Cork County Council is to consult the public on three possible routes for a proposed €500 million northern ring road of the city.
The development is likely to incorporate a €300 million tunnel that would take an estimated 25,000 vehicles a day out of Cork city.
Cork county engineer Mr Ned Flynn said the council was looking at three options, all involving the construction of a tunnel of between 2.5km and 5km in length. All three will be put on display next month in Ballincollig, Blarney and Glanmire.
Mr Flynn said the most likely starting point for the road would be on the Poulavone roundabout on the Ballincollig bypass. It would then cross the River Lee somewhere near Leemount before hitting a high escarpment at the back of the Lee Road.
"The escarpment there is quite high and given that we are constrained by road design regulations to a 5 per cent gradient - we rise up one metre for every 20 metres we travel - the most attractive option would be a tunnel," he said.
Depending on the arc it takes, the tunnel would continue underground north of Clogheen before exiting in the Killeens/Monard area, where a major interchange would be built to link the ring road with the main Cork-Mallow-Limerick road.
The ring road would then continue overground to Dublin Pike and Kilcully before connecting via another interchange with the main Cork-Dublin road somewhere on the mile-long stretch between the end of the Glanmire bypass and Sarsfield's Court hospital.
"Nothing is definite yet - the Poulavone roundabout is the most probable starting point but we could also look at starting it further west near the Ovens Bridge and crossing the River Lee further upstream," said Mr Flynn.
He said the tunnel would be the biggest ever construction project in the State, including twin tunnels with two lanes each, a major bridge over the River Lee and two interchanges at Killeens and Glanmire.
"The escarpment on the northern side of the Lee is made up of a combination of sandstone and limestone, but tunnelling through rock may not be that difficult - rock gives a very solid foundation and is preferable to earth which can be more variable," he said.
He said that the council would seek to minimise disruption by avoiding houses on whichever line was chosen.
The likely line the tunnel will follow is all under agricultural land as all three possible routes take arcs of varying width to avoid the urban areas of Knocknaheeny and Hollyhill on the north-western side of the city, he said.
"We would hope to have an interchange with the Mallow road in the Killeens and Monard area, and that would tie in well with the plans for a railway station in the Monard area and the creation of a park and ride system there."
He pointed out that the ring road was included in the National Roads Authority's five-year plan, but only at the design stage.
This means that no funding will be available for the construction work in the NRA's current spending programme which concludes in 2009.
Construction on the project is likely to take three years, with work on the dual-carriageway and the tunnel sections likely to be carried out simultaneously, said Mr Flynn.
He added that the numbers using the route would likely increase from the estimated 25,000 when it opens.
