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A staunch ally in the campaign for walking and cycling lanes over the Auckland Harbour Bridge has dropped out after learning that such facilities could cost $43 million and affect structural integrity.

Since late 2007, North Shore City Council's infrastructure and environment committee cheered on the Getacross campaign with its support for modifications to the bridge.

But yesterday committee chairman Ken McKay said that if 750 cyclists rode to work on the bridge it would cost $500,000 a cyclist to add lanes.

Deputy Mayor Julia Parfitt said: "It's throwing good money after bad to retrofit lanes."

Investigations into the feasibility of special lanes and safety and structural issues with the bridge's clip-on lanes came at considerable cost.

"To keep this as a live issue will be a considerable cost to taxpayers and ratepayers."

She backed Mr McKay's proposal that the council stop lane investigations - pending progress on providing a third Auckland Harbour crossing.

She supported a look into fitting $2000 bike racks on the buses that go across the bridge.

Outgunned by nine votes to four, councillors Chris Darby and Ann Hartley insisted the council reaffirm its support for lanes, subject to financial and structural problems being resolved. Mr Darby said he accepted, in light of the economic climate, the NZ Transport Agency's board decision to shelve the project for at least the next three years.

However, 11,000 people had signed up to the Getacross campaign.

"There's a massive shift to walking and cycling in this city and this council should [back] further investigations."

Mrs Hartley said: "The Transport Agency should get off its bike and get on with serious leading issues of the network ... It's imperative we keep this to the fore."

However, councillor Kevin Schwass had another view: "This is ludicrous.

It's just been driven by passionate enthusiasts and with a bill of half a million dollars per cyclist, I say don't consider it any further."

Earlier, Transport Agency regional manager Tommy Parker briefed councillors on why the board left the lanes off its three-year list.

Mr Parker said the preferred option of adding 1.2m-wide lanes, endorsed by the Auckland Regional Transport Authority, would cost $43 million.

He said also that the added weight of the lanes would have an impact on the projected life of the clip-on extensions of 1969. They would mean instant restrictions on heavy traffic despite a current $45 million project to strengthen clip-ons.

Without them, the safe loading capacity of the bridge would not be exceeded for trucks for 15 years.

"Without cycleways we can manage 30 years' life out of the bridge but we would have to maintain restrictions."