Dual carriageway

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A typical British dual carriageway, this one being the A63(T) near Hull

A dual carriageway or divided highway 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 or median. This type of road is usually able to carry a great deal more traffic than normal "single carriageway" roads and boulevards.

[edit] United Kingdom

In the UK, although the term dual carriageway applies to any road with physically separated lanes, it is frequently used as a descriptive term for major routes 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 long-distance 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 (eg. "A1(M)").

[edit] 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.^[1] 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.

[edit] 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:

[edit] Ireland

A typical modern Irish dual carriageway (opened 2004) along the N11, south of Newtownmountkennedy. On motorways, the yellow hard shoulder markings are unbroken.

Although in the Republic of Ireland 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 demarkated 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 2006, the N2 north of the M50 is the only route section with such a special limit). 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 development on National Primary or National Secondary roads where the speed limit exceeds 60 km/h (37 mph), this is at the direction of the National Roads Authority, however a County Council 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 may also be used 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. These 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) are being fitted to existing routes.

As of 2007 three major types of dual carriageway are being built on national road schemes in the Republic of Ireland:

• High Quality Dual Carriageway - this is being built mainly on inter-urban routes, and is to full motorway standard but without the regulations that go with the motorway classification. The Roads Act 2007 allows for these roads to be designated motorway by ministerial order.
• Standard dual carraigeway of the traditional type is mainly planed for schemes on the N11 road, N18 road, N20 road, and N25 road. Traditionally these 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 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 was the Ennis bypass.
• 2+2 road - These will be created by widening existing roads, and will have two lanes in each direction with a crash barrier in the middle but no hard shoulder. Junctions will be at grade. See 2+1 road for details of how this type came into usage. The first 2+2 scheme, the N4 Dromod Roosky bypass, opened on 7 December 2007 [1].

[edit] North America

In the U.S. this type of road may be called a divided highway and has a median strip between the traffic directions. Many, but by no means all, divided highways in the United States are part of the Interstate Highway System, funded by the federal government. In addition to the Interstate system, some portions of the older United States Numbered Highways as well as certain State highways (such as Pennsylvania Route 60).

In Canada, 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); however, the segment between the roadways is always a median strip rather than a central reservation. On some portions of Ontario's 400-series highway network, the median may be an "Ontario tall-wall" barrier rather than an unpaved strip.

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.

[edit] 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.

[edit] Croatia

Dual carriageways or expressways in Croatia (Croatian: brza cesta) are non-tolled roads with 2 or more lanes in each direction, but without emergency lanes. Because of the lack of tolls normally seen on many Croatian highways, the use of dual carriageways is highly encouraged. 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 D28 expressway, connecting capital Zagreb to a satellite town, Vrbovec. The D28 is currently finished up to the Gradec interchange. It is undergoing extensions which will increase the traffic traversing it.

[edit] 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.

[edit] Australia

A typical dual carriageway in Melbourne, Australia.

Examples of dual carriageway on non-urban roads in Australia include the Hume Highway and the Pacific Highway (Australia); 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 today, 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 Goulburn and Canberra 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 110km/h (100km/h for heavy vehicles), except for a short section in the Australian Capital Territory on the Federal Highway which is state limited to 100km/h.

Sources:

• AusLink
• Hume Highway
• Pacific Highway (Australia)
• DIVIDED HIGHWAY ACHIEVED BETWEEN BRISBANE AND BYRON BAY TURNOFF - MEDIA RELEASE, The Hon Jim Lloyd MP
• NSW South West construction projects, RTA
• Pacific Highway upgrade, RTA

[edit] 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 Rome and its port Ostia 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. ^[2][3]

The London end of the Great West Road became Britain's first dual carriageway when it was opened in 1925 by King George V [2].

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 Naples 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 Europeon 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 1,300 km (800 mi), and the Soviet Union had 400 km (250 mi).

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

In 1940 the Pennsylvania Turnpike was opened to traffic and at 160 miles (257 km) long it 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.

[edit] See also

• Highway
• Freeway
• Motorway
• Autostrasse
• Expressway
• Single carriageway

[edit] External links

• Department for Transport. Speed: Know your limits (PDF)

[edit] References