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2+1 road is a specific category of three-lane road, consisting of two lanes in one direction and one lane in the other, alternating every few kilometres, and separated usually with a steel cable barrier. Traditional roads of at least 13 metres (43 ft) width can be converted to 2+1 roads and reach near-motorway safety levels at a much lower cost than an actual conversion to motorway or dual carriageway. Denmark and Sweden have been building 2+1 roads since the 1990s.
In Ireland, 2+1 refers to a particular class of divided carriageway that for the most part has three lanes, officially called Type 3 Dual Carriageways. Such roads alternate between two lanes on one side, and one lane on the other; although there is only one lane in each direction for short distances at changeover points, and for the mainline where turning lanes are present on one or both sides of the carriageway approaching junctions. The road type is distinct from ordinary two lane single carriageway roads (S2) or wide two lane roads (WS2) that have sporadic additional lanes on one side for hills (climbing lanes) or right turns at junctions.
Many national primary roads in the Republic were upgraded in the 1990s and 2000s (decade) to wide two lane road (two lane road with space for three lanes, in addition to hard shoulders) to allow more space for overtaking (a very common maneuver in a country that had little dual carriageway until the early 2000s (decade)). However, due to the deceptive perception of safety given by such roads, many future upgrade projects are intended to be constructed as 2+1 road where traffic volume suits.
A pilot installation was used on the N20 near Mallow, County Cork. As of 2006, the National Roads Authority had decided to install 2+1 on other routes; including some under construction in 2006. As of 2006, existing 2+1 roads in the Republic of Ireland use a central crash barrier of similar design to that installed from 2004 onwards for straight sections of dual-carriageways and motorways (prior to 2004, only narrow median and obstacles such as flyover supports and embankments used crash barriers). These barriers consist of closely spaced poles, fixed below ground, carrying three high-tension thick wound cables. There are interim plans for maintaining these barriers daily, as they require prompt attention in the event of their being damaged in an accident.
In July 2007, it was announced that the National Roads Authority would no longer be building 2+1 roads, and instead replacing them with 2+2 roads (officially known as Type 2 Dual Carriageways  )- at grade dual-carriageways with a narrow median and no hard shoulders. A large portion of the country's 'N' road network will be upgraded to either motorway or dual carriageway by 2013–2015. Existing 2+1 pilot schemes will be left intact for now. Ordinary two lane, and even wide two lane, will still be used on routes requiring lower capacity. High quality dual carriageway/motorway is being installed on the major high capacity national primary roads.
|Examples of signage used in Ireland for 2+1 roads:|
Two lanes ahead
Merge to single lane ahead
Single lane only
Two way traffic (opposing lanes)
(Returning to undivided two lane road)
In Sweden, many 13 metres wide roads have been built, especially in the period 1955–1980. These have two 3.5 metres (11 ft) wide lanes, and two 3 metres (9.8 ft) wide shoulders, in the beginning planned as emergency strip, due to the relative unreliability of autos of that period.
Around 1990, the idea emerged to build fences in the middle of them and to have 2+1 lane. This would be a cheap way of increasing traffic safety, since these roads have had a bad safety record. The width invites high speeds. Some people were, for example, overtaking against meeting traffic assuming meeting cars would go to the side. The roads are a little narrow for 3 lanes, but trials were carried on a few roads. It turned out that not only did safety improve, but it was also easier to overtake than before as the 2-lane sections provide safe overtaking opportunities. After the year 2000, more than 1000 km of roads in Sweden have been converted from wide ordinary roads into 2+1-road, all with barriers.
Until recently, the roads had the original 90 kilometres per hour (56 mph) speed limit in use on most highways. As a result of this, many people drove at 90 km/h at 1-lane parts but 110 kilometres per hour (68 mph) at 2-lane parts, this being the speed limit on motorways. The speed limit has now been changed to 100 kilometres per hour (62 mph) with a notably smoother traffic flow.
In New Zealand, 2+1 roads, known as 'passing lanes', exist throughout the country, mainly in heavy trafficked areas and on hills. The majority of them have no median barriers separating the flows of traffic, and most of these allow overtaking in the opposite direction provided the lane is clear of oncoming traffic.
The "2+1-System" refers to expressways with three lanes on a single carriageway where bypassing on the lane of the opposite direction is prohibited so that speed restriction is not required to increase safety. These expressways are grade-separated with a design speed of 100 to 120 km/h and the side of two lanes (allowing to bypass lower speed vehicles) alternates about every 1.5 to 2 kilometers. In hilly country the uphill direction is usually the one with two lanes to allow overtaking of heavy, slow vehicles. In many cases there is no median barrier or it consists of concrete blocks - additional safety measures are mostly needed near the end of the two lane section where some motorists tend to pull in very late so that a longer no-traffic section needs to be inserted on the middle lane.
After some good experiences with test roads the system has been used often in places where the amount of traffic does not justify construction of a dual carriageway expressway but remote rural areas should be connected to major towns with a high speed road. Existing examples are B 1, B 4 near Uelzen, B 16, B 20, B 31n near Stockach (120 km/h), B 33, B 54, B 56n, B 67 between Bocholt and Rhede, B 72, B 210, B 300, B 482.
Divided 2+1 roads are rare outside of Europe, though they are seen in a few places, such as Ontario, Quebec, New Hampshire and Australia. The most prominent example is Interstate 93 in Franconia Notch State Park (before dropping down to a single lane in each direction, plus steel divider). In Australia the Pacific Highway features sections of 2+1 along its length between Brisbane and Sydney. The mode is not unknown in the United States off of the Interstates, including for example Arizona State Route 77 north of Oracle.
Sometimes, during freeway reconstruction, a barrier transfer machine will be used on one half of the freeway while the other is being reconstructed.
A similar concept is utilized with 3-lane setups where 2 lanes have right of way, and the opposing side has to yield to the oncoming side on major highways that are mostly 2 lanes, of which those are more common.
- Swedish Wikipedia
- Google Streetview, e.g. at
- 2+1 roads - Danish experiences, report from Danmarks Transportforskning, November 16, 2001
- National Roads Authority:
- NCHRP Research Synthesis #275 on 2+1 Roadway Design Standards