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Traffic experts claim motorway tunnels through Waterview in Auckland will produce less pollution than surface roads, and the bigger the tunnels, the cleaner they say the air will be.
But neighbours of giant "tail-pipes" towering over each end of the proposed $1.89 billion to $2.14 billion Waterview link
remain alarmed that the Transport Agency is considering pumping unfiltered tunnel fumes into the atmosphere near schools, homes and health centres.
The agency has issued air-quality assessment reports before a community workshop to be held on Thursday, while preparing to lodge land designation and resource applications before the end of next month.
These draw on computer modelling by Australian experts of traffic estimates and weather patterns to predict concentrations of pollutants likely to end up at 29 "sensitive receptor" sites within 3km of the proposal twin tunnels - mainly schools and childcare facilities, but also health centres such as the Mason psychiatric hospital.
Transport Agency principal project manager Clive Fuhr says the modelling shows that pumping vehicle emissions through 25m stacks for dispersal into the atmosphere would have a "negligible" effect on local air quality.
"The evidence is that tall vents are very effective at dispersing all pollutants, including those for which there is no treatment method," he said.
Tunnel emissions would add less than 0.3 per cent to the level of particulates - potentially carcinogenic fine particles prevalent in diesel fumes - at most affected locations and considerably smaller contributions of nitrogen dioxide, carbon monoxide and benzene.
One report predicts air quality at most sites will be better than if the tunnels are not built by a target date of 2015, and surface routes such as Great North Rd left to become more congested by vehicles with their individual tail-pipes close to nose level.
It says parts of the project area already have "significant air-quality impacts from traffic".
But extra vehicles using the Northwestern Motorway and above-ground ramp connections with twin 3.2km tunnels carrying two lanes of traffic each way are predicted to cause some deterioration of air quality from particulate pollution at two Pt Chevalier sites - St Francis Primary School and a childcare centre.
Although most public concern about the tunnels has been raised by parents of Waterview Primary School, with a boundary just 38m from a venting stack proposed for their northern end, the reports predict a slight reduction of particulates around there.
The school is still in line for more than 5 per cent extra nitrogen dioxide pollution, but that is also blamed on heavy traffic on the above-ground Waterview interchange, rather than the tunnels.
Despite initially proposing two-lane tunnels costing $1.89 billion, the Transport Agency is also assessing calls from organisations such as the Automobile Association and the Auckland Business Forum to spend a further $250 million to build three-lane tubes.
But these are predicted to add only 1450 vehicles to a daily projected traffic load of 74,500 in 2016 for twin two-lane tunnels, and 5800 to a 2026 expectation of 90,500 cars and trucks.
The Transport Agency's consultants also predict less pollution from larger tunnels, because of extra air pushed through them by ceiling fans assisting a "piston" effect from freer-flowing traffic.
A parent of primary and kindergarten children at Waterview, Margie Watson, said the Transport Agency was obliged to filter fumes pumped out of the tunnel, even if it could prove overall pollution was less than from dispersing noxious substances over a wider area of surface roads.