A dual carriageway (BrE) or a divided highway (AmE) is a class of highway with carriageways for traffic travelling in opposite directions separated by a central reservation (BrE) or median strip (AmE). Roads with two or more carriageways which are designed to higher standards with controlled access are generally classed as motorways, freeways, etc., rather than dual carriageways.
A road without a central reservation is known as a single carriageway regardless of how many lanes there are. Dual carriageways have improved road traffic safety over the years and over single carriageways and typically have higher speed limits as a result. In some places, express lanes and local or collector lanes are used within a local-express-lane system to provide more capacity and to smooth out traffic flows for longer-distance travel.
History 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.
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.Henry S. Griffith, History of the Town of Carver, Massachusetts: Historical Review, 1637-1910, New Bedford, Massachusetts: E. Anthony & Sons, 1913. 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 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 Road 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 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 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 of dual carriageway roads, Italy had nearly , and the Soviet Union had .
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 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.
European implementations
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 (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.
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". 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 UKType of vehicleSpeed limitCar, motorcycle or a car-based van up to 2 tonnesCar with caravan or trailerBus or coach up to 12 m longGoods vehicle up to 7.5 tGoods vehicle over 7.5 t (England and Wales) (Scotland and Northern Ireland)
A dual carriageway in a built up area will have a statutory speed limit of unless otherwise sign-posted. It is common for such urban dual carriageways to have an increased speed limit of . 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.
Ireland
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 for national routes usually applies (by default the limit is for non-national roads, even if dual carriageway). Local authorities have the power to apply a limit of up to 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 of 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 .
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: