weaving is avoided. Cloverleaf interchanges that involve a non-highway can be changed to parclos without too much reconstruction. Although the interchange's capacity is reduced, weaving is eliminated, increasing the safety and efficiency of the interchange.

Diverging diamond interchange[link]

Diverging diamond

A diverging diamond interchange is similar to a traditional diamond interchange, except that it uses directional lanes for the non-highway to cross over each other on either side of the highway, altering the direction of travel on the over/underpass through the use of traffic lights. This allows all turns to and from the highway to be made without crossing the opposite direction of travel, increasing the capacity when compared to a typical diamond interchange.

The idea for the diverging diamond interchange came from a freeway-to-freeway connection north of Baltimore, where I-695 has an interchange with Interstate 95. Whereas this interchange has been replaced with a four-level stack, it was once of a design known as the "synchronized split-phasing interchange".^[2]

The first diverging diamond interchange in the United States opened on July 7, 2009, in Springfield, Missouri, at the junction of Interstate 44 and Missouri Route 13.^[3]

Single-point urban interchange[link]

Single point urban interchange (SPUI)

Single point urban interchange in Florida, United States.

• The Single-point urban interchange, often abbreviated to SPUI, is a modification of the diamond interchange and has its ramps meet at one point, usually on the overpass/underpass of the non-highway. This requires only one set of traffic signals, increasing its efficiency and capacity when compared to a diamond.

Freeways in the Phoenix Metropolitan area are great examples of the utilization of SPUI interchanges. Some examples of SPUI are along AZ-51 from Downtown Phoenix all the way to Loop 101, another location where SPUI is common.

Other/hybrid interchanges[link]

Highway/non-highway hybrid interchanges consist of diamond and partial cloverleaf elements. Their construction can consist of multiple interchange designs such as loop ramps, flyovers, and roundabouts.

One form of the roundabout interchange can be used to connect a highway with a non-highway. It uses a single roundabout, rotary, or traffic circle which spans the highway as a over/underpass. Such junctions can be improved by adding a flyover for straight-through traffic on the non-freeway, creating the Roundabout interchange.

The three-level diamond/volleyball interchange is three levels high, and it handles interchanging ramps via four intersections. This kind of an interchange can also be used to connect two highways together, but due to the use of intersections, its traffic is not free-flowing.

• 

  Four ramp partial cloverleaf (B-2)

• 

  Six ramp partial cloverleaf (A-4)

• 

  Folded diamond/parclo

• 

  Three level diamond/volleyball

• 

  Roundabout

See also[link]

• Free-flow interchange
• Grade separation
• Intersection (road)
• Junction (traffic)
• Unused highway

References[link]

External links[link]

Wikimedia Commons has media related to: Road junctions

• Kurumi.com U.S. interchanges directory
• Glossary - Part of the publication Highway Design Handbook for Older Drivers and Pedestrians by the Turner-Fairbank Highway Research Center branch of the U.S. Federal Highway Administration
• Detailed history of interchanges with diagrams (German)

A road is a route, thoroughfare or way that supports travel by a means of conveyance.

Definitions[link]

For purposes of international statistical comparison, the Organisation for Economic Co-operation and Development (OECD) 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," which includes "bridges, tunnels, supporting structures, junctions, crossings, interchanges, and toll roads, but not cycle paths."^[1] In urban areas roads may diverge through a city or village and be named as streets, serving a dual function as urban space easement and route.^[2] Modern roads are normally smoothed, paved, or otherwise prepared to allow easy travel.^[3] Historically many roads were simply recognizable routes without any formal construction or maintenance.^[4]

United States[link]

In the United States, laws distinguish between public roads, which are open to public use, and private roads, which are privately controlled.^[5]

United Kingdom[link]

In the United Kingdom there is some ambiguity between the terms highway and road. The Highway code details rules for "road users".^[6] For the purposes of the English law, Highways Act 1980, which covers England and Wales but not Scotland or Northern Ireland, the term road is defined to be "any length of highway or of any other road to which the public has access, and includes bridges over which a road passes."^[7] This includes footpaths, bridleways and cycle tracks, and also road and driveways on private land and many car parks.^[8] Vehicle Excise Duty, a road use tax, is payable on some vehicles used on the public road.^[8]

