Dual carriageway
From Free net encyclopedia
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.
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United Kingdom
In the UK, although the term dual carriageway applies to any road with 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.
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. All such 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)").
The national speed limits on dual carriageways not in built-up areas are as follows. Local speed limits, where indicated by signs, take precedence over these.
| Type of vehicle | Speed limit |
|---|---|
| Car up to 2 tonnes/motorcycle | 70 mph (about 110 km/h) |
| Car with caravan or trailer | 60 mph (about 95 km/h) |
| 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 (about 80 km/h) |
Ireland
Although in the Republic of Ireland the term dual carriageway technically applies to any road with separated lanes, it is usually used only to refer to those routes that do not have a motorway designation. Generally only national primary routes are built as or upgraded to dual-carriageway. These routes have an "N" number (e.g. N8).
Dual carriageways of this class differ from motorways in a number of ways. The standard speed limit of 100 km/h for national routes usually applies, though local authorities have the power to apply a limit of 120km/h as used on most motorways. Traffic lights and junctions are permitted at grade on dual carriageways. For older sections of dual carriageway, this has resulted in less 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. Dual carriageways that are not motorway classified, also do not need to be equipped with emergency phones.
Motorway restrictions do not apply to Irish dual carriageway or "N" routes. Irish planning policy means that such routes frequently have houses, schools and other developments fronting on to them. Accordingly, hard shoulders are included wherever feasible to provide for the resulting pedestrian and cyclist traffic. 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 the route being motorway or not, and median width) are being fitted to existing routes.
United States
In the U.S. this type of road is called a divided highway and has a median strip between the traffic directions.
Junctions may be at-grade or grade-separated, and there may be gaps in the median strip to allow turning and crossing.
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.
Switzerland
Swiss dual carriageways are referred to as Autostrasse. There may or may not be a central reservation.
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.
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.