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A dual carriageway (British English) or divided highway (North American English) is a road or highway in which the two directions of traffic are separated by a central barrier or strip of land, known as a central reservation (British English) or median (North American English). This type of road is usually able to carry a great deal more traffic than normal single carriageways (British English) or two-lane roads (North American English) and boulevards.

United Kingdom

In the UKmarker, although the term dual carriageway applies to any road with physically separated lanes, it is frequently used as a descriptive term for major route built in this style. Such major dual carriageways usually have two lanes of traffic in each direction, with the lane nearest the centre being reserved for overtaking. Occasionally dual carriageways have only one lane in each direction, or more than two lanes each way (usually to permit easier overtaking of slower uphill traffic). Different speed limits apply on dual carriageway sections from those that apply on single carriageway sections of the same class of road, except in cities and built-up areas where the dual carriageway is more of a safety measure, often intended to prevent pedestrians from crossing a busy road.

Diagram of types of road in the UK
When first constructed, many dual carriageways - including the first motorways - had no crash or other barriers in the central reservation. Hence in the event of delays on the road, or if a driver missed his exit, there was a widespread problem of drivers making a U-turn onto the other carriageway; many accidents were caused as a result of their misjudging the speed of approaching traffic on the other carriageway when doing so. The majority of dual carriageway roads now have barriers. Some are heavy concrete obstructions which can have the effect of bouncing a vehicle back into the path of other traffic; others are made from steel ropes mounted on moderately weak posts, where the rope cuts into the vehicle body to slow the vehicle while keeping it against the barrier until it has stopped.

Turning right (that is, across the line of traffic heading in the opposite direction) is usually permitted only at specific locations. Often the driver will be required to turn left (away from the dual carriageway) in order to loop around to an access road that permits crossing the major road. Roundabouts on dual carriageways are relatively common, especially in cities or where the cost of a grade-separated junction would be prohibitive.

A dual carriageway with grade-separated junctions and which meets other requirements may be upgraded to motorway standard, denoted as an (M) added after the road number (e.g. "A1marker" or "A38").

Confusion

While most drivers are clear about what a motorway is, some are confused about the definition of a dual carriageway. For a road to be classed as a dual carriageway, the two directions of traffic flow must be physically separated by a central reservation. A road where the two directions of flow are separated only by lines painted on the road surface is a single carriageway, regardless of the number of traffic lanes that may be available to the traffic in each direction. So a road with three or four lanes is not a dual carriageway if there is no central reservation.

Speed limits

The national speed limit applies on dual carriageways (unless it is in a 'built-up area', or a lower limit is posted), which is as follows:

National speed limits on dual carriageways in the UK
Type of vehicle Speed limit
Car, motorcycle or a car-based van up to 2 metric tonnes 70 mph
Car with caravan or trailer 60 mph
Bus or coach up to 12 m long 60 mph
Goods vehicle up to 7.5 t 60 mph
Goods vehicle over 7.5 t 50 mph


Ireland

An example of a 2+2 dual-carriageway in Ireland.
This type is similar to many found in the UK.
Although in the Republic of Irelandmarker the term dual carriageway technically applies to any road with physically separated lanes, it is usually used only to refer to those route sections that do not have a motorway designation. Most often it is national roads (roads with a route number prefix of N; e.g. N8) that are built as or upgraded to dual carriageway. A number of non-national roads (for example, regional roads) are dual carriageway, for example in urban areas near or in cities, or where the road was formerly part of a national route.

Dual carriageways of this class differ from motorways in a number of ways. The hard shoulder is demarcated with a dashed yellow line (as opposed to an unbroken yellow line on motorways). The standard speed limit of 100 km/h (62 mph) for national routes usually applies (by default the limit is 80 km/h (50 mph) for non-national roads, even if dual carriageway). Local authorities have the power to apply a limit of up to 120 km/h (75 mph) as used on most motorways (as of 2008, the dual carriageway section of the N2 north of the M50 and the High Quality Dual Carriageway]] section of the N1 between the end of the M1 and the border with Northern Irelandmarker are the only route sections with such special limits). Traffic lights and junctions are permitted at grade on dual carriageways. For older sections of dual carriageway, this has resulted in fewer flyover junctions. Newer dual carriageway sections are usually near motorway standard, with grade-separated junctions, but may not be designated as motorways due to the need to preserve access to adjoining property or to the absence of a non-motorway alternative route. Also, dual carriageways that are not motorway classified do not need to be equipped with emergency phones.

Motorway restrictions only apply to motorway sections, rather than all dual carriageway sections of national roads (these are signposted with the N prefix on the route number, rather than M). Some national secondary roads, and regional roads in particular often have houses, schools and other developments fronting on to them. Less important national primary roads, and older sections not yet upgraded may also feature such developments built before the introduction the Irish Planning system in 1964. Today Irish planning policy prohibits such development on National Primary or National Secondary roads where the speed limit exceeds 60 km/h (37 mph). This policy results from concerns expressed by the National Roads Authority. However, a local authority is not obliged to implement this policy and can disregard this policy at it own discretion. This would usually only occur in exceptional circumstances or where planners are over ruled by elected councillors using section 140 of the Local Government Act 2001. Accordingly, hard shoulders are included wherever feasible to provide for the resulting pedestrian and cyclist traffic, and are present on much of the national route network. These hard shoulders may also be used as running lanes by motorised traffic under certain conditions.