The definition of a road depends on the definition of a highway, however there is no formal definition for a highway in the relevant Act. A 1984 ruling said "the land over which a public right of way exists is known as a highway; and although most highways have been made up into roads, and most easements of way exist over footpaths, the presence or absence of a made road has nothing to do with the distinction.^[9][10] Another legal view is that while a highway historically included footpaths, bridleways, driftways, etc., it can now be used to mean those ways that allow the movement of motor-vehicles, and the term rights of way can be used to cover the wider usage.^[11]

Historical road construction[link]

The Porta Rosa is a Greek street dating from the 3rd to 4th century BC in Velia, Italy with a paved surface and gutters.

A paved Roman road in Pompeii, Italy

The assertion that the first pathways were the trails made by animals has not been universally accepted, since in many cases animals do not follow constant paths.^[4] Others believe that some roads originated from following animal trails.^[12][13] The Icknield Way is given as an example of this type of road origination, where man and animal both selected the same natural line.^[14] By about 10,000 BC, rough pathways were used by human travelers.^[4]

• The world's oldest known paved road was laid in Egypt some time between 2600 and 2200 BC.^[15]

• Stone-paved streets are found in the city of Ur in the Middle East dating back to 4000 BC.^[4]

• Corduroy roads (log roads) are found dating to 4000 BC in Glastonbury, England.^[4]

• The timber trackway; Sweet Track causeway in England, is one of the oldest engineered roads discovered and the oldest timber trackway discovered in Northern Europe. Built in winter 3807 BC or spring 3806 BC, tree-ring dating (Dendrochronology) enabled very precise dating. It has been claimed to be the oldest road in the world.^[16][17]

• Brick-paved streets were used in India as early as 3000 BC .^[4]

• In 500 BC, Darius I the Great started an extensive road system for Persia (Iran), including the famous Royal Road, which was one of the finest highways of its time.^[18] The road remained in use after Roman times.

• In ancient times, transport by river was far easier and faster than transport by road,^[17] especially considering the cost of road construction and the difference in carrying capacity between carts and river barges. A hybrid of road transport and ship transport beginning in about 1740 is the horse-drawn boat in which the horse follows a cleared path along the river bank.^[19][20]

• From about 312 BC, the Roman Empire built straight^[21] strong stone Roman roads throughout Europe and North Africa, in support of its military campaigns. At its peak the Roman Empire was connected by 29 major roads moving out from Rome and covering 78,000 kilometers or 52,964 Roman miles of paved roads.^[17]

• In the 8th century AD, many roads were built throughout the Arab Empire. The most sophisticated roads were those in Baghdad, Iraq, which were paved with tar. Tar was derived from petroleum, accessed from oil fields in the region, through the chemical process of destructive distillation.^[22]

• The Highways Act 1555 in Britain transferred responsibility for maintaining roads from government to local parishes.^[17] This resulted in a poor and variable state of roads. To remedy this, the first of the "Turnpike trusts" was established around 1706, to build good roads and collect tolls from passing vehicles. Eventually there were approximately 1,100 trusts in Britain and some 36,800 km (22,870 miles) of engineered roads.^[17] The Rebecca Riots in Carmarthenshire and Rhayader from 1839 to 1844 contributed to a Royal Commission that led to the demise of the system in 1844,^[23] which coincided with the development of the UK railway system.

Design[link]

Structural road design is the science of designing a road for its environment in order to extend its longevity and reduce maintenance. The Shell pavement design method is used in many countries for the design of new asphalt roads.

Construction[link]

A road being torn up.

Surveyor at work with a leveling instrument.

