The Garda Commissioner has said the interim report into the false breath tests controversy has not yet identified a single cause for the exaggerations but that does not excuse it.

The report by Assistant Commissioner Michael O'Sullivan was delivered to the Policing Authority last night.

Commissioner Nóirín O'Sullivan is appearing at a public meeting of the Policing Authority today.

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Commissioner O'Sullivan told the authority that the indicators in the report point to a number of factors but insisted the gardaí will get to the bottom of it.

She accepted that the exaggerations were simply not good enough.

Authority member Maureen Lynott pointed out there were huge variations in the over-reporting of breath tests such as 5% in Wexford and 495% in Dublin West and asked was it wholesale fraudulent reporting.

The commissioner said she cannot say at this point what the reasons were for the exaggerations.

Assistant Commissioner John Twomey said the investigation was continuing, that investigators have spent a number of days in those areas, they have gathered data and information, and rank-and-file gardaí and supervisors have been interviewed.

Ms Lynott also said that the wholesale over-recording represented a weakness in management and supervision consistent with failures identified in the O'Higgins Commission and the Garda Inspectorate reports.

She also said this was a much bigger problem that the issue of breath tests, larger than individual failures and a very serious critical issue of supervision and local management which affects everything.

Earlier, the Policing Authority said that the effectiveness of road traffic policing has deteriorated in the past four years.

Chairperson Josephine Feehily told the Garda Commissioner that road deaths had increased by 4% while serious injuries were up by 34% over the past four years.

Ms Feehily also said the controversy over false breath tests and wrongful convictions had raised serious questions and undermined the effectiveness of traffic policing and undermined confidence in it.

Deputy Commissioner Twomey told the authority that reductions in personnel in recent years has been a key issue.

The number of gardaí in roads policing had dropped from 1,200 to 700, he said.

There was a need to upgrade equipment and the breathalyser units were to be updated this year, he added.

Gardaí have told the Policing Authority that they have written to 10,000 of the 14,500 people who were wrongfully convicted of speeding.

Assistant Commissioner Michael Finn said of those they have had 4,500 responses.

Of those, 1,500 letters came back undelivered and gardaí would have to follow these up in person, as the people did not accept the registered letters.

Of the 3,000 who engaged with gardaí, the Assistant Commissioner said that about half of those people said they were content for the gardaí to go ahead and appeal the conviction on their behalf.

The other half, he said, wanted further information or were seeking legal advice.