A single carriageway (

British English British English (BrE, en-GB, or BE) is, according to Lexico, Oxford Dictionaries, "English language, English as used in Great Britain, as distinct from that used elsewhere". More narrowly, it can refer specifically to the English language in ...

) or Undivided highway (

American English American English, sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. English is the most widely spoken language in the United States and in most circumstances i ...

) is a

road A road is a linear way for the conveyance of traffic that mostly has an improved surface for use by vehicles (motorized and non-motorized) and pedestrians. Unlike streets, the main function of roads is transportation. There are many types of ...

with one, two or more

lane In road transport, a lane is part of a roadway that is designated to be used by a single line of vehicles to control and guide drivers and reduce traffic conflicts. Most public roads (highways) have at least two lanes, one for traffic in each ...

s arranged within a one

carriageway A carriageway (British English) or roadway (North American English) consists of a width of road on which a vehicle is not restricted by any physical barriers or separation to move laterally. A carriageway generally consists of a number of tra ...

with no central reservation to separate opposing flows of traffic. A

single-track road A single-track road or one-lane road is a road that permits two-way travel but is not wide enough in most places to allow vehicles to pass one another (although sometimes two compact cars can pass). This kind of road is common in rural areas ...

has a single lane with passing places for traffic in both directions.

Road traffic safety Road traffic safety refers to the methods and measures used to prevent road users from being killed or seriously injured. Typical road users include pedestrians, cyclists, motorists, vehicle passengers, horse riders, and passengers of on-ro ...

is generally worse for high-speed single carriageways than for dual carriageways due to the lack of separation between traffic moving in opposing directions.

Countries

Ireland

The term ''single carriageway'' is used for roads in the

Republic of Ireland Ireland ( ga, Éire ), also known as the Republic of Ireland (), is a country in north-western Europe consisting of 26 of the 32 Counties of Ireland, counties of the island of Ireland. The capital and largest city is Dublin, on the eastern ...

Speed limits Speed limits on road traffic, as used in most countries, set the legal maximum speed at which vehicles may travel on a given stretch of road. Speed limits are generally indicated on a traffic sign reflecting the maximum permitted speed - expres ...

on single-carriageway roads vary depending on their classification:

national primary road A national primary road ( ga, Bóthar príomha náisiúnta) is a road classification in Ireland. National primary roads form the major routes between the major urban centres. There are 2649 km of national primary roads. This category of ro ...

s and

national secondary road A national secondary road ( ga, Bóthar Náisiúnta den Dara Grád) is a category of road in Ireland. These roads form an important part of the national route network but are secondary to the main arterial routes which are classified as national ...

s have a general speed limit of , while regional roads and local roads have a general speed limit of . In urban areas, the general speed limit is .

United Kingdom

The maximum UK speed limit for single-carriageway roads is lower than the maximum for dual-carriageway roads. The National Speed Limit, which is lower for built-up areas, only applies in places where a lower numeric speed limit is not in place. The UK has one major single-carriageway motorway, the A38(M), but a number of link roads at motorway interchanges are single-carriageway.

United States

No equivalent term exists in American English. A single carriage motorway in the U.S. would be termed an "undivided highway"; this is likely to mean a multi-lane road with only striping (paint) (but no median) between the two directions of

traffic flow In mathematics and transportation engineering, traffic flow is the study of interactions between travellers (including pedestrians, cyclists, drivers, and their vehicles) and infrastructure (including highways, signage, and traffic control dev ...

. A road with two lanes of traffic moving in opposite directions would be called a two-lane road. In keeping with the

U.S. Department of Transportation The United States Department of Transportation (USDOT or DOT) is one of the executive departments of the U.S. federal government. It is headed by the secretary of transportation, who reports directly to the President of the United States and ...

's ''

Manual on Uniform Traffic Control Devices The ''Manual on Uniform Traffic Control Devices for Streets and Highways'' (usually referred to as the ''Manual on Uniform Traffic Control Devices'', abbreviated MUTCD) is a document issued by the Federal Highway Administration (FHWA) of the Unit ...

'' (MUTCD), since the early 1970s, all numbered highways in the U.S. are striped by color to show the direction of traffic flow. Two-way undivided roads have a yellow center line (and, if there are baseline shoulder stripes, they are solid white on both sides). This center line may be solid, broken, or a combination of the two, with the different styles, denoting whether passing (which requires a driver to move into the lane used by oncoming traffic) is permitted at a given location. Multilane roads use broken white lines between lanes moving in the same direction; at least one solid yellow line lies to the left of the lane which borders traffic moving in the opposite direction, and the right sideline is solid white. Drivers can always tell the direction of the traffic flow by looking at the striping coloration.

Undivided highways with central turn lanes

Since successful experiments in the late 1960s, some urban undivided highways in the U.S. have had a central left-turn lane used by both directions of flow. Essentially, this configuration puts a turning lane in the position of where a median would be if the road were divided. These roads almost always have an odd number of lanes overall, usually five (two lanes in each direction with a central turning lane), but three-lane and seven-lane versions are not uncommon. Central turn lanes are most frequently built in suburban commercial areas where there are a large number of closely spaced driveways (or minor streets).

References