Last Saturday's traffic ordeal highlighted Auckland's vulnerability to complete and protracted gridlock. A single crash can bring the core of the city's transport network to its knees. It's not as if it was the first time, and more is to come.

As far back as December 1999, one truck caused city-wide chaos when it capsized on the Auckland Harbour Bridge.

The cost to the region's economy of traffic delays is estimated to be many billions of dollars a year, which does not include the mental anguish caused to frustrated and angry drivers.

Over one-third of the people in New Zealand live in or around the Auckland urban area. Given that the region's population continues to expand by the size of Dunedin every three to four years, the vulnerability to traffic snarl-ups will grow exponentially.

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An efficient and congestion-resistant transport network is just as essential as clean air and drinking water, safe swimming beaches, adequate and dependable supplies of energy and reliable wastewater disposal systems. There is a big burden of responsibility on local and central government planners to ensure that its future requirements are met.

Government has the responsibility to minimise social and economic vulnerability. They have a duty to promote community resilience through enlightened planning. And, above all, they have an obligation to prepare us for the future.

Public transport itself will not ease the region's traffic crisis. Auckland's geography, history and politics make it a unique case for infrastructure planning. Its long, thin shape led to the earliest transport routes developing along a narrow north-south axis. Strategic arterial roads followed this pattern. The result is a highway system that is not yet part of a fully integrated network. It is linear with no alternative routes around major bottlenecks. Traffic that would want to bypass the city is forced through Spaghetti Junction, adding to the vulnerability of the system to gridlock.

In the event of blockages on motorways, surrounding non-strategic arterial roads are not a practical alternative. Nearly all are designed for 50km/h speeds and capacity is limited more by intersection operation than by the number of lanes available. These routes easily become congested.

The region's most strategic arterial roads are vulnerable during earthquakes. Older multi-span bridges and abutments along motorways such as around Spaghetti Junction would be most vulnerable to damage from ground liquefaction. Even minor damage to these would bring city traffic to a halt.

After years of discussion, there are still no agreed plans for a second or third Waitemata Harbour crossing. Given the limited capacity of ferries, a single road crossing means there is no option should the vital north-south connection become clogged, or even worse, fail.

Even a modest earth tremor could fracture supports for the harbour bridge. Both the St Mary's Bay and Northcote approaches would become impassable as they are built on reclaimed land which is more susceptible to ground shaking. Approaches to the upper harbour bridge would suffer similar problems.

The vulnerability of a city is to a large extent a function of the adequacy of preparedness planning. How soon could Auckland be evacuated?

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There is limited motorway access out of the isthmus that is the Auckland urban area, so there few alternative exits. Main feeder roads head for one major harbour crossing and easily become congested.

Approach lanes to the harbour bridge are close to the high-tide mark and susceptible to flooding from storm surges.

A report some time ago on the vulnerability of the harbour bridge showed even a modest earth tremor could fracture supports for the four clip-on lanes. The result would mean traffic chaos.

The economic effects would be immense. It is only after calamities occur that we recognise how vulnerable we are.

Chris de Freitas is an associate professor in the School of Environment at the University of Auckland.