|The carriageway reference used for maintenance and emergency services (United Kingdom).|
|Used on these elements|
|Tools for this tag|
carriageway_ref is used to distinguish the two separated carriageways of a dual carriageway road. These markers allow emergency services and maintenance agencies to locate where incidents may be occurring, and people calling for breakdown services to locate themselves to their breakdown operator. This is very different from the ref=* tag, which is used to indicate the road number.
In the United Kingdom the main carriageways of a road are usually designated "A" and "B", with the off sliproad from the "A" side being "J" and the on sliproad being "K", with "L" and "M" being used for corresponding "B" carriageway sliproads. "C" and "D" are used for where motorways or other divided highways have collector/distributor lanes, e.g. the M20 near Maidstone or M60 around Manchester. Some more complicated junctions use other references.
The reference values can be considered common knowledge because they are given on wikipedia:Driver location signs signs (e.g. M27 B 2.8, as seen in the image right) and are also printed upon the emergency telephones.