Dual carriageway

Missing image
This early German Autobahn uses a dual carriageway design.

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.


United Kingdom

In the U.K., 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 than 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.

Turning right (that is, across the line of traffic heading in the opposite direction) is usually only permitted 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.

National speed limits on dual carriageways in the UK
Type of vehicleSpeed limit
Car up to 2 tonnes/motorcycle70 mph (about 110 km/h)
Car with caravan or trailer60 mph (about 95 km/h)
Bus or coach up to 12 m long60 mph
Goods vehicle up to 7.5 t60 mph
Goods vehicle over 7.5 t50 mph (about 80 km/h)


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 100km/h for national routes applies, as opposed to the speed limit of 120 km/h for motorways. Traffic lights and junctions are permitted at grade on such routes. 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 frequency of junctions (necessitating the lower speed limit). Dual carriageways that are not motorway classified, also do not need to be equipped with emergency phones.

Motorway restrictions do not apply to dual carriageways, and even cyclists may use such routes (although international practice would dictate this as inadvisable), and indeed often do use the hard shoulder on dual carriageways.

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 retrofitted 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.


The best example 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.


Swiss dual carriageways are referred to as Autostrasse. There may or may not be a central reservation.


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.

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