Dr Garret FitzGerald must be terribly disappointed with the conclusion of the W.S. Atkins study that Luas should go ahead on-street, rather than underground, in the city centre. Others who favour banishing the light rail system beneath the ground are also shaken.

Contrary to expectations, not to mention wild speculation in the media, Atkins did not find in favour of the underground option. Had it done so, all of the "undergrounders" who called for this independent study would have said: "There, we told you so!"

Instead, they have lost the argument and now can't bring themselves to concede defeat.

Dr FitzGerald has described the consultants' recommendation to proceed with CIE's on-street system as "timorous", saying that the only reason given to justify it was that the underground option would cost almost twice as much. The truth, however, is that numerous other reasons are advanced for an on-street system.

According to Atkins - and this must have come as a shock to the ex-Taoiseach - the on-street system would carry more passengers per annum (29.2 million), as against 23.2 million for the underground option. The consultants say it would reduce peak-hour car use by 3,300 trips per day, compared to 2,800 for the underground.

A lot of eyebrows have been raised by the revelation that either option would only result in a shift from cars to public transport of 1 per cent. But this refers to the percentage of total trips across the whole city. Obviously, no car-user in, say, Blanchardstown is going to transfer to a light rail system which serves Dundrum and Tallaght.

Although Atkins says there is a better economic case for going underground, this is largely because delays to other road-users such as car commuters are counted as a negative in evaluating the on-street system. But the same would be true of QBCs (quality bus corridors) which also take road space away from cars.

The cost of the two alternatives must be an important factor in assessing their merits. Atkins says going underground would cost £500 million, compared to £263 million for the on-street scheme. But the report clearly indicates that the underground estimate could vary by up to 30 per cent, bringing the cost to £650 million.

"To justify this additional level of expenditure . . . it would be necessary to demonstrate that the underground option represents a more cost-effective way of meeting Dublin's long-term transport needs than the surface option. The analysis undertaken for this study suggests that this is not the case," the British consultants conclude.

Interestingly, the price they give for going underground is very similar to the estimate provided by CIE's own consultants, Semaly/Ewbank Preece O hEocha, nearly two years ago. Their report was dismissed at the time by the underground lobby as little more than special pleading on behalf of their clients; now they have been vindicated.

Atkins also warns that the extra cost of the underground option poses a potential risk to the funding of other elements of the Dublin Transportation Initiative strategy, which puts forward a package of measures designed to meet the city's future transport needs. An on-street light rail system is an integral part of the DTI strategy.

Instead of spending £250 million to £400 million on an underground, the money could be invested in extending light rail to other areas - such as Ballymun and Finglas, as the DTI proposed - as well as providing a rail link to Dublin Airport. The north side, therefore, would have every right to look askance at the profligacy of going underground.

The Atkins report points out that the underground option would not perform the same role as an on-street system in terms of "enforcing" public transport priority within the city centre. Thus, its ability to implement the wider DTI strategy to achieve a city-wide shift from cars to public transport could be questionable.

Apart from being "far superior in simple financial terms", the study found that the on-street system - and this is a point ignored by Dr FitzGerald - would achieve much more in terms of urban regeneration and the planned western extension of the city centre towards Smithfield, the National Museum at Collins Barracks and Heuston Station.

He made much in his article yesterday of the Atkins finding that an underground was the best environmental option. But the consultants also say that, if it was implemented without measures to remove traffic from the central area, this would bring about a significant deterioration in the inner city environment.

Instead, he concentrated on painting a nightmare scenario of long trams, "half the length of Croke Park", blocking traffic on city-centre streets at 75-second frequencies. However, this thesis appears to be based on a false premise that Luas would be inserted into the present traffic-choked environment of the central area.

In fact, as the Atkins report makes clear, its recommendation in favour of the on-street system is based on the introduction of a completely new traffic management regime in the city centre. Indeed, it says this regime, including, for example, "environmental cells" from which through-traffic would be excluded, is crucial to the equation.

It is evident that Dublin Corporation's plan to implement the new traffic management system heavily influenced the consultants in reaching their conclusion. Because without such measures, as they say, "unacceptable levels of delay would be imposed upon other road traffic (including buses)" - and Dr FitzGerald would be right.

But with the new regime in place, and much of the existing traffic rerouted away from the central area through which Luas will run, the performance of the street network in the morning peak period between O'Connell Street and Lower Grafton Street would reduce delays to traffic by up to 85 per cent compared to the existing situation.

Even with Luas in place, saturation at key junctions in the area is reduced dramatically, and the main report has a table which shows this quite clearly. Another table shows that the number of vehicles queuing at these junctions would be cut from figures in the region of 36 and 44 to just three and two. So much for the nightmare scenario.

Thus, with the new traffic management system in place, Atkins was able to conclude: "Under these circumstances, the analysis has indicated that the surface option can be accommodated on-street without significant impact upon the performance of the city-centre highway network or on the reliability of the LRT service itself."

The case for the on-street system, therefore, "rests upon the implementation of measures proposed by Dublin Corporation to remove general road traffic from key sections of the central area alignment", the report makes clear. In other words, what we're talking about is a completely different city-centre environment.

Neither did Dr FitzGerald deal with any of the disadvantages associated with his underground option, which is really a swish, south County Dublin way of looking at the world. For example, a sizeable chunk of the much-loved park in St Stephen's Green would be devastated to dig an immense hole, some 20 metres deep, where one of the stations would be built. The same fate would lie in store for the Iveagh Gardens.

These works could last for two years and six months. The same timescale would apply to the large excavations needed for underground stations in O'Connell Street and College Green, in front of Trinity College. Temporary road decks, known as "umbrellas", would be installed to cover the huge holes, but traffic would have to be restricted.

The Atkins report also suggests that some buildings may experience "differential settlement" (i.e. subsidence) while a tunnel is bored underneath. "The risk of building damage . . . during the construction phase requires detailed consideration if the underground option is progressed further," according to the independent consultants.

And as for his claim that the report confirms his view that the capacity of an on-street Luas would not meet passenger demand, the consultants say that phase one of the currently proposed surface system "has sufficient capacity to carry forecast demand for the foreseeable future - at least until 2025".

One gets the feeling that Dr FitzGerald believes Atkins would be a brilliant report if only it had come to a different conclusion, one that would suit his argument.