Asphalt layer and roller

Road construction requires the creation of a continuous right-of-way, overcoming geographic obstacles and having grades low enough to permit vehicle or foot travel.^{[24] (pg15)} and may be required to meet standards set by law^[25] or official guidelines.^[26] The process is often begun with the removal of earth and rock by digging or blasting, construction of embankments, bridges and tunnels, and removal of vegetation (this may involve deforestation) and followed by the laying of pavement material. A variety of road building equipment is employed in road building.^[27][28]

After design, approval, planning, legal and environmental considerations have been addressed alignment of the road is set out by a surveyor.^[21] The Radii and gradient are designed and staked out to best suit the natural ground levels and minimize the amount of cut and fill.^{[26] (page34)} Great care is taken to preserve reference Benchmarks ^{[26] (page59)}

Roads are designed and built for primary use by vehicular and pedestrian traffic. Storm drainage and environmental considerations are a major concern. Erosion and sediment controls are constructed to prevent detrimental effects. Drainage lines are laid with sealed joints in the road easement with runoff coefficients and characteristics adequate for the land zoning and storm water system. Drainage systems must be capable of carrying the ultimate design flow from the upstream catchment with approval for the outfall from the appropriate authority to a watercourse, creek, river or the sea for drainage discharge.^{[26] (page38 to 40)}

Road construction on Marquette Avenue in Minneapolis, Minnesota, United States

A borrow pit (source for obtaining fill, gravel, and rock) and a water source should be located near or in reasonable distance to the road construction site. Approval from local authorities may be required to draw water or for working (crushing and screening) of materials for construction needs. The top soil and vegetation is removed from the borrow pit and stockpiled for subsequent rehabilitation of the extraction area. Side slopes in the excavation area not steeper than one vertical to two horizontal for safety reasons.^{[26] (page 53 to 56 )}

Old road surfaces, fences, and buildings may need to be removed before construction can begin. Trees in the road construction area may be marked for retention. These protected trees should not have the topsoil within the area of the tree's drip line removed and the area should be kept clear of construction material and equipment. Compensation or replacement may be required if a protected tree is damaged. Much of the vegetation may be mulched and put aside for use during reinstatement. The topsoil is usually stripped and stockpiled nearby for rehabilitation of newly constructed embankments along the road. Stumps and roots are removed and holes filled as required before the earthwork begins. Final rehabilitation after road construction is completed will include seeding, planting, watering and other activities to reinstate the area to be consistent with the untouched surrounding areas.^{[26] (page 66 to 67 )}

Processes during earthwork include excavation, removal of material to spoil, filling, compacting, construction and trimming. If rock or other unsuitable material is discovered it is removed, moisture content is managed and replaced with standard fill compacted to 90% relative compaction. Generally blasting of rock is discouraged in the road bed. When a depression must be filled to come up to the road grade the native bed is compacted after the topsoil has been removed. The fill is made by the "compacted layer method" where a layer of fill is spread then compacted to specifications, the process is repeated until the desired grade is reached.^{[26] (page 68 to 69 )}.

Line marking

General fill material should be free of organics, meet minimum California bearing ratio (CBR) results and have a low plasticity index. The lower fill generally comprises sand or a sand-rich mixture with fine gravel, which acts as an inhibitor to the growth of plants or other vegetable matter. The compacted fill also serves as lower-stratum drainage. Select second fill (sieved) should be composed of gravel, decomposed rock or broken rock below a specified Particle size and be free of large lumps of clay. Sand clay fill may also be used. The road bed must be "proof rolled" after each layer of fill is compacted. If a roller passes over an area without creating visible deformation or spring the section is deemed to comply.^{[26] (page 70 to 72 )}

Geosynthetics such as geotextiles, geogrids and geocells are frequently used in the various pavement layers to improve road quality. Geosynthetics perform four main functions in roads: separation, reinforcement, filtration and drainage; which increase the pavement performance, reduce construction costs and decrease maintenance.^[29]

The completed road way is finished by paving or left with a gravel or other natural surface. The type of road surface is dependent on economic factors and expected usage. Safety improvements like Traffic signs, Crash barriers, Raised pavement markers, and other forms of Road surface marking are installed.

