The construction of a full motorway between Dublin and Cork could "cancel out" improvements in rail services on the route unless the average speed of trains is increased to 120kph (75mph), according to Prof Austin Smyth's analysis.
Noting that the two cities are 255km apart and the motorway will cut driving time by almost 40 minutes, his report said: "The fundamental concern is that rail journey time stands at around 2.5 hours and that little reduction in this time is planned."
Assuming an average speed of 90kph, driving time between Dublin and Cork would be reduced to two hours and nine minutes. This would encourage more car trips, some of which would switch from rail, and could reduce train use by about 12 per cent.
Road/rail competition "is critical to Iarnród Éireann's commercial position", but Prof Smyth said this "does not seem to have been considered in Transport 21" - even though encouraging rail use is consistent with environmental sustainability.
"Inter-city rail also has significant potential in helping to meet spatial development needs. Once again, however, under envisaged rail journey times, the extent of its contribution under this heading will be reduced significantly by the large road programme." Because of its implications for rail serving the same corridors, he warned that developing a high-quality road network "will significantly increase car travel and its market share with all of the attendant implications for greenhouse gas emissions".
Although the proposed upgrading of the N8 under the National Development Plan was not subject to cost-benefit appraisal, targets were set for achieving "level of service" and average speeds on the road network - but it was not done for the railways.
This was because Transport 21 did not consider road and rail schemes in an integrated fashion. As a result, "inter-city rail's commercial future could be undermined by road investment, just when Ireland needs a secure rail network to promote sustainability in transport".
His analysis noted that typical rail journey times of two hours 45 minutes on the Dublin-Cork route represents an average speed of 91kph - compared to typical inter-city rail speeds of up to 139kph on mainline services in Britain and 220kph in France.
Despite an increase in frequency to one train per hour in each direction, he said "the real opportunity for rail lies with higher speed [ and] there is a fundamental need for faster journey times on this inter-city route if rail market share is to be increased".
"If average speed on the Dublin-Cork line could be raised to 120kph, which is still below inter-city speeds in Britain, the journey time could be cut to two hours and five minutes. This does not feel like an ambitious target and should not need new trains," he said.
According to his calculations, the combination of higher frequency and faster operation could boost patronage by 25-30 per cent. He also concludes that more people would make use of other long-distance rail services from Dublin if trains ran faster.
Prof Smyth's report, Transporting Ireland, is to be officially launched at a Chartered Institute of Logistics and Transport conference on Transport 21 in Dublin on October 18th.
