Some detailed information follows on the definition of different standards of road used in Ireland.
Many motorways in Ireland are opened as dual carriageways and subsequently have motorway regulations applied to them. From a legal viewpoint, this is easier. During planning for the interurban motorway network, it was first announced that all the roads would be motorways, but as a concession to the farmers of the country many of them were downgraded to dual carriageways. This meant that the roads would be physically identical to motorways, but farming vehicles, bicycles, pedestrians etc. would be allowed the use of the road. It would also be possible that local accesses be provided to farms, following an application being lodged with the local authority. The only roads to escape this downgrade were the ones already under construction or completed, namely the M1, M3, M4 as far as Kinnegad, and M7 as far as Mountrath southwest of Portlaoise. Oddly too, the Fermoy Bypass north of Cork would be motorway, but not any other part of the road.
Thankfully, this unacceptable situation was reversed when a new law was passed in 2007, known by the unwieldy name of "SI No. 279 of 2008 Roads Act 2007 (Declaration of Motorways) Order 2008". This law would make it possible in future to declare an already existing road a motorway. Previously, a road could only be a motorway if it was referred to as such in the legal documents dating from the planning phase.
In August 2008, it was announced that most of the interurban motorway network would in fact be opening as motorway after all. Sections which had already been completed would be changed over, which would require signage removal. The NRA were pushing this issue to protect the network they were building.
It could be argued that this was a bit underhanded as many of the roads had passed community consultation on the understanding that road users usually barred from using motorways would be given access to it. However, to allow local access, farm access, and agricultural vehicle and bicycle access to trunk routes like this would have greatly eroded their function as facilitators of long distance traffic, as well as their safety record. Local traffic is not supposed to be using motorways, development should be located in towns located along the route and not in isolated areas (where it contributes to urban sprawl), and slow vehicles such as bicycles and tractors are a traffic hazard. By imposing this change of status, the NRA were protecting the country's new motorways from inappropriate development which would have disimproved their safety level and necessitated widening projects in the near future. As it stands, it is not envisaged that road widening will be necessary for several decades.
In fact, many motorways being built currently do not even have enough traffic to justify their existence - particularly the M9, which has AADTs between 8,000 and 20,000 [as of 2010] - and are being progressed for the sake of having long, continuously signed routes. In addition, it should be noted that most acutely in the case of Ireland, the conventional road network is of an exceptionally poor standard. Little or no upgrades occurred for decade upon decade. Strategic roads such as the N18 between Ennis and Galway, the central section of the N20, and the N9 north of Waterford, are little more than winding country lanes. Since these roads cannot be upgraded by widening (known as online upgrade), a new route would have to be built on a greenfield site anyway - and since land is cheap in rural areas, it is about the same cost and effort to build motorway as a lesser standard. The same thinking was what guided the development of the Interstate network in the USA. Although many sections of this network, such as those passing through the Midwest, did not have traffic levels on the conventional road network that justified motorway construction, the roads were built to that standard regardless so as to create a network of uniform quality. In addition, as land and construction projects always get increasingly expensive with the passage of time, it is better to construct the whole network in one go rather than taking an incremental approach whereby the projects get progressively dearer as the years pass.
In October 2008, another list of roads ready for upgrade was published. These will be changed over to motorways in Spring 2009. Notable inclusions in this list were sections of the N11 - an attempt is being made to make as much of this road motorway as possible, though the environmentally sensitive sections near Dublin which cut through areas such as Glen of the Downs, mean that there will always be stretches that cannot be upgraded.
In an apparent attempt to reduce costs, from around 2004 onwards the NRA started building motorways to a slightly reduced spec. The roads would be slightly narrower, which over a long distance would save land. In particular, the central median would be only 2.6 metres wide instead of around 15 m for older motorways. Opposing traffic flows would be separated by only a concrete Jersey barrier. This tends to give these motorways a more claustrophobic feel, though the lanes widths are still the same.
