From the first week of August motorists using the M50 will face traffic restrictions along a 7km (four miles) stretch between the N4 interchange and Ballymount as construction starts on the upgrade of Dublin's chronically congested orbital route, writes David Labanyi.
The N4 and N7 interchanges within this section will be redesigned to create a "free flow" system that will not require traffic lights. An extra lane is being added in each direction on the M50 along with a fourth lane linking adjacent interchanges.
This extra lane is to allow traffic going from one interchange to the next to remain separate from the main body of traffic on the M50. When complete, the M50 will have a hard shoulder.
John Glynn, project manager with lead contractor SIAC-Ferrovial, said the speed limit on this section of the M50 will be reduced to 60km/h (37mph) during the construction.
He admitted that trying to minimise traffic congestion on a route handling up to 100,000 vehicles a day was the most complicated part of the project and that disruption was "unavoidable" during the construction stage.
However, by using "quite a complex system" of road layout changes and giving motorists regular updates on restrictions, Mr Glynn said capacity on the route would not be significantly affected.
To reduce driver confusion, details of layout changes at junctions and along the M50 will be published in local and national media and will also be available on a website, he said.
Mr Glynn said two lanes in each direction would remain open to traffic between 6am and 10pm but outside these peak times intermittent traffic restrictions would apply.
To reduce traffic congestion much of the building work will take place at night.
One of the first changes motorists will experience will be M50 traffic pushed on to the hard shoulder and the slow lane to allow construction begin along the central median.
What is now the inside lane will be reserved for emergency vehicles, allowing them to reach any possible crash.
The land required to widen the M50 will come primarily from placing additional lanes on what is now the central median and also narrowing existing lanes slightly to 3.5 metres. This is the standard lane width. Once building is complete, north-bound and south-bound traffic will be separated by a concrete barrier.
Hugh Creegan, NRA head of programme management, said the Ballymount interchange would also be upgraded under the first phase of works but not to a "free flow" system.
Access lanes to and from all the interchanges along the M50 will also be upgraded.
Mr Glynn said when work on the central median was completed, traffic would be temporarily switched to one side of the motorway while work was completed on the other, before being switched back again.
Because the additional M50 lanes will run along what is now the median, no extra land is required for the upgrade, although extra land has been purchased to accommodate the new, larger interchanges. However, once completed, the current upgrade on the M50 is the limit of extra capacity possible.
If future demand runs over the capacity of the upgraded M50, Mr Creegan said demand management measures would be required. A number of these were currently being assessed, he said.
Mr Glynn added that barriers would be erected near housing to reduce the impact of noise and dust during construction.
Work on the section between the N4 and Ballymount is expected to last two years and will cost €244 million, including land acquisition costs, according to the NRA.
The section between the N4 and Ballymount will be the first part of the 32km-long M50 upgraded to six lanes. The overall project is expected to be completed by 2010.