According to a May 2009 report by the American Association of State Highway and Transportation Officials (AASHTO) and TRIP—a national transportation research organization—driving on rough roads costs the average American motorist approximately $400 a year in extra vehicle operating costs. Drivers living in urban areas with populations more than 250,000 are paying upwards of $750 more annually because of accelerated vehicle deterioration, increased maintenance, additional fuel consumption, and tire wear caused by poor road conditions.

When a single carriageway road is converted into dual carriageway by building a second separate carriageway alongside the first, it is usually referred to as duplication,^[30] twinning or doubling. The original carriageway is changed from two-way to become one-way, while the new carriageway is one-way in the opposite direction. In the same way as converting railway lines from single track to double track, the new carriageway is not always constructed directly alongside the existing carriageway.

Maintenance[link]

"Road works ahead" sign, typically used in Europe

Like all structures, roads deteriorate over time. Deterioration is primarily due to accumulated damage from vehicles, however environmental effects such as frost heaves, thermal cracking and oxidation often contribute.^[31] According to a series of experiments carried out in the late 1950s, called the AASHO Road Test, it was empirically determined that the effective damage done to the road is roughly proportional to the Fourth power of axle weight.^[32] A typical tractor-trailer weighing 80,000 pounds (36.287 t) with 8,000 pounds (3.629 t) on the steer axle and 36,000 pounds (16.329 t) on both of the tandem axle groups is expected to do 7,800 times more damage than a passenger vehicle with 2,000 pounds (0.907 t) on each axle. Potholes on roads are caused by rain damage and vehicle braking or related construction works.

Manual road repair taking place in Howrah, India.

Pavements are designed for an expected service life or design life. In some UK countries the standard design life is 40 years for new bitumen and concrete pavement. Maintenance is considered in the whole life cost of the road with service at 10, 20 and 30 year milestones.^[33] Roads can be and are designed for a variety of lives (8-, 15-, 30-, and 60-year designs). When pavement lasts longer than its intended life, it may have been overbuilt, and the original costs may have been too high. When a pavement fails before its intended design life, the owner may have excessive repair and rehabilitation costs. Many concrete pavements built since the 1950s have significantly outlived their intended design lives.^[34] Some roads like Chicago, Illinois's "Wacker Drive", a major two-level viaduct in downtown area are being rebuilt with a designed service life of 100 years.^[35]

Virtually all roads require some form of maintenance before they come to the end of their service life. Pro-active agencies use pavement management techniques to continually monitor road conditions and schedule preventive maintenance treatments as needed to prolong the lifespan of their roads. Technically advanced agencies monitor the road network surface condition with sophisticated equipment such as laser/inertial Profilometers. These measurements include road curvature, cross slope, asperity, roughness, rutting and texture (roads). This data is fed into a pavement management system, which recommends the best maintenance or construction treatment to correct the damage that has occurred.

Maintenance treatments for asphalt concrete generally include crack sealing, surface rejuvenating, fog sealing, micro-milling and surface treatments. Thin surfacing preserves, protects and improves the functional condition of the road while reducing the need for routing maintenance, leading to extended service life without increasing structural capacity.^[36]

Failure to maintain roads properly can create significant costs to society, in a 2009 report released by the American Association of State Highway and Transportation Officials (USA) about 50% of the roads in the USA are in bad condition with urban areas worse. The report estimates that urban drivers pay an average of $746/year on vehicle repairs while the average US motorist pays about $335/year. In contrast, the average motorist pays about $171/year in road maintenance taxes (based on 600 gallons/year and $0.285/gallon tax).

Slab Stabilization[link]

Distress and serviceability loss on concrete roads can be caused by loss of support due to voids beneath the concrete pavement slabs. The voids usually occur near cracks or joints due to surface water infiltration. The most common causes of voids are pumping, consolidation, subgrade failure and bridge approach failure. Slab stabilization is a non-destructive method of solving this problem and is usually employed with other Concrete Pavement Restoration (CPR) methods including patching and diamond grinding. The technique restores support to concrete slabs by filing small voids that develop underneath the concrete slab at joints, cracks or the pavement edge. The process consists of pumping a cementitous grout or polyurethane mixture through holes drilled through the slab. The grout can fill small voids beneath the slab and/or sub-base. The grout also displaces free water and helps keep water from saturating and weakening support under the joints and slab edge after stabilization is complete. The three steps for this method after finding the voids are locating and drilling holes, grout injection and post-testing the stabilized slabs.

