INTRODUCING an on street LRT service requires major traffic disrupting engineering works along the streets affected. The prospect of such engineering works on Dublin streets has evoked considerable concern among city centre businesses. Business people fear the temporary diversion of traffic during construction could cause permanent rather than temporary loss of business.

If an on street LRT could be shown to be the best way to alleviate the city's traffic problem, this difficulty would, of course, have to be faced, while taking steps in close co operation with the business interests affected to minimise and mitigate the effects of such temporary disruption.

However, if as was suggested in yesterday's article, on street LRT would be incapable of handling the traffic volume on at least one of the two proposed routes, such "temporary" disruption becomes impossible to justify to those whose businesses will be adversely affected by the track laying.

But there is an even more serious problem, to which no serious attention seems yet to have been given: that is the permanent congestion that the operation of an on street LRT in Dublin will cause.

The scale of this disruption may not be fully appreciated. It will take three quite different forms.

First, there will be a reduction in the number of traffic lanes on main thoroughfares such as Harcourt Street, St Stephen's Green, Dawson Street, Westmoreland Street, O'Connell Bridge, Lower O'Connell Street and Middle Abbey Street - as well as on streets between Heuston Station and the Naas Road. In certain cases where a street is at present not quite wide enough to accommodate three lines of traffic, this could halve the existing traffic flow, and in most cases it would cut it by a third.

These reductions in the number of lanes available to other traffic will be necessary because to give the LRT the priority required to enable it to operate without obstruction through the city centre, its track will have to be kept clear of other vehicles.

In the second place, cross traffic will have to be halted whenever an LRT vehicle is approaching. This will disrupt the present traffic control system which depends on adjusting the traffic light system to accommodate fluctuating traffic flows. Moreover, because, as was demonstrated in the first of these articles, in order to cope with the demand at realistic vehicle occupancy rates, the frequency of the service will have to be substantially higher than hitherto envisaged, and so the scale of disruption of cross traffic will be much greater than had been thought. Thus, even a frequency of 17 vehicles per hour, inadequate for the demand, would involve halts to cross traffic at an average interval of 1 minute 45 seconds - halts additional to those required for road traffic flow reasons.

The third way in which city traffic will be disrupted is by the complete closure of certain city centre streets to cars. These Include Upper Abbey Street, leading from Middle Abbey Street to Capel Street, which is now very heavily used, as the only available east west route between Parnell Street and the southern quays. But another route to be closed to cars is the crucial section of Nassau Street and lower Grafton Street that links the bottom of Dawson Street to College Green.

Now this is one of only two south north routes between Westland Row and Christchurch Place - a distance of over three quarters of a mile across the south centre of the city. The halving of intra city car flows on this key axis is an extraordinarily drastic measure to be taken, not on its merits, but as an almost casual - and unannounced - by product of a decision to introduce an on street LRT service.

The huge increase in congestion that this halving of the centre city south north traffic flow will create will far exceed any possible relief from the reduction in car traffic entering the city from Dundrum and Tallaght brought about by the LRT. And all that has been offered to date in vague justification of this huge net increase in traffic congestion has been an assertion that it will discourage cars from coming into the city! A much more serious approach than this is required towards our city traffic problem.

There are, of course, good reasons for seeking to redress the imbalance between private and public transport in cities like Dublin. But to do this intelligently it is necessary first of all to address the real reason for this imbalance. This is the fact that road space in urban areas is scarce, especially on entry and exit routes at peak periods but also throughout the day in inner city areas where commuters and shoppers seek to park.

Now the only charge made for the scarce space on entry and routes at is charged on petrol used by commuting vehicles. And while this fuel duty may represent an appropriate charge for the provision and maintenance of non urban road space, it is self evidently far below the level needed to balance the supply of and demand for peak period urban commuter road space.

As would be the case with any other commodity or service in limited supply, the consequence of gross under pricing of road space, when not accompanied by some kind of quantitative control, can only be chaos. What happens is that private car owners, encouraged by the provision of road space at a fraction of its real value, and discouraged from using bus transport by the fact that road congestion prevents buses from operating efficiently, further congest the urban roads at peak periods, slowing down public transport even more to a fraction of the speed it can attain under uncongested conditions.

A huge cost is thus imposed on commuters. For where public transport can operate efficiently, a clear majority of commuters prefer it. Thus, in the hinterland of the DART, 60 per cent of vehicle using commuters travel by rail, and 40 per cent by car. By contrast, from other areas, 70 per cent come by car and only 30 per cent use public transport.

We can see, therefore, that where the only transport available is bus transport - which cannot operate efficiently because of the private car congestion caused by under pricing of road space - 30 per cent of vehicle users who would use an efficient rail based system are prompted as a second best to commute by private car.

AS a result of this, everybody loses: not just the 30 per cent who would use an efficient public transport service if it existed, but also the 40 per cent who prefer to commute by car even when an efficient public transport service is available, and also the 30 per cent who, because of the location of their homes, prefer to commute by bus: all have their journeys greatly lengthened as a result of the absence of an efficient rail based service.

Nevertheless, Singapore apart, and with the limited exception of motorways from outer suburban areas in certain countries, public authorities throughout the world, fearful of the reaction of car owners, have been unwilling to introduce road pricing for commuters - although this is now technically feasible.

Various expedients have been resorted to in order to limit the chaotic consequences of this reluctance to apply market principles in this area. Parking controls are one such expedient, which, however, is of limited effectiveness because, if car access to a city is free, optimism about parking induces far more people to drive in than can be accommodated without congestion.

Some relief can also be provided by introducing priority lanes for buses. But the preferred and by far the most effective way of getting around the consequences of the unwillingness to control demand by road pricing, is to introduce railbased commuter services, which may run on streets where demand is relatively light and/or streets are wide, but which go underground in congested city centres.

I am not aware, however, of any other city where it has been seriously suggested that the answer to traffic congestion is to make it worse by using onstreet LRT services to block principal thoroughfares. Only in Dublin has this been suggested.

The simple truth is that even if in the case of Dublin on street LRT, it had the required capacity to carry the commuters - which it demonstrably has not - it is effectively ruled out by the particular urban geography of Dublin's city centre. For the centre of Dublin is bisected by a west east river and on the south side it is narrowly hemmed in to a 350 metre wide space between Dublin Castle and Trinity College, within which the key streets are all quite narrow. Failure to face these realities would be a fatal blunder of urban planning.