A £3.6m road revamp scheme to help cut pollution has so far failed to bring the sustained improvements in nitrogen dioxide levels it was designed to deliver.

But officers behind the St Augustine's gyratory system - which saw changes to roads in the Anglia Square area of Norwich - say it is too early to jump to conclusions, without longer term studies.

The St Augustine's gyratory scheme, which was completed in January 2011, saw a number of streets made one-way, with one of the key objectives to improve air quality by stopping vehicles from pumping out fumes while idling at traffic lights.

In 2011 there was a 'significant improvement' in air quality at one of the main monitoring points in St Augustine's Street, with 47.5 microgrammes of nitrogen dioxide per cubic metre

(µg/m3), compared to 56.2 µg/m3 in 2009 and 55 µg/m3 in 2010.

But members of the Norwich Highways Agency Committee, made up of city and county councillors, yesterday heard that last year saw nitrogen dioxide levels go back up to 51.6 µg/m3, to the disappointment of officers.

Officers said, in the report which came before councillors, that: 'It was hoped that the significant improvement shown in 2011 would be continued, though annual variation is normal.'

They said air pollution trends are affected by factors such as temperature, pressure, humidity and global circulation patterns, so 'caution needs to be taken over simple year on year comparisons'.

'A truer picture of the ambient pollution levels will be revealed over the longer term,' officers said.

Councillors said the scheme had brought improvements in general. County councillor Tony Adams, chairman of the committee, said: 'From a personal point of view, in terms of improving traffic flow and congestion, I think it is brilliant. The only slightly disappointing thing is that it doesn't seem to have improved the business situation there.

'There still seem to be a lot of shops which are run down.'

The committee also heard how the redesigned roads had increased traffic by about a fifth in Patteson Road and Buxton Road.

City councillor Mike Stonard, vice-chairman of the committee said: 'It's unfortunate and in hindsight it seems inevitable that those run throughs would be created.

'I think it's really sad we cannot come up with a solution to deal with that.'