Believe it or not, the M50 motorway was intended to provide a bypass of Dublin for national traffic converging on the city. At least, that's what was in the minds of the engineers who conceived it all of 30 years ago. They could hardly have imagined that it would become the curved spine of an American-style "edge city".

It's another world out there - a parallel universe of interchanges, slip-roads, high-tech business parks and shopping malls with colour-coded parking zones. There are very few landmarks, apart from the triple-decker interchange near Blanchardstown, with the Royal Canal and the Dublin-Sligo railway line running through it.

Because of the proliferation of development along its route, the M50 has radically changed the pattern of commuting in Dublin, which is now as much suburb-to-suburb as suburb to city centre. Work commuters account for 70 per cent of journeys during the morning peak, with four out of five cars occupied by one person.

It is also clogged on Saturdays by families flitting from shopping centres to multiplex cinemas. Traffic volumes have increased more than fivefold from an average daily flow of 13,000 vehicles in 1990 when the first section of the M50 opened, to 68,500 last year. The Southern Cross will merely add another element to this merry-go-round.

Notorious traffic jams that built up at the makeshift junction with the N81 through Tallaght will now move to a similar junction in Ballinteer, pending completion in 2003 of the M50's final leg - the South Eastern Motorway. Gardai will be on duty for the first few weeks in what will probably be a vain effort to keep traffic moving.

But Eamonn O'Hare, director of transportation with Dun Laoghaire-Rathdown County Council, which supervised construction of the Southern Cross on an agency basis, anticipates about half of the traffic on the route will "disperse" before it reaches Ballinteer - at least until the South Eastern Motorway is completed.

He also pointed out that most of the other road schemes associated with the Southern Cross - such as Leopardstown Road, Wyckam Way and Brehon Field Road (otherwise known as the Green Route) - have already been completed, while both Brewery Road and the long-awaited Dundrum bypass should be finished next spring.

Originally conceived in 1971, the M50 (where are the other 49?) has taken 30 years to bring to virtual completion, a remarkably long time compared to 12 years each for the second Severn bridge linking England with Wales and the even longer Oresund bridge/tunnel stretching across the Baltic between Copenhagen and Malmo.

The Dublin city manager, John Fitzgerald, has complained that major infrastructural projects here are "dogged by delays prior to the construction stage, thus exacerbating the problems they were designed to alleviate. Often, these delays are predicated on legal challenges which can literally add years to the completion of a project".

The Southern Cross route was held up for two years by judicial review proceedings initiated by three affected landowners - Major T.B. McDowell, chairman of the Irish Times Trust; Edward Fitzachry; and the Select Vestry of Whitechurch. It was then held up for a further year by Pat Fitzgerald, lessee of the Maxol filling station in Sandyford. Their legal actions were settled at the doors of the High Court on terms which have never been disclosed.

A separate challenge by Jackson Way Properties against the South Eastern Motorway held it up for more than a year, and the true identity of those behind this company is still being investigated by the Flood tribunal.

But even with its first two legs completed - the Western Parkway and the Northern Cross - the M50 had become so congested at peak periods that the National Roads Authority dusted down contingency plans to widen it from four to six lanes, including the construction of a second high-level bridge over the Strawberry Beds.

This is all good news for National Toll Roads plc, which built the original West Link bridge and will now construct the second one in parallel. Increasing volumes of traffic on the M50 has boosted the company's annual toll revenue to £21 million-plus, a quarter of it going in licence fees to the Department of the Environment.

But unlike the continuing controversy in London over a succession of proposals to widen its even more congested M25 orbital motorway to 10 lanes in places, there has been no public debate about the NRA's proposed improvements to the M50. At one stage, these included new flyovers on the woefully designed roundabout interchanges.

The Dublin Transportation Office is opposed to flyovers because of the danger that they would smooth the path, at least temporarily, for more car commuters to gain access to the city. Even as it is, roundabouts on the route have had to be equipped with traffic lights and new sliproads to make them work more efficiently.

Why the standard international cloverleaf interchange was not adopted is something of a mystery. The most credible explanation is penny-pinching by the Department of the Environment, which wanted to minimise the land "take" required for junctions.

Plans to widen the M50 to accommodate three lanes of traffic in each direction - prepared for the NRA by Arup's, the consulting engineers - are on display for public consultation. And while there is enough space for the extra lanes on running stretches, yet more land will need to be acquired to accommodate larger interchanges.

Meanwhile, construction work is expected to start this autumn on the South Eastern Motorway, which will link the rest of the M50 with the Shankill bypass on the N11. Again, Ascon has been appointed as the main contractor. Negotiations are ongoing with Dun Laoghaire-Rathdown over the price, previously estimated at £113 million.

Land acquisition for this last leg of the M50 turned out to be hellishly expensive. Not only does this motorway run through some of the most sought-after parts of south Co Dublin, but soaring property values over the past five years will leave the authorities with a bill of £180 million to buy the strip required for this 11-km route.

And all the road planning is not over yet, not by a long shot. Dun Laoghaire-Rathdown has already agreed on a preliminary design for a dual-carriageway running north-south from the N11 at St Helen's to the join the M50 at Drummartin, near Sandyford. Ultimately, this would become the southern leg of a full Eastern Bypass motorway.

With the Dublin Port Tunnel (its northern leg, in effect) now finally under construction, and the Sandyford-St Helen's link in planning, only one last piece of the jigsaw would remain to be put in place - a route linking the planned tunnel interchange in the north port area with Booterstown, running in tunnel beneath Sandymount Strand.

Though funding for the Eastern Bypass is not included in the National Development Plan, the realisation of this Holy Grail of Dublin's road engineers for 30 years was brought a step closer in March 1999 when it was inserted in the current City Plan - six years after being officially ditched by the Government. Now it is back on the agenda.

According to the city manager, it is still no more than "a concept, a line on a map" and further detailed studies would have to be undertaken before a final decision was taken. But he said it would be "crazy to deny ourselves the option", especially as there was considerable demand for it from Dublin businesses.

Neither Dublin Corporation nor the NRA can say what it might cost to run a four-lane dual-carriageway in a tunnel beneath Sandymount Strand and Booters

town Marsh, but it is obvious that the "serpent that refuses to die", as Liz O'Donnell once called it, would come with price tag of at least £500 million, and probably a lot more.

When the Eastern Bypass is finally built, the road engineers will have achieved their long-standing objective, first articulated in the 1971 Dublin Transportation Study, of encasing the city in a "motorway box". So instead of the M50's C-ring, Dublin would end up with a full orbital ring road and thus a neat equation in engineering terms.

But Tim Brick, the corporation's disarmingly frank chief road engineer, is on record as saying that although it would undoubtedly bring relief to the Sandymount area and make Dublin Airport much more accessible to southsiders, "anyone who thinks that it will remove a solitary vehicle from St Stephen's Green is living in cloud-cuckoo-land".

In the meantime, plans are being reactivated for an Outer Ring Road, beyond the M50, with the ostensible purpose of linking the three western "new towns" of Tallaght, Lucan-Clondalkin and Blanchardstown. However, given the continuing sprawl of Dublin and the congestion this is creating, it is likely to become an M50 bypass.