Until 2004/2005, many motorways and dual carriageways in Ireland did not have crash barriers in the central reservation, the policy being to use a wider median instead. Crash barriers are now mandatory for such routes, and wire cabling or full crash barriers (depending on whether or not the route is a motorway, and median width) have been fitted to existing routes.

 three major types of dual carriageway are being built on national road schemes in the Republic of Ireland:


  • High Quality Dual Carriageways (HQDC) - these are being built mainly on the major inter-urban routes, to full motorway standard but without motorway regulations. The Roads Act 2007 allows for these roads to be redesignated as motorways by ministerial order. Many of the sections of HQDC on the major inter-urban routes have been redesignated as motorways and full motorway regulations will apply when the redesignations come into effect.
  • Standard dual carriageway of the traditional type is mainly planned for schemes on the N11 road, the N18 road and the N25 road. Plans for this type of dual carriageway on the N20 road have been superseded by newer plans to build a motorway, the M20, to replace most of this route. Traditionally this type of dual-carriageway had a mixture of at grade junctions (including roundabouts), grade separated junctions, and median crossings. Nowadays they are similar to HQDCs, but minor at grade exits - generally left turn only - are allowed and the design speed (by Irish standards) is only 100 km/h. Median crossings and roundabouts are no longer generally found on these schemes. An example of a standard dual carriageway scheme, opened in 2006, is the Ennismarker bypass although this road has grade separated junctions and no median crossings. This route has now been upgraded to motorway status.
  • 2+2 roads - officially these roads are designated as Type 2 dual carriageways by the National Roads Authority (NRA). They will be created by widening existing roads or building new roads, and will have two lanes in each direction with a steel cable crash barrier in the middle but no hard shoulder. Most junctions will be at grade. With the exception of the restricted median width and the lack of lay-bys, this type of dual carriageway is similar to many dual carriageways found in the UK. The first 2+2 scheme (and the only example as of 2008), the N4 Dromodmarker Rooskymarker bypass, opened on 7 December 2007.
  • 2+1 roads - officially these roads are designated as Type 3 dual carriageways by the NRA. They have two lanes in one direction and one lane in the other, alternating every few kilometres, and usually separated with a steel cable barrier. Sections of 2+1 road have been built on the N20 and the N2. In July 2007, the NRA announced that it would no longer build 2+1 roads and 2+2 roads will be built instead.


United States

In the U.S. this type of road may be called a divided highway and has a median between the traffic directions. With few exceptions, all roads in the federally funded Interstate Highway System are fully-controlled access divided highways known as freeways. A broader definition, expressways, includes both freeways and partial limited-access divided highways, and "expressway" is often used specifically to refer to the latter. United States Numbered Highways, state highways and other locally maintained highways may also be divided. Speed limits on rural divided highways in the United States can be as fast as 75 mph and on some freeways the speed limit is 80 mph. Urban divided highways which are at grade and typically have much lower speed limits are sometimes called boulevard.

In keeping with the U.S. Department of Transportation's of Uniform Traffic Controls and Devices (MUTCD), since the early 1970s all divided highways are striped by color to show the direction of traffic flow. Two-way undivided roads have an amber broken center line and solid white baseline shoulder stripes. But multilane one-way carriageways use broken white lines between lanes; the median-side baseline is solid amber, and the right sideline is solid white. Frequently in the U.S. the two carriageways are separated by some distance (wide medians with small forests or even hills in them), but drivers can always tell whether the roadway is two way or one way—and, if one way, the direction in which the traffic flows—by looking at the striping coloration. For an example, see inset showing US Route 52 near Lafayette, Indianamarker.



Canada

In Canadamarker, both "divided highway" and "dual carriageway" may be used for this type of road, although "divided highway" is more common (as "dual carriageway" is an older term being used less than it used to be); however, the segment between the roadways is always a "median" rather than a "central reservation". On some portions of Ontariomarker's 400-series highway network, the median may be either steel guardrail or an Ontario tall-wall barrier rather than an unpaved strip.

Like the US, there are two types of divided highways, fully-controlled access divided routes known as freeways, while expressways includes both freeways and partial limited-access divided highways, and "expressway" is often used specifically to refer to the latter. Compared to the US, Canadians often use "highway" to refer to freeways, especially in the Greater Toronto Area. Partial limited-access divided highways such as the Hanlon Parkway and Black Creek Drive have at-grade intersections and private entrances but have sufficient right-of-way to convert them to full freeways if traffic warrants. There are also RIRO expressway, such as a portion of Highway 35, which are not full freeways since they allow access to existing properties, but traffic speeds are faster than regular roads due to a median barrier preventing left turns (motorists have to use a "turnabout" overpass to access exits on the opposing direction).