The technical name for this new standard was Type 1 Dual Carriageway, previously referred to as High Quality Dual Carriageway. It should be noted that this standard of road is identical in all respects to motorway with two exceptions: there are no restrictions on traffic, and a parallel access route does not have to be provided. Upon designation to motorway however, if such a road does not exist, it must be provided. In other words, building a Type 1 or High Quality Dual Carriageway and subsequently declaring it a motorway is exactly the same as building a motorway.
Probably due to the belief that "motorways" in Ireland would in fact be opening as dual carriageways, a small number of schemes were opened in the mid-2000s which are in fact slightly substandard for motorway status. For example, they were excessively bendy, or had unusually steep grades. Notable examples include the 7km Cashel Bypass section of the M8 which opened in 2004 and was declared a motorway in September 2008 and the 23 km N11 Gorey Bypass, which was redesignated in Spring 2009. (On the other hand, the N6 Athlone Bypass, which is twisty and has very closely spaced exits, was too controversial and remains dual carriageway). However, there is no safety issue here, as these sections are simply given lower speed limits to compensate - and there are precedents in other countries where motorways very occasionally have minor deviations from motorway standard at locations where it would have been difficult or unreasonably expensive to adhere to the full standard. For example, there are a small number of locations on the American Interstate Highway network in remote parts of the Midwest where access to farms is provided directly from the motorway, as building individual junctions for all of these would have cost too much. Also, in the UK and elsewhere, motorways usually have no hard shoulder (stopping lane) at all when in dense urban areas, especially when the road is elevated - and junctions are usually very close together.
At the same time as the announcement of the Type 1 standard, Types 2 and 3 were also defined. Type 2 is also referred to as "2+2" due to the number of lanes each way. A 2+2 is a dual carriageway with no stopping lanes at the sides and a narrow median. It also features low standard or non existent junctions, which could just be roundabouts. However, junctions with minor roads are of the left-in-left-out type, or are bridged as overpasses. Median breaks are not allowed. This makes 2+2s slightly high spec than old dual carriageways, which rarely had grade separated junctions, and were riddled with median breaks and crossing traffic. Type 3 is usually referred to as "2+1" and is a type widely deployed throughout the world. Two traffic lanes run in one direction and one in the other, with a switchover occurring every 2 km. There is also a median barrier. Sadly, after building a few pilot schemes around the country, the standard fell out of favour and was abandoned. In July 2007 the Government announced that all schemes which were defined as 2+1 in the planning phase would now be progressed as 2+2s - a move which would create a large network of dual carriageways throughout the country. There will eventually be over 600 km of these. At the same time, the Wide Single Carriageway standard was deleted and any road such planned road would also go ahead as 2+2 in most cases.
In Ireland, the standard of road selected for an upgrade scheme is based on the above considerations of condition of the existing road, degree of safety improvement needed and the desire to open up isolated areas of the country, but also a metric known as Level of Service (wiki) which is originally American but used in several other countries too. This decides road type by starting with the desired service level - traffic throughput, lack of congestion, and sense of driver security. Driver security is related to how safe drivers feel from becoming involved in a collision. For example, on a single-carriageway road, if a lot of oncoming vehicles are overtaking and encroaching on the centre line, or crossing it, this causes driver insecurity, and on a multilane road, it would often be due to a lot of vehicles constantly switching lanes. They may be doing so due to closely spaced exits or poor driving discipline.
From this, 6 grades of service are defined, labelled from A to F (worst), in which traffic throughput is decreasing and congestion and driver insecurity is increasing. In Ireland, during the 1990s it was generally considered that LOS D would be appropriate to aim for, but around the late 90s-early 2000s it became policy to provide level C. The traffic capacity of C was defined as around 20,000 AADT for the lower bound and 55,000 for the upper for a wide-median motorway and 52,500 for a narrow-median one. From these, it can be inferred that the capacity of a 6-lane narrow-median motorway such as the M1 north of Dublin Airport is around 78,750 AADT and for an 8-lane (such as much of the M50) it would be 105,000. It is interesting to note that these levels are frequently breached at rush hour on those two roads.