Slab stabilization does not correct depressions, increase the design structural capacity, stop erosion or eliminate faulting. It does, however, restore the slab support, therefore, decreasing deflections under the load. Stabilization should only be performed at joints and cracks where loss of support exists. Visual inspection is the simplest manner to find voids. Signs that repair is needed are transverse joint faulting, corner breaks and shoulder drop off and lines at or near joints and cracks. Deflection testing is another common procedure utilized to locate voids. It is recommended to do this testing at night as during cooler temperatures, joints open, aggregate interlock diminishes and load deflections are at their highest.

Testing[link]

Ground penetrating radar pulses electromagnetic waves into the pavement and measures and graphically displays the reflected signal. This can reveal voids and other defects.

The epoxy/core test, detects voids by visual and mechanical methods. It consists of drilling a 25 to 50 millimeter hole through the pavement into the sub-base with a dry-bit roto-hammer. Next, a two-part epoxy is poured into the hole—dyed for visual clarity. Once the epoxy hardens, technicians drill through the hole. If a void is present, the epoxy will stick to the core and provide physical evidence.

Common stabilization materials include pozzolan-cement grout and polyurethane. The requirements for slab stabilization are strength and the ability to flow into or expand to fill small voids. Colloidal mixing equipment is necessary to use the pozzolan-cement grouts. The contractor must place the grout using a positive-displacement injection pump or a non-pulsing progressive cavity pump. A drill is also necessary but it must produce a clean hole with no surface spalling or breakouts. The injection devices must include a grout packer capable of sealing the hole. The injection device must also have a return hose or a fast-control reverse switch, in case workers detect slab movement on the uplift gauge. The uplift beam helps to monitor the slab deflection and has to have sensitive dial gauges.^[37][38]

Joint Sealing[link]

Also called joint and crack repair, this method’s purpose is to minimize infiltration of surface water and incompressible material into the joint system. Joint sealants are also used to reduce dowel bar corrosion in Concrete Pavement Restoration (CPR) techniques. Successful resealing consists of old sealant removal, shaping and cleaning the reservoir, installing the backer rod and installing the sealant. Sawing, manual removal, plowing and cutting are methods used to remove the old sealant. Saws are used to shape the reservoir. When cleaning the reservoir, no dust, dirt or traces of old sealant should remain. Thus, it is recommended to water wash, sand-blast and then air blow to remove any sand, dirt or dust. The backer rod installation requires a double-wheeled, steel roller to insert the rod to the desired depth. After inserting the backer rod, the sealant is placed into the joint. There are various materials to choose for this method including hot pour bituminous liquid, silicone and preformed compression seals.

^{[37][39][40][41]}

Safety considerations[link]

Pedestrian crossing, line markings and street furniture.

Careful design and construction of roads can increase Road traffic safety and reduce the harm (deaths, injuries, and property damage) on the highway system from traffic collisions.

On neighborhood roads traffic calming, safety barriers, pedestrian crossings and cycle lanes can all protect pedestrians and cyclists.

Lane markers in some countries and states are marked with Cat's eyes or Botts dots, bright reflectors that do not fade like paint. Botts dots are not used where it is icy in the winter, because frost and snowplows can break the glue that holds them to the road, although they can be embedded in short, shallow trenches carved in the roadway, as is done in the mountainous regions of California.

For major roads risk can be reduced by providing limited access from properties and local roads, grade separated junctions and median dividers between opposite-direction traffic to reduce likelihood of head-on collisions.