Junctions may be at-grade or grade-separated, and there may be gaps in the median strip to allow turning and crossing. Divided highways are seldom equipped with traffic circles, roundabouts or rotaries.

China

The best examples of dual carriageways in mainland China can be seen on the China National Highways. On some routes, such as China National Highway 106, there is a central reservation.

Croatia

Dual carriageways or expressways in Croatiamarker ( ) are non-toll roads with 2 or more lanes in each direction, but without emergency lanes. The main highways/motorways in Croatia are also dual carriageways, but they have emergency lanes and tolls.

Many bypasses and beltways of smaller cities in Croatia have been recently constructed or planned as dual carriageways. All dual carriageways in Croatia house a central median, usually fitted with guardrails.

The most heavily used dual carriageway in Croatia is the B28 expressway, connecting capital Zagrebmarker to a satellite town, Vrbovecmarker. The D28 is currently finished up to the Gradec interchange. It is undergoing extensions which will increase the traffic traversing it.

Singapore

A high proportion of roads in Singapore are dual carriageways with central reservations; examples include Clementi Road, Commonwealth Avenue and Holland Road. Often there might be railings erected on the central reservation to prevent pedestrians from dashing across the road. These usually have traffic lights along the way but flyovers and road tunnels (or 'underpasses') can be built to minimise the use of traffic lights; for example, at the Holland Road-Farrer Road-Queensway junction there are three levels of roads. Before the 1980s roundabouts were popular but since then many have been changed to traffic-light controlled junctions.

These dual carriageways are to be distinguished from motorways, known in Singapore as expressways such as the Pan-Island Expressway (PIE) and Ayer Rajah Expressway (AYE) where no traffic lights are used.

Australia

Examples of dual carriageways on non-urban roads in Australia include the Hume Highway and the Pacific Highway ; the Hume Highway by 2012, will be 100% dual carriageway and the Pacific Highway by 2016 will also be 100% dual carriageway. Today, 90% of the Hume Highway is dual carriageway and only 40% or 280 km (174 mi) of the Pacific Highway is dual carriageway, plus 10% of the Pacific Highway or 78 kilometres (48 miles) is under construction. The Federal Highway between the Hume Highway at Goulburnmarker and Canberramarker is 100% dual carriageway, completed before the 2000 Summer Olympic Games. Some parts of the Princes Highway, Great Western Highway (A32) and the Barton Highway are also dual carriageway. Most non-urban dual carriageway highways/freeways are speed limited to 110 km/h (100 km/h for heavy vehicles), except for a short section in the Australian Capital Territorymarker on the Federal Highway which is state limited to 100 km/h.

Sources:

History

A very early example (perhaps the first) of a dual carriageway was the Via Portuensis, built in the 1st century by the Roman emperor Claudius between Romemarker and its port Ostiamarker at the mouth of the Tiber.

In 1907 the Long Island Parkway opened and roughly 20% of it featured a semi-dual carriageway design. The New York City parkway system, which was built between 1907 and 1934, also pioneered the same design. However the majority of it featured concrete or brick railings as lane dividers as opposed to using grass medians.

In 1924 the first Italian autostrada was opened running 55 km (34 mi) from Milan to Varese. It featured a broad road bed and did not feature lane dividers except near cities and through the mountains.

The London end of the Great West Roadmarker became Britain's first dual carriageway when it was opened in 1925 by King George V.

In 1927 the Rome bypass was opened. It ran 92 km (57 mi) bypassing Rome to the east. Almost the entire length featured a dual carriageway design. In the early 1930s it was extended southward all the way to Naplesmarker and northward to Florence. Most of the original routing was destroyed by the Allies in the Second World War.

A German dual carriageway in the 1930s


By 1930 several American and European cities had built dual carriageway highways mostly to control traffic jams and/or to provide bypass routes for traffic.

In 1932 the first German Autobahn opened between Cologne and Bonn. It ran 21 km (13 mi) and paved the way for future highways. Although it, like the first Autostrada, did not feature a dual carriageway design, it inspired the mass construction of future high speed roadways.

During the 1930s, Germany, Italy, and the Soviet Union began construction of a network of dual carriageway expressways. By 1942, Germany had over 3,200 km (2,000 mi) of dual carriageway roads, Italy had nearly , and the Soviet Union had .

What may be the world's first long-distance intercity dual carriageway/freeway is the Queen Elizabeth Way in Southern Ontario in Canadamarker, initially linking the large cities of Torontomarker and Hamiltonmarker together by 1939, with construction on this stretch of the present-day Queen Elizabeth Way beginning in 1936 as "Middle Road".

Opened to traffic in 1940, the 160 mile (257 km) long Pennsylvania Turnpike was the first rural dual carriageway built in the United States. By 1955 several states had built dual carriageway freeways and turnpikes and in 1957 the Interstate Highway System began. Completed in 1994, the major highway system links all the major cities of the United States.

External links



References

  1. The Traffic Signs Regulations and General Directions 2002 opsi.gov.uk



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