分隔式道路

双车道或分隔式公路是一类公路，其车道用于沿相反方向行驶的交通，并由中央预留区或隔离带分隔开。具有两条或更多车道且设计具有受控通道的更高标准的道路通常被归类为高速公路、高速公路等，而不是双车道。 没有中央预留的道路被称为单一车道，无论有多少车道。多年来，双车道比单车道改善了道路交通安全，因此通常具有更高的速度限制。在一些地方，高速车道和地区车道均使用交替道路系统以容纳更高的容量和平顺长途距离的车流量。

历史

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A very early (perhaps the first) example of a dual carriageway was the Via Portuensis, built in the first century by the Roman emperor Claudius between Rome and its harbor of Portus. The route between the city and the port experienced a great deal of commercial and pedestrian traffic, as Portus served as the primary avenue for the grain shipments of the Cura Annonae into Rome, as well as transporting the majority of goods imported from across the Mediterranean world. This influx of road activity necessitated the construction of a dual thoroughfare as the road approached the Porta Portese, the corresponding gate in the Aurelian Walls of Rome for the Via Portuensis.^[1]

One claim for the first divided highway in the United States was Savery Avenue in Carver, Massachusetts, first built in 1860, where the two roadways were separated by a narrow strip of trees down the middle.^[2] In 1907 the Long Island Motor Parkway opened, and roughly 20% of it featured a semi-dual-carriageway design. The New York City Belt 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 instead of grass medians.

In the year of 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.^[3][4]

The London end of the Great West Road became Britain's first dual carriageway when it was opened in 1925 by King George V.^[5]

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 World War II.

By 1930 several US 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 became a precedent 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（810 mi）, and the Soviet Union had 400 km（250 mi）.

What may have been the world's first long-distance intercity dual carriageway/freeway was 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". It was gradually upgraded to a freeway from the 1950s to 1970s.

Opened to traffic in 1940, the 160-英里-long（260-千米） 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.

应用

亚洲

东南亚

 马来西亚

各道路类型图表

位于马来西亚半岛西海岸的南北大道

欧洲

 英国

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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.^[6] Occasionally dual carriageways have only one lane in each direction, or more than two lanes each way (sometimes 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.

Sign informing motorists of an upcoming section of dual carriageway in Seacroft, Leeds

When first constructed, many dual carriageways—including the first motorways—had no crash- or other barriers in the central reservation. In the event of congestion, or if a driver missed their exit, some drivers made U-turns onto the opposite 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 bounce 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. Often on urban dual carriageways where the road has been converted from a four-lane single carriageway the central reservation will not be substantial: often just a small steel divider to save space.

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. Where space is even more limited, intersections may be controlled by traffic lights. Smaller residential roads adjoining urban dual carriageways may be blocked off at one end to limit the number of junctions on the dual carriageway; often other roads will pass over or under the dual carriageway without an intersection.

A dual carriageway with grade-separated junctions and which meets other requirements may be upgraded to motorway standard, denoted by an (M) added after the road number (e.g. "A1(M)" or "A38(M)"). Unlike in Ireland, there was no official terminology for 'high-quality dual carriageways' until April 2015, when in England a new standard was set to designate certain high-quality routes formally as "Expressways".^[7] Many roads such as the A1, the A14, the A19 and the A42 are built to a high quality, with grade-separated junctions, full barriers at roadside and central reservations and, in some cases, three lanes of traffic. They may still fall short of motorway standard in terms of hard shoulders, the height of overpasses or the quality of intersecting junctions.

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^[8][9]

Type of vehicle	Speed limit
Car, motorcycle or a car-based van up to 2 tonnes	70 mph（113 km/h）
Car with caravan or trailer	60 mph（97 km/h）
Bus or coach up to 12 m long	60 mph（97 km/h）
Goods vehicle up to 7.5 t	60 mph（97 km/h）
Goods vehicle over 7.5 t	60 mph（97 km/h） (England and Wales) 50 mph（80 km/h） (Scotland and Northern Ireland)

A dual carriageway in a built up area will have a statutory speed limit of 30 mph（48 km/h） unless otherwise sign-posted. It is common for such urban dual carriageways to have an increased speed limit of 40 mph（64 km/h）. A road deemed to be in a built up area is indicated by the presence of street lights; on lit dual carriageways that are not considered to be in a built-up area, the speed limit will be clarified with intermittent signs.

 爱尔兰

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

An example of a 2+2 dual-carriageway in Ireland. This type is similar to many found in the UK.

Although in 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 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 (The High Quality Dual Carriageway section of the N1 between the end of the M1 and the border with Northern Ireland and the N25/N22 Ballincollig Bypass in Cork 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. A local authority is not obliged to implement this policy and can disregard this policy at its own discretion. This would usually only occur in exceptional circumstances or where planners are overruled 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 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.

Between 2000 and 2010, three major types of dual carriageway were built on national road schemes in Ireland:

• High Quality Dual Carriageways (HQDC) – these were 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 were applied when the redesignations came into effect.
• Standard dual carriageway of the traditional type was mainly used 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 Ennis bypass although this road has grade separated junctions and no median crossings. This route was upgraded to motorway status in 2009.
• 2+2 roads—officially these roads are designated as Type 2 dual carriageways by the National Roads Authority (NRA). They were created by widening existing roads or building new roads, and have two lanes in each direction with a steel cable barrier in the middle but no hard shoulder. Most junctions are 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 was the N4 Dromod Roosky bypass, opened on 7 December 2007.^[10]
• 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 were 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 were built instead.