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Newcomers to the community of road enthusiasts (or roadgeeks, if you prefer) usually become bewildered by the various abbreviations that are discussed. The following are mostly terms used in highway engineering and appear in government design standards. Some forms are adaptations or extensions to the system created by users of the SABRE forums.
The abbreviations are all ways of describing the formation of a road - its physical appearance and layout. It's an informal shorthand and isn't particularly logical.
Definitions of other terms are in the Dictionary.
Single carriageway roads
Single carriageways are described with an S prefix, followed by a number representing to the total number of lanes on the road.
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S2 is a standard two lane road, usually one lane each way, though the term describes only the physical characteristics of the road, so it could just as easily be a one-way road with two lanes travelling in the same direction. |
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WS2 is a wider than usual two lane road. Many new rural roads are built to this standard. It often permits overtaking without vehicles having to fully cross into a lane of oncoming traffic. |
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S3 is a single carriageway road divided into three lanes, with the centre lane intended for overtaking, but with neither direction having priority in using it. They were popular for major rural roads from the 1950s through to the 1970s but now few sections survive - most re-marked as WS2. This type of road is often described as a "suicide lane", on the grounds that it has three lanes: left-side, right-side and sui-cide. |
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S2+1 describes a three lane road where the one direction takes two lanes and the other takes one. WS2+1 is quite experimental - three lanes, with the middle lane switching direction every so often to provide an overtaking lane to each side in turn. |
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S4 is a four lane road with two lanes each way. It does not qualify as a dual carriageway as there is no physical divide between the two directions of flow. These may be divided by double-white lines to prevent vehicles crossing, but in practice there is often a dashed white line down the middle, meaning that vehicles could theoretically overtake in both lanes of oncoming traffic. |
Dual carriageway roads
Dual carriageways are shown with a D prefix followed by a number referring to the number of lanes on each carriageway - not the total number of lanes on the road.
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D2 is a dual carriageway with two lanes each way. |
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D2H is a dual carriageway with two lanes each way, that is also equipped with hard strips. |
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D3M is a dual carriageway with three lanes each way and is equipped with full hard shoulders. The 'M' refers to the fact that this is a motorway formation, but a non-motorway road equipped with full hard shoulders is described this way too (i.e. it is an engineering term, not a legal one). |
Non-standard terms
There are several other terms that are at best "under discussion" or "disputed", and at worst likely to provoke the response "what?".
S1 is a narrow single carriageway with one lane each way. It's not a design standard - you'd never have cause to build one as a public road (though you might if you were building a farm access or a driveway!) - and it's debatable whether an abbreviation to refer to it is required. It could be used to refer to existing roads - a one way street, for example.
D2AP is a subject of some discussion. AP stands for All Purpose and it is therefore essentially the same as saying D2. This abbreviation is not particularly popular but is occasionally encountered.
D2+3 might be used in the same way as S2+1 above to refer to an uneven number of lanes on each half of a dual carriageway.
D2+2+2+2 is an attempt to refer to four carriageways - usually a dual carriageway with collector-distributor carriageways on the outside. It falls apart because of discussion over whether a different symbol should denote the central reserve, making D2+2-2+2 or even D2:2+2:2. One Highways Agency project referred to D3+D2, which suggests it's treated as two separate roads.
Q2 is another attempt to refer to a four carriageway road, with Q standing for "Quadruple". This works fine when each of the four carriageways has the same number of lanes but falls apart thereafter. Perhaps you could write Q2+3 to imply two carriageways of each number.
With thanks to Graham Mackay and Toby Speight for information on this page.
