What century are we living in? The New Zealand Transport Agency is adding two lanes, plus a cycleway to a 9.5km stretch of the Southern Motorway between Manukau and Papakura, but sees no need to wrap it in noise-reducing barriers as part of the deal.

A report to Auckland Council says the road builders plan noise barriers eventually along "the majority of its length", but the 900,000 visitors to Auckland Botanic Gardens each year, for example, will just have to bring their ear plugs.

To put the boot in, NZTA wants to seize a 2.4ha lump off the side of the gardens to house a stormwater treatment facility. This means a 30-year-old screen of native plants will have to be removed, forcing visitors to not only hear the incessant traffic but see it as well.

In mitigation, NZTA is proposing a 1.1m concrete safety barrier alongside the park that its acoustic advisers say will reduce the roar from 64.1 decibels to 61.5 decibels - a barely discernible drop.

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Across the motorway, NZTA is planning a 5m high noise barrier to protect the residents living there. This should result in a small drop in noise from the current 71.3 decibels to 66.6 decibels.

Let me add that all these decibel readings are hell-on-earth territory, alongside which no one in a civilised society should be allowed to live or play.

World Health Organisation guidelines for outdoor living areas, for example, rate 50 decibels as of "moderate annoyance, daytime and evening" and 55 decibels, "serious annoyance".

Those with long memories will be saying, but haven't we been here before? Indeed we have. Well sort of.

In January 2001, the Botanic Gardens spent $300,000 on a 3.5m by 440m steel barrier along its motorway boundary. It was to abate the conversation-stopping noise faced by staff and visitors alike. And it did, by a very noticeable 15 decibels.

Unfortunately, similarly afflicted neighbours to the west were not consulted and they claimed the new wall was reflecting sound across the motorway, adding to their misery.

The Botanic Gardens' acoustic adviser, Marshall Day Acoustics - now NZTA's consultants, rejected this. If there was extra noise, it said, it was the result of seasonal wind patterns.

It also suggested it was a perception problem - instead of looking across the motorway to green bush, the neighbours were now seeing an ugly, shiny steel fence.

After two years of shadow boxing, the neighbours had their day in the Environment Court. Judge Frederick McElrea ordered the wall demolished within 42 days, choosing to believe the "straightforward, genuine and credible" residents over the "unnecessarily adversarial" and "not always reliable" experts.

Unfortunately for acoustician Chris Day, he had not taken noise readings in the residential backyards before the wall went up, because the science told him the wall would not be reflective.

Upset by the judge's criticisms, he decided to prove his point by testing in the complainants properties before and after the the wall came down, and measuring the effect of the wall.

When the wall came down, the complainants were jubilant, one claiming noise had dropped 50 per cent.

Like the judge, I'd sympathised with the victims, so Day invited me along to before and after nocturnal measuring sessions - nocturnal to avoid extraneous local noise. He won the argument. In one place, 30m from the motorway, the noise averaged a head-aching 72 decibels with the barrier and 73 decibels after it went.

In two other sites there was little or no change in the mid-40 decibel readings. In both these sites there were variations of up to 17 decibels because of changes in traffic flow and weather.

The only effect of the wall falling was noise levels rocketing back up in the public park. And that remains the situation.

Despite adding two extra lanes, NZTA is now refusing, according to a council briefing, "to provide additional mitigation to the effects of existing traffic noise".

Presumably it sees the proposed 1.2m safety barrier as covering the effect of the extra lanes. Instead, it has agreed to monitor noise levels once the 5m wall on the neighbours' side has been installed "and will erect additional noise attenuation structures on the Gardens side ... if this proves necessary".

The noise is peaking in the mid-70 decibels in the gardens now. NZTA should get real. Better noise protection along its motorways is necessary now.