Dual carriageway

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The dual carriageway N11 in Ireland.

A  dual carriageway (known as a divided highway in American English) is a road with two carriageways (roads in each direction) that are physically divided by a non-road barrier, median, or other obstruction. This is done to separate high speed traffic, allow usage of the middle of the road, or accommodate new lanes. Motorways/expressways, trunk roads, and even lower classifications of roads are often split and defined as dual carriageways. This page explains the definition and mapping of dual carriageways in OSM. Government transportation departments or other organizations may classify dual carriageways differently.

A dual carriageway with a median that adjusts in width to accommodate turn lanes.

A dual carriageway does not have to maintain a median or barrier throughout its entire extent to be mapped as a dual carriageway. For example, primary and secondary roads can have landscaped medians that are meant to separate ways of traffic until a turn lane is accommodated and the median stops. The median usually continues after the intersection, so the road should continue to be mapped as a dual carriageway as long as the median is generally present.

Dual carriageways are not appropriate for for roads that have no physical separation. For example, roads with single-marked centerlines[1], flush medians[2], channelized left turn lanes[3], or two way left turn lanes[3].

How to map

A dual carriageway is mapped with two oneway way Ways tagged with oneway=yes. The median in the center could be tagged as area:highway=traffic_island according to Proposed_features/Street_area (traffic_calming=island + area=yes is often used without regards to whether it may be considered as "traffic calming" by causing lateral deflection). Any u-turn links in between the two roads should be connected with highway=(highway type)_link.

Examples

Dual carriageways

Multiple dual carriageways present at I-405 at CA 19.

I-405 at CA 19.jpg

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Not dual carriageways

East Grant Road in Tucson, Arizona is not a dual carriageway because of it does not have any physical barrier in between the lanes.

NotCarriage.png

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Exceptions

If a road contains any of the non-appropriate elements for some short distance, but is still separated by a physical obstructions along other parts of the road in the vast majority of its length, then the road can continue to exist as a dual carriageway for the moment. These exceptions are typically easy to recognize, and may be kept in-thought of ease of recognition, routing, presence, and functionality of the road, in lieu of more detail. Other exceptions have been made by some users for dual carriageways such as roads like Super twos found in the U.S. Later on, when they are to be micromapped, the sections concerned can be changed back to a single way with appropriate tags such as change=*.

That being said, when a new dual carriageway is created, extra attention can be taken to avoid drawing 2 way pass the physically separated sections. This would more accurately represent the road's real layout from the beginning, eliminating the need to wait for iterative refinement at some future time.

Further reading

  • Dual vs. Single Carriageways - A guide explaining the government-defined classification of carriageways in the U.S. and how they vary state-to-state by DOT. These classifications do not follow OSM's classification though it explains that OSM-like classification is useful and used in some instances.

References