The placement of energy attenuation devices (e.g. guardrails, wide grassy areas, sand barrels) is also common. Some road fixtures such as road signs and fire hydrants are designed to collapse on impact. Light poles are designed to break at the base rather than violently stop a car that hits them. Highway authorities may also remove larger trees from the immediate vicinity of the road.

Environmental performance[link]

Air pollution along Pasadena Highway in Los Angeles, United States

Careful design and construction of a road can reduce any negative environmental impacts.

Water management systems can be used to reduce the effect of pollutants from roads.^{[citation needed]} Rainwater and snowmelt running off of roads tends to pick up gasoline, motor oil, heavy metals, trash and other pollutants and result in Water pollution. Road runoff is a major source of nickel, copper, zinc, cadmium, lead and polycyclic aromatic hydrocarbons (PAHs), which are created as combustion byproducts of gasoline and other fossil fuels.^[42] De-icing chemicals and sand can run off into roadsides, contaminate groundwater and pollute surface waters;^[43] and road salts can be toxic to sensitive plants and animals.^[44] Sand applied to icy roads can be ground up by traffic into fine particulates and contribute to air pollution. Sand can alter stream bed environments, causing stress for the plants and animals that live there.^{[citation needed]}

Roads are a chief source of environmental noise generation. In the early 1970s it was recognized that design of roads can be conducted to influence and minimize noise generation.^[45] Noise barriers are used to reduce Noise pollution, in particular where roads are located close to built-up areas. Regulations can restrict the use of Engine braking.

Motor vehicle emissions contribute air pollution. Concentrations of air pollutants and adverse respiratory health effects are greater near the road than at some distance away from the road.^[46] Road dust kicked up by vehicles may trigger allergic reactions.^[47]

Terminology[link]

The A22(T) with line markings near Summer Hill, East Sussex, England, United Kingdom

Road with guard rails in Kaluga Oblast, Russia

Road with traffic signs in the outskirts of Bern, Switzerland

Highway 401, a route with a collector / express setup.

