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Dual carriageway
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Dual carriageway
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The dual carriageway N11 in Ireland.
A dual carriageway with a median that adjusts in width to accommodate turn lanes.
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 obstacle. 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 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.
Roads are
not
to be mapped as dual carriageways if the two directions are only separated
by paint. This includes single or multiple painted centre lines
, hatched areas, channelized turn lanes
, two-way turn lanes
by flush medians
or by any other means that is not a physical barrier to vehicles.
Likewise, a
highway link
should only be drawn alongside the main highway when physically separated from it, with the link starting or ending at the theoretical
gore
point (where the lanes diverge). Lane change restrictions (like solid lines) preceding links should be mapped using
change:lanes
=*
. The
turn:lanes
=*
tag may also apply to the preceding sections.
Contents
How to map
Examples
2.1
Dual carriageways
2.2
Not dual carriageways
Exceptions
Further reading
References
How to map
A dual carriageway is mapped with two oneway
Ways tagged with
oneway
yes
. The median in the center could be tagged as
area:highway
traffic_island
according to
proposed features/Street area
. If the traffic island serves as a traffic calming device, the tag
traffic_calming
island
can be added to the two
highway
=*
ways.
Link roads
that connect the two carriageways (e.g, a u-turn lane through the median) can be mapped as the appropriate
highway
_link
Some mappers use
dual_carriageway
yes
on dual carriageway
highway
=*
lines (applicable to all road categories) to make them clearly identifiable.
For mapping dual carriageway intersections, see
Junctions#Dual carriageway intersections
An example of an
intersection with two dual carriageways
An example of a dual carriageway with a
highway
secondary_link
linking the roads. The link should start or end at the theoretical
gore
point, and
turn:lanes
=*
and
change:lanes
=*
should be added to the preceding road as appropriate. Drawing the link along the length of a turn lane should only be done if it is physically separated.
An diagram of an intersection of two dual carriageways. Take note that these links are for both directions of traffic so they don't get
oneway
yes
. On the contrary, may be helpful to explicitly tag
oneway
no
Examples
Dual carriageways
Multiple dual carriageways present at I-405 at CA 19.
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.
Große Langgasse in Mainz, Germany isn't a dual carriageway because its central reservation, despite the paving stone surface, is flush with the asphalt.
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 two
s found in the U.S. Later on, when they are to be micromapped, the sections concerned can be changed back to a single
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
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
See section
Two-Lane Roadways
2.0
2.1
See section
Multi-Lane Roadways
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