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Single carriageway

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Single Carriageway
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Birmingham's A38(M) is an example of a multi-lane single carriageway road
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A Single carriageway road is a road where there is a single continuous surface to the road, with no physical barrier separating the directions of travel. It is the most common form of road standard in the United Kingdom and Ireland. The term does not refer to the number of lanes - whilst most single carriageway roads have one lane per direction of traffic (known as S2), two lanes per direction of traffic (known as S4) can be found in many locations. Up until the early 1980s, it was common to find three-lane single carriageway roads (known as S3), where the central lane was designed for use by traffic wishing to overtake slower vehicles but these roads (colloqually known as suicide lanes for obvious reasons) are now very rare - probably the best example is on the A6 near Shap, Westmorland.

The widest single carriageway road in the United Kingdom is the A38(M) Aston Expressway in Birmingham, which is 7 lanes wide and uses Tidal Flow.

Single carriageway roads are generally found on routes with low to medium volumes of traffic.

Single Carriageway Types

These designations can be used in multiple ways and have multiple implications. The basic rules are:

  • If there is an even amount of lanes in both directions (and/or non-priority shared lanes) OR are 1-way streets they are designated as Sx or WSx, x being the total amount of lanes.
  • If one direction has more lanes than the other than the larger number of lanes come first then the smaller number after such as Sx+y or WSx+y, with x being the larger amount of lanes and y being the smaller amount of lanes.
  • If the carriageway is purposefully wide, except for turning lanes then the S is substituted for WS.
  • Personal discretion if bus lanes are included in the amount of lanes.
  • Turning lanes are not included in the number, unless they form from a running lane, or are significantly long.
  • Single Carriageways with a hard shoulder are extremely rare, and normally gets a WS destination.
  • Cycle lanes are discarded from the number of lanes, including contraflow lanes.


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S1 refers to normally a Single Track, it's only wide enough for 1 vehicle in 1 direction. In rural areas with very low traffic these can be two way roads, however on busier routes or urban areas it can make up parts of 1 way streets.


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S2 is a typical road, normally provides both directions of travel with their own dedicated lane, normally overtaking is permitted. Two-lane 1 way streets come under S2


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S3 roads are hard to come-by now, they were common as they allowed overtaking in both directions, even when there was oncoming traffic (as long as oncoming traffic wasn't over taking). Most S3s were converted into WS2 or S2+1. Most S3 nowadays are found on Gyratories or 1-way roads which have three lanes.


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S2+1 Are S3 roads, however one side of the road has priority or dedication of using the middle lane.


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S4 are single carriageway roads with two lanes of traffic in both directions.


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WS2 are roads which have two lanes of traffic, 1 lane in either direction. However are extra wide which have some benefits such as vehicle separation being larger, or if a vehicle or something is partially blocking the carriageway, it may not affect the flow of traffic. However does encourage faster traffic and potentially unsuitable overtaking.


Single carriage roads with more than 4 lanes are most common as brief stretches near junctions. Where they exist, they can be described with an 'S' followed by the total number of lanes. The A38(M) is the widest single carriageway road in Great Britain, and is an S7; a notable S6 is The Mall in London between Trafalgar Square and Buckingham Palace. Another example of an S6 is the A60 Trent Bridge at the boundary of West Bridgford and Nottingham.

Single carriageway
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