The week before Christmas saw Auckland City's Mayor and councillors give approval in principle to construction of the Avondale extension of State Highway 20, after pressure from Transit New Zealand.

With a price tag of $1.2 billion and the potential loss of 300 homes, this motorway will be Auckland's largest single roading project since the building of the harbour bridge. But is it a sensible one?

How valid are the arguments in favour of SH20 motorway - planned from Mt Roskill, through Owairaka, Avondale and Waterview to the Northwestern Motorway at Pt Chevalier?

One key Transit NZ justification for SH20 is that Auckland needs an alternative route to SH1 to relieve congestion on the central motorway network and take pressure off Spaghetti Junction.

Once SH20 is built, north- and southbound vehicles will be able to bypass the city centre and traffic volumes will be eased.

This appears to be logical, but in reality is not. New roading encourages new vehicle journeys. Induced traffic demand and the growth in new vehicle numbers will quickly defeat any advantages gained.

Vehicles switching to a completed SH20 would merely join commuters opting off trains and buses because of a perceived new faster vehicle route. Add in Auckland's predicted vehicle growth - around 100 a day - and an increase in optional trips, and the new arterial route will inevitably become as congested as existing motorways. SH1, perceived as a result of SH20 to be less jammed , will attract new traffic as commuters change route, travel times or travel mode, and quickly return to its former snail's pace in peak hours.

Case studies of new arterial roading projects being defeated by growth in traffic volumes can be found around the globe. In the early 1990s, Washington DC's Interstate 270 was expanded from six to 12 lanes at a cost of US$200 million ($293 million). Yet within eight years the highway was again reduced to a "slow moving carpark", as congested as it had been a decade earlier.

By OECD standards Auckland already has one of the most extensive motorway networks in the world, measured by population size and kilometres of lanes.

We are far ahead of Australian cities in the motorway count and yet our congestion is as bad, if not worse. Brisbane and Perth are similarly sprawling suburban cities, but have fewer motorways. Both cities are enticing increasing numbers of commuters off roads by investing in modern electric trains, co-ordinated with buses, integrated ticketing and realistic destination choices. By comparison Aucklanders lack modern, integrated public transport alternatives.

Another key proposition for SH20 is that it will relieve local traffic congestion and help provide safer streets. Looking at Auckland's motorway expansion over the past few decades there is good evidence that such benefits are barely discernable and temporary at best.

The Northwestern Motorway was built to relieve a congested Great North Rd. Yet today SH16 and Great North Rd are equally congested.

Even with the constant widening of SH16, Great North Rd carries 60,000 vehicles a day along the Waterview straight, making it a hazardous exercise for locals seeking to cross the carriageway or turn right from side roads. What little benefit was gained from the construction of SH16 has been quickly lost.

Would the city's freight and business traffic benefit from SH20? Another route is appealing but if it is congested too, it's hardly a solution. Interestingly London, with its ever increasing population and business traffic, hasn't built a new urban motorway in 25 years.

The answer to goods and service traffic woes lies in reducing vehicle numbers using the roading network. Every commuter who boards public transport leaves the roadway free for freight vehicles. Sweden's capital, Stockholm, has constructed public and alternative transport systems which carry a phenomenal 70 per cent of commuters, freeing up the roading network for business vehicles.

Auckland's Third World public transport attracts less than 8 per cent of commuters out of their cars. The best thing business leaders could do would be to advocate for SH20's billion-plus dollars to be invested in a modern, integrated public transport system.

A completed SH20 is seen as providing an alternative route across the region in case a terrorist attack or an earthquake severs SH1. This proposal seems to discount using the local roading networks should such a disaster strike, but exactly why is not clear.

Evidence suggests that motorways are not effective in dealing with such emergencies. When Hurricanes Ivan and Katrina approached New Orleans last year, thousands sought to escape inland via the Interstate. This quickly jammed, trapping occupants in their near-stationary vehicles for up to eight hours. In emergencies motorways quickly seize up with high vehicle volumes and few entry and exit points, making their use limited.

Worryingly, none of the arguments for SH20 seriously takes account of the likely community, health, environmental and amenity costs of constructing such a project. The proposed route runs through hundreds of city homes, severing existing neighbourhoods and increasing noise and carbon monoxide levels.

The motorway seriously threatens Oakley Stream, the city's longest urban stream and walkway. Regardless of whether SH20 is built on the surface or in a cut-and-cover fashion, up to 60ha of irreplaceable recreation and amenity space will be degraded or destroyed.

Such a loss would be the largest in Auckland City's history and fall hardest on the wards with the lowest ratio of parks to people - Mt Eden, Mt Albert and Avondale. The consequential social and health costs to the city will be determined only in the years to come.

In terms of community liveability, decongesting Auckland's roads and wise use of our public transport dollar, SH20 is the wrong way to go.

* Phil Chase, a former transport planner, is a member of Auckland City's Eden-Albert Community Board.