An accident which blocked all four of the Southern Motorway's southbound lanes forced major roads to back up throughout the city. Photo / Brett Phibbs 

An accident which blocked all four of the Southern Motorway's southbound lanes forced major roads to back up throughout the city last week. Photo / Brett Phibbs 

Auckland's worst traffic gridlock for years was caused by an extra squeeze of pressure on a stretched transport network - and it is not over yet.
Although the city has struggled through its busiest traffic week of the year, culminating in Thursday's chaos after a serious crash closed all four southbound lanes of the main motorway out of town, Auckland Transport warns of a difficult weekend.
It is urging Aucklanders and their visitors to consider using public transport or share car rides with friends or neighbours as hundreds of thousands of people throng to a raft of events over the weekend.
Commuters stewed in traffic queues over three successive afternoons, but the longest were caused by the cascading impact of a 2-hour closure of Newmarket Viaduct's southbound carriageway at the height of Thursday's peak travel period.
The viaduct is the country's busiest section of motorway, normally carrying 7000 southbound vehicles an hour during afternoon peaks, and the closure from a serious crash could not have come at a worst time for what the Transport Agency acknowledges is a highly sensitive urban traffic network.
"We have a really, really sensitive network because of the traffic volumes and the limitations of geography in a harbour-borne city,"
regional traffic operations manager Kathryn Musgrave told the Weekend Herald.
"This happened at 3.50pm, right smack in the peak, so it was during the worst time."
Not only that, but Auckland Transport says this was already the busiest traffic week of the year, as students hasten to the first classes of term joined other commuters trying to make an earnest start back at work from the summer holidays.
The phenomenon known as "March madness" happens every year, and tends to ease off after the first frenzied week, but Automobile Association traffic spokesman Phil Allen says he has never seen a worst example of gridlock than on Thursday afternoon.
"When you have your biggest traffic arterial flowing out of the centre of the city suddenly stopped, it's like a river," he said.
"You block a river and the water just keeps building up and up with a ripple effect coming back and back and it tries to ooze out into other tributaries and blocks up those.
"The whole network got constrained and blocked itself."
Ms Musgrave said recent major projects such as the Victoria Park motorway tunnel and the Newmarket Viaduct replacement had increased capacity in the network, and improved the reliability of journey times during most times of day.
But she said all big cities suffered congestion during peak periods, and there was no way to design a network of constantly free-flowing traffic without ending up with "20 lanes of motorway".
The best her organisation could do was to keep developing a range of travel choices for people by investing in projects such as rail electrification and busways, and improving lines of communication to give drivers early knowledge of unplanned incidents, so they could alter travel plans.
That would include a new service to be introduced before Easter in which drivers could subscribe to email alerts to their smart-phones.

By Mathew Dearnaley Email Mathew