The wide Coastal Road in the Philippines now called the Manila Cavite Expressway

Road in the Jujuy Province Argentina

• Alignment - the route of the road, defined as a series of horizontal tangents and curves.
• Adverse camber - where a road slopes towards the outside of a bend, increasing the likelihood that vehicles travelling at speed will skid or topple. Usually only a temporary situation during road maintenance.
• All-weather road - Unpaved road that is constructed of a material that does not create mud during rainfall.
• Banked turn
• Camber or Crown - the slope of the road surface downwards away from the centre of the road, so that surface water can flow freely to the edge of the carriageway, or on bends angling of the surface to lean traffic 'into the bend' reducing the chance of a skid.
• Cant - superelevation
• Cross slope The slope of the pavement, expressed as units of rise per unit of run, or as a percentage.
• Grade - longitudinal slope
• Belisha Beacon - an orange globe, lit at night, used to highlight a pedestrian crossing.
• Bollard - Rigid posts that can be arranged in a line to close a road or path to vehicles above a certain width
• Byway - Highway over which the public have a right to travel for vehicular and other kinds of traffic, but is used mainly as a footpath or bridleway
• Bypass Road that avoids or "bypasses" a built-up area, town, or village
• Bottleneck - Section of a road with a carrying capacity substantially below that of other sections of the same road
• Botts' dots - Non-reflective raised pavement marker used on roads
• Cat's eye - reflective raised pavement marker used on roads
• Central reservation - On dual carriageway roads, including controlled-access highways, divided highways and many limited-access roads, the central reservation (British English), median (North American English), median strip (North American English and Australian English), neutral ground [Louisiana English] or central nature strip (Australian English) - Area that separates opposing lanes of traffic
• Chicane - Sequence of tight serpentine curves (usually an S-shape curve or a bus stop)
• Chipseal - Road surface composed of a thin layer of crushed stone 'chips' and asphalt emulsion. It seals the surface and protects it from weather, but provides no structural strength. It is cheaper than asphalt concrete or concrete. In the U.S. it is usually only used on low volume rural roads
• Corniche - Road on the side of a cliff or mountain, with the ground rising on one side and falling away on the other
• Curb (kerb) - A raised edge at the side of the roadway.
• Curb extension - (also kerb extension, bulb-out, nib, elephant ear, curb bulge and blister) Traffic calming measure, intended to slow the speed of traffic and increase driver awareness, particularly in built-up and residential neighborhoods.
• Drainage gradient
• Farm-to-market road - a state road or county road that connects rural or agricultural areas to market towns.
• Fork - (literally "fork in the road") Type of intersection where a road splits
• Green lane - (UK) Unsurfaced road, may be so infrequently used that vegetation colonises freely, hence 'green'. Many green lanes are ancient routes that have existed for millennia.
• Guide rail - Prevents vehicles from veering off the road into oncoming traffic, crashing against solid objects or falling from a road. Also called a guard rail or traffic barrier.
• Gutter (UK) - a drainage channel usually at the edge of the road or along a median.
• Interstate Highway System (United States) - System of Interstate and Defense Highways
• Layby (Pullout, pull-off) - A paved area beside a main road where cars can stop temporarily to let another car pass.
• Loose chippings - the hazard of stone chippings that have come loose
• Median - On dual carriageway roads, including controlled-access highways, divided highways and many limited-access roads, the central reservation (British English), median (North American English), median strip (North American English and Australian English), neutral ground [Louisiana English] or central nature strip (Australian English) - Area that separates opposing lanes of traffic
• Mountain pass - A relatively low level route through a range of mountains
• Milestone - One of a series of numbered markers placed along a road, often at regular intervals, showing the distance to destinations.
• National Highway - Road built and maintained by a national authority.
• Pavement - The road regarded as a geoconstruction. In the UK the term is road surface and the pavement is a pedestrian walkway alongside the road.
• Pedestrian crossing - Designated point on a road where road marking or other means helps pedestrians cross safely
• Pelican crossing - (officially Pelicon crossing) UK a PEdestrian LIght CONtrolled crossing.
• Private highway - Highway owned and operated for profit by private industry
• Private road - Road owned and maintained by a private individual, organization, or company rather than by a government
• Profile - the vertical alignment of a road, expressed as a series of grades, connected by parabolic curves.
• Public space - Place where anyone has a right to come without being excluded because of economic or social conditions
• Ranch road - U.S. road that connects rural and agricultural areas to market towns
• Road number - Often assigned to identify a stretch of public roads— often dependent on the type of road, with numbers differentiating between interstates, motorways, arterial thoroughfares, etc.
• Road-traffic safety - Process to reduce the harm (deaths, injuries, and property damage) that result from vehicle crashes on public roads
• Roadworks - Part or all of the road is occupied for work or maintenance
• Roughness - Deviations from a true planar pavement surface, which affects vehicle suspension deflection, dynamic loading, ride quality, surface drainage and winter operations. Roughness have wavelengths ranging from 500 mm up to some 40 m. The upper limit may be as high as 350 m when considering motion sickness aspects; motion sickness is generated by motion with down to 0.1 Hz frequency; in an ambulance car driving 35 m/s (126 km/h), waves with up to 350 m will excite motion sickness.
• Roundabout UK a road junction where typically three or more roads are joined by a circular section of road. Traffic 'on the roundabout' has priority over traffic on approach roads, unless indicated otherwise. In countrys where traffic drives on the left the roundabout is travelled in a clockwise direction. Also known as an island in parts of the UK.
• Shoulder (also hard shoulder) - A clear, level area to the side of the roadway available for stopping if needed.
• State highway - Road numbered by the state, falling below numbered national highways (like U.S. Routes) in the hierarchy or a road maintained by the state, including nationally-numbered highways
• Traffic - Pedestrians, ridden or herded animals, vehicles, bicycles, and other conveyances using any road for purposes of travel. ^[48].
• Texture (roads) - Deviations from a true planar pavement surface, which affects the interaction between road and tire. Microtexture have wavelengths below 0.5 mm, Macrotexture below 50 mm and Megatexture below 500 mm.
• Traffic calming - Set of strategies used by urban planners and traffic engineers to slow down or reduce motor vehicle traffic, thereby improving safety for pedestrians and bicyclists and improving the environment for residents
• Traffic island - UK a small raised area used to help define the traffic flow, which may also act as a refuge for pedestrians crossing the carriageway or a location for signs, barriers or lights—a synonym for roundabout in some parts of the UK
• Traffic light - Also known as a traffic signal, stop light, stop-and-go lights—a signaling device at a road intersection, pedestrian crossing, or other location that assigns right of way to different approaches to an intersection
• Zebra crossing (UK) - a pedestrian crossing marked by black and white stripes on the carriageway

