Speed limits will be reviewed as part of the move to all-metric road signage, the Minister for Transport said last night.
Speaking to The Irish Times at the end of the first weekend of the penalty points system, Mr Brennan said that "if we're insisting people respect the speed limits, it's important that those limits be sensible".
In most places they were, he added, but road improvements made the 30 miles per hour restriction unrealistically low on certain stretches, while the 60 miles per hour limit was