HAVING said the benefit of two courteous and very open briefings by CIE and the consultants to the Luas scheme, Semaly, and Ewbank, Preece and O hEocha, and of another by the Dublin Transportation Office, none of which, of course, in any way engages the responsibility of those involved for any of what follows, I can now address more precisely two of the key issues that divide the tunnel wise from the street wise in the Luas controversy.

These are the questions of traffic demand and system capacity; and the assessment of the likely net cost of tunnelling the city sections of the LRT lines, taking account of the offsetting savings in capital and operating costs that would arise from the shortening of the route that tunnelling would make possible.

The initial point to be made about traffic demand is that the estimates for the LRT route have been based upon out of date assumptions about the growth of population, employment, and car ownership between 1991 and 2001.

In fact, half way through this period, the 2001 estimates for all three of these have already been reached or surpassed. Consequently the traffic estimates currently in use for the purpose of the Luas project need to be drastically revised.

Such a revision, using the ESRI projected growth rates to 2006, and a reduced growth rate thereafter, and applying the 1991-1996 relationship between economic growth and the expansion of employment and, car ownership, yields car ownership and employment levels in 2011 which are about 40 per cent higher than the figures used in the Luas traffic projection.

The Dublin Transportation Office argues that much of the transport generated by this ownership will be confined to the suburbs, rather than increasing commuter traffic to the city.

This is indeed probable, but even if one assumes that by 2011 commuter traffic will have been boosted by 20 per cent rather than 40 per cent, that would still increase the morning peak traffic inwards from Ranelagh on the Dundrum route in the year 2011 from the present forecast of 3,000 to 3,600.

But the Dublin Transport Initiative traffic projections have also shown that the planned extension of the Luas from Dundrum to, Cabinteely will increase the morning peak hour traffic on this route by two thirds, which would bring the morning peak hour load in 2011 - up to 6000.

Such a figure would be in line with experience with the DART. For last year the DART was already carrying almost 5,000 passengers city bound past Merrion Gates in the morning peak hour. And the populations of the immediate binterlands of the DART from Bray and of a Luas from Cabinteely are similar, viz. around 95,000.

Thus it would clearly be indefensible to build a Luas system incapable of handling comfortably 6,000-7,000 passengers in the peak hour; although, equally clearly, the initial capacity of the vehicle fleet on a network smarting at Dundrum rather than Cabinteely might not need to be more than about half this figure.

WHAT frequency would be required to handle such a load? Obviously this depends upon vehicle capacity. The January 1995, CIE's document Dublin LRT, after natives to Tallaght, Multicriteria Analysis, proposed to operate, the route using 30 metre vehicles with a stated capacity of "200 passengers for each unit of rolling stock, nominally... in normal conditions of comfort . . . 25-30 per cent of passengers seated, four standing passengers per square metre".

Now, however, a document issued in response to criticisms of the inadequacy of proposed on street capacity has suggested that these 30 metre vehicles could carry more than 200 passengers, an outcome to be achieved by raising belatedly the standing room density from four per square metre to five per square metre.

More recently, growing recognition of the previously ignored capacity problem has led to a proposal to use larger 40 metre long vehicles, with a theoretical capacity of, 317 passengers at the higher standing density of five people per square metre. And the peak hour capacity has been calculated on the assumption all services during that hour will carry 3,17 people.

It may be possible to achieve this load of 317 passengers at the busiest moment of the peak hour but average peak hour commuter loads are always well below the largest load carried by an individual service: this is because commuter peaks are not - flat on top like Table Mountain in South Africa, they are genuine peaks, like our Big Sugarloaf.

Thus when a census was carried out on the DART on November 25th last year, the busiest train from Howth carried 825 passengers (a figure, incidentally, well below the DART theoretical capacity of 1,000), but the average load during that hour was 617. Similarly the average load on Dublin buses during the peak two hours is about halt of the theoretical bus capacity.

For the morning peak hour on Luas it cannot be assumed that the average load will be higher than 70 per cent of the theoretical capacity, which in the case of a vehicle with a theoretical capacity of 317 passengers, means an average load of 225 during this peak hour.

Thus, even if, as is also now being suggested, the peak frequency - is raised from the original proposal of 12 per hour to 20 per hour (viz. a three minute interval instead of a five minute one, which will interrupt cross traffic on average every 90 seconds), the peak hour capacity of Luas would on that basis still be no more than 4.500 per hour.

So, even on these drastically revised assumptions, one third larger vehicles, a two third, higher frequency, and a standing room density one quarter greater than was proposed before I raised this capacity issue, an on street Luas does not appear to be able to cater for about one third of the likely peak hour demand on a Cabinteely route in the year 2011.

Moreover, it now seems quite likely that the Ballymun Luas, to be, constructed starting in the year 2000, will be extended to the airport. For the alternative, a service operated on a spur from the main Belfast line may well prove to be impracticable because of a potential conflict with mainline and Malahide commuter services. And airport traffic seems likely to be even heavier than traffic to and from Cabinteely, and would thus also be likely to be well beyond the capacity of an on street Luas.

But, it may be asked, would not precisely the same capacity problem arise if the LRT operated in a tunnel? The answer is no, because an LRT operating throughout either on a street level segregated way or in a tunnel could double up in peak periods, with one driver operating two coupled vehicles, just as is done in the case of the DART.

SUCH a coupled use of 40 metre vehicles has not been proposed and is not part of the current project, as is evident from the recent announcement that the length of the on street stations will be 40 metres.

Coming on top of a substitution of 40 metre for 30 metre vehicles, an increase in the standing density beyond the "comfort" point, and a huge increase in frequencies to 20 per hour in each direction, a decision to deploy coupled 40 metre vehicles around right angled corners and through some of the narrow streets of our city, blocking sections of these streets with 80 metre stations, would be an ultimate admission of the bankruptcy of the on street concept in our capital city.

Why has there been such a persistent refusal to face the issue of the capacity deficit of an on street Luas?" One reason seems to be the manner in which the Dublin Transportation Office, which is responsible for coordinating the implementation of the traffic strategy, is seeking to apply its vision of a Dublin city centre dedicated to pedestrians, who it sees as being protected by user friendly on street light rail services against cars congesting the city centre. The DTO envisages public transport ensuring a full utilisation of the potential capacity of a low rise city centre.

But, beyond that point of a fully utilised potential capacity, it does not want more people coming into the city. Consequently it does not, seem to have been preoccupied with the issue of adequate LRT capacity and there is a clear reluctance to contemplate a tunnelled alternative to its favoured on street LRT.

Now, I sympathise strongly with its objective, but I feel its approach is too ideological. First of all it involves a rejection in principle of, underground rail services, which are a valued feature of almost all major cities, including many which do not have Dublin's problem of a dearth of traffic routes through the city centre. By contrast, the DTO approach seems to attribute to on street light rail system an almost mystical role in the evolution of our city.

For my part, I cannot see how this vision of Dublin is to be helped by building an inadeqate LRT system, with a capacity deficit which will ultimately force many people to continue to drive into the city, thus impeding the realisation of the planners' vision. I take the simple view that if you want an uncongested city, you must ensure that your public transport systems efficient and endowed with sufficient capacity to carry all who are willing to abandon their cars in favour of such a system.

Nothing, I feel, could be more disastrous than to build an on street system and then, later, because of its inadequate capacity, to have to scrap it in favour of a belatedly introduced tunnelled system, at an extra cost of some hundreds of millions of pounds. That is the issue that must now, however belatedly, be addressed. A refusal by the public authorities, from the Minister downwards, even to debate this issue seriously would be in defensible.