Ireland is increasingly congested. Luas is meant to help in Dublin but is grossly uneconomic. Company parking only exacerbates the problem. In the first of a two-part series, James Nix considers the alternatives to the car

The opening of Luas will be seminal for Dublin. The first line, from Tallaght to Connolly Station, is due to take passengers in June.

The second, from Sandyford to St Stephen's Green, is set to open in August. They have been promoted as an efficient answer to the city's traffic problems. Yet Luas costs 11 times more a kilometre than the best bus lanes and will carry almost a third fewer passengers.

The city's two tram lines are set to cost €33 million a kilometre, including the vehicles and the depots. They are among the more expensive in Europe - Montpellier's equivalent cost about €23 million a kilometre - but on a par with the cost of projects in the US, whose Department of Transportation backed a study as long ago as 1994-95 showing the average was $42 million (€34 million) a kilometre.

Luas will add 24 kilometres of tram lines to a city that already has 102 kilometres of quality bus corridors. A QBC has a capital cost of €3 million a kilometre, again including vehicles and depots. The cost of a QBC in Dublin is difficult to benchmark, because the city has been to the fore in dedicating existing road space to buses, but that figure can be broken down into categories.

First, a typical 15-kilometre stretch of road will require a €15 million investment to be turned into a QBC, including raising kerbs at bus stops, realigning footpaths and cycleways and doing associated work. Second, the 65 buses required to serve a high-frequency QBC each cost €300,000, a total of €19.5 million. Depots, the final capital element, cost €10.5 million - in all, €45 million for the 15-kilometre corridor.

Bus or tram: which carries more? On Luas's Line A, 12 trams will leave Tallaght for Connolly Station between 8 a.m. and 9 a.m. Each will be able to carry 207 people, or just under 2,500 passengers each way in an hour.

Dublin has nine QBCs. Four of them, serving Blanchardstown, Lucan, Malahide and Stillorgan, are each used by about 45 inbound buses during the morning rush hour. With buses designed to carry 98 people, each of those QBCs can transport 4,410 passengers in an hour. In other words, Line A has only 56 per cent of the capacity of a QBC.

The picture is better on Line B, which will use longer trams, of 40 metres rather than 30. As the 12 vehicles will each be able to carry 310 passengers - a design capacity of 3,720 passengers an hour - it will have 84 per cent the capacity of a QBC.

But averaging the figure for both tram lines gives 3,102 passengers an hour in each direction - or 70 per cent the capacity of the top-performing QBCs.

What about boosting the capacity of the tram line by running long trams at high frequencies? The difficulty is that they would disrupt street life. Heavily used bus corridors benefit from buses' short length - double-deckers are between 10 and 12 metres long. Buses also score better in terms of braking distance, ability to overtake and interaction with cyclists.

Which system is cheaper to operate? Trams cost between 40 and 60 per cent more to run than buses, according to Prof David Hensher of Sydney University. A great deal of ancillary equipment comes with a tram service - track, overhead line, substations and the like - all of which must be maintained.

And, of course, only trams can use tram lines. QBCs are also used by bicycles, taxis and, at designated times, private cars - and the cost of maintaining Dublin's QBCs is shared between four local authorities.

So why build tram line? The plans for Luas date from the early 1990s, the era of the belching bus. But buses are far less polluting than they were a decade ago, with emission-reducing technology advancing rapidly in the past five years. Hybrid engines, powered by a combination of fossil fuels and electricity, emit a fraction of the pollutants of similarly sized buses built in the 1980s.

You can build hydrogen-powered vehicles that produce no emissions when they move - but fossil fuels are typically used to generate the hydrogen in the first place and a hydrogen bus is eight to nine times more expensive than a diesel vehicle. A hybrid electric bus is just 25 per cent more expensive to buy than its diesel competitor and, critically, 30 per cent cheaper to run. TransBus, a UK-based manufacturer, started taking orders for hybrid electric buses this year. Wrightbus, which is based in Ballymena, is looking at mass-producing its hybrid electric prototype, the Electrocity.

Surface transport in cities usually loses money when it is provided at low cost. Dublin Bus covers only 75 per cent of its costs with its fares; the other 25 per cent is subsidy.

The situation is similar under competitive tendering: London will soon start to pay almost €1.5 billion a year to subsidise bus companies that provide services on bundles of routes.

Underground transport is much more expensive to build but can be cheaper to run. The city of Lille, in north-eastern France, has an automated metro system. Instead of drivers it has roving attendants who supervise the system. Relative to the number of people it employs, the network handles more than twice as many passenger journeys as a conventional underground railway.

Automated metros are sometimes likened to horizontal lifts: short trains arrive as often as once a minute. This way of doing things, according to the Lille metro, means operating and maintenance costs are covered by fares, with a small contribution made to capital. Barcelona, Copenhagen, Milan, Paris and Prague have adopted automated metro over the past decade, and any new line in Madrid is set to be automated.

Automated metro faces a high capital barrier, a barrier most cities don't leap. And this brings back into focus the need for good cost-benefit analysis in allocating surface-transport spending.

Transport debates often begin with a remark along the lines that cities that abandoned on-street tram networks in the 1950s were short-sighted. The suggestion does not stand up to scrutiny. After the second World War trams cost three times more than buses in terms of their floor areas. City authorities were unable to justify renewing their fleets. Even today Europe sees no more than 500 on-street tram vehicles built each year - a fraction of the number of new buses. In terms of their total floor areas, trams now costs five times more than buses.

It makes sense to rank modes of transport according to how sympathetic they are to other street users. There was a time when buses were highly polluting and, to passers-by, nothing more than menace. Times have changed.

James Nix is a transport and land-use researcher at Dublin Institute of Technology.

E-mail james.nix@dit.ie