2+1 road

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Image:Motortrafikled1.jpg 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 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.

1 Introduction of 2+1 in Ireland
2 North America
3 References
4 See also

[edit] Introduction of 2+1 in Ireland

In the Republic of Ireland, 2+1 refers to a particular class of divided carriageway that for the most part has three lanes. 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 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 manoeuvre in a country that had little dual carriageway until the early 2000s). 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 has decided to install 2+1 on other routes; including some under construction in 2006. Full policy concerning 2+1 roads has not been finalised, as some pilot schemes are still in operation. It is likely that some current wide two lane roads, i.e. many of the new routes built in the 1990s and 2000s, will be retro-fitted with 2+1. Ordinary two lane, and even wide two lane, will still be used on routes requiring lower capacity. Dual carriageway/motorway is being installed on the major high capacity national primary roads.

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.

Examples of signage used in Ireland for 2+1 roads:
Image:Diamond road sign lane splits.png Two lanes ahead	Image:Diamond road sign merge to single lane.png Merge to single lane ahead	Image:Diamond road sign single lane.png Single lane only	Image:Diamond road sign two-way traffic lanes.png Two way traffic (opposing lanes) (Returning to undivided two lane road)

[edit] North America

In North America; there are similar 2-lane highways with occasional segments where a passing lane is added to only one side at a time (2:1 lane ratio). The 2+1 roads in North America are often similar to having a 3-lane reversible lane road, but with a yield:right-of-way setup where the oncoming side has a sign stating "yield to oncoming traffic".

In the Rocky Mountains and other areas with steep grades, 2+1 roads are common, with the 3rd lane being a passing lane for the uphill traffic.