Regulation[link]

Right- and left-hand traffic[link]

Traffic flows on the right or on the left side of the road depending on the country.^[49] In countries where traffic flows on the right, traffic signs are mostly on the right side of the road, roundabouts and traffic circles go counter-clockwise/anti-clockwise, and pedestrians crossing a two-way road should watch out for traffic from the left first.^[50] In countries where traffic flows on the left, the reverse is true.

About 33% of the world by population drive on the left, and 67% keep right. By road distances, about 28% drive on the left, and 72% on the right,^[51] even though originally most traffic drove on the left worldwide.^[52]

Economics[link]

A city street in Mumbai, India with left-hand traffic.

Transport economics is used to understand both the relationship between the transport system and the wider economy and the complex network effects when there are multiple paths and competing modes for both personal and freight (road/rail/air/ferry) and where Induced demand can result in increased on decreased transport levels when road provision is increased by building new roads or decreased (for example California State Route 480). Roads are generally built and maintained by the public sector using taxation although implementation may be through private contractors).^[53][54] or occasionally using road tolls.^[55]

Public-private partnerships are a way for our communities to address the rising cost by injecting private funds into our infrastructure. There are four main ones:^[56]

• design/build
• design/build/operate/maintain
• design/build/finance/operate
• build/own/operate

Economics and society depend heavily on efficient roads. In the European Union (EU) 44% of all goods are moved by trucks over roads and 85% of all persons are transported by cars, buses or coaches on roads.^[57] The term was also commonly used to refer to roadsteads, waterways that lent themselves to use by shipping.

Construction costs[link]

According to the New York State Thruway Authority,^[58] some sample per-mile costs to construct multi-lane roads in several US northeastern states were:

• Connecticut Turnpike — $3,449,000 per mile
• New Jersey Turnpike — $2,200,000 per mile
• Pennsylvania Turnpike (Delaware Extension) — $1,970,000 per mile
• Northern Indiana Toll Road — $1,790,000 per mile
• Garden State Parkway — $1,720,000 per mile
• Massachusetts Turnpike — $1,600,000 per mile
• Thruway, New York to Pennsylvania Line — $1,547,000 per mile
• Ohio Turnpike — $1,352,000 per mile
• Pennsylvania Turnpike (early construction) — $736,000 per mile

Statistics[link]

The United States has the largest network of roads of any country with 4,050,717 miles (6,518,997 km) as of 2009.^[59] The People's Republic of China is second with 3,583,715 kilometres (2,226,817 mi) of road (2007) See List of countries by road network size. The Republic of India has the third largest road system in the world with 3,383,344 kilometres (2,102,312 mi) (2002). When looking only at expressways the National Trunk Highway System (NTHS) in People's Republic of China has a total length of 45,000 kilometres (28,000 mi) at the end of 2006, and 60,300 km at the end of 2008, second only to the United States with 90,000 kilometres (56,000 mi) in 2005.^[60][61]

Many human settlements are not reachable by road alone. Road vehicles must be carried by boat or ferry to islands, and some locations are intentionally car-free. Even on the mainland, some settlements have no roads connecting with the main continental network, due to natural obstacles like mountains or wetlands, remoteness, or general expense. Unpaved roads or lack of roads are more common in developing countries, though even large cities like Iquitos in Peru and Juneau, Alaska in the United States are disconnected.

See also[link]

References[link]