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Peak traffic across the Auckland Harbour Bridge has declined since 1991, feeding doubt among some regional councillors about a need for a $3 billion tunnel.

But overall daily traffic volumes across the bridge have risen 38 per cent in that time, suggesting many commuters are setting out earlier or later than the traditional two-hour morning travel peak of 7am to 9am.

A survey reported yesterday to the Auckland Regional Council's transport policy committee counted 16,032 vehicles crossing the harbour bridge southbound in the morning peak late last year, compared with 17,048 in 1991.

It recorded 16,759 vehicles heading back across the bridge to North Shore City in the evening peak, down from 17,092 in the council's 1991 five-yearly survey of traffic flows on main routes across 10 regional grid-lines.

Traffic heading in counter-peak directions grew steadily over the 15 years, as did vehicles travelling both ways across the bridge between 11am and 1pm.

Transport planners see that as a reflection of "peak spreading" in which people make trips before or after the official peak periods to avoid traffic jams.

More evidence of that is found in a difference between overall traffic volumes across the region, which have increased by about 1 per cent a year since 2001, and inbound morning peak traffic - which grew by just 1 per cent over the five-year period to 2006.

The decline in peak traffic drew questions from committee members about the need for a $3 billion tunnel across the harbour, for which Transit NZ says it will work with the regional council as well as Auckland and North Shore cities on choosing a preferred corridor.

Transit's first choice is between the Northern Motorway at Northcote and the Tank Farm, through which it may bury the tunnel under a street-park before bringing it out through a portal near Cook St on Spaghetti Junction.

The feasibility of other routes such as between Northcote and Stanley St and between Bayswater and Mechanics Bay will also be investigated.

Although Transit does not believe another crossing will be needed until at least 2020, North Shore MP Wayne Mapp fears the harbour bridge will reach its full daily capacity of about 180,000 vehicles long before then, and says a tolled tunnel should be built as a partnership between the Government and the private sector.

But regional councillor Robyn Hughes said the harbour bridge vehicle counts raised the question of whether there was a point at which traffic "manages itself".

"If that's the case, why do we need a $3 billion tunnel from the Tank Farm to Northcote?"

She said building more roads for more cars was no answer to Auckland's congestion problems, and that $3 billion could buy 10 years of free public transport including capital spending.

Regional council deputy chairwoman Christine Rose said trends showed the merits of travel demand management measures, given that congestion was often a social rather than engineering problem.

"Too often we blame roads rather than people's behaviour - people using more public transport or travelling off-peak are showing an ability to respond to those pressures which we shouldn't under-estimate."

Committee chairman Joel Cayford said the survey was limited by counting vehicles rather than numbers of people being carried in them.

The fact that many buses were crossing the bridge with standing room only, carrying perhaps 50 times more people than the average car, was not reflected in the survey.

Dr Cayford noted that bottlenecks were not so much on the bridge as on the motorway approaches to it, and feared that a tunnel with a portal at Northcote would not address that problem.

It would probably create a need for three more lanes to be added to the Northern Motorway, he said.

Transit regional manager Peter Spies said those were all issues to be investigated between now and March, with the help of traffic modelling which would also include the impact of a completed western ring route.

He said the morning peak period on the motorway network began well before 7am, and he often heard radio traffic reports at 6.30am of queues stretching back to Greville Rd, about 10km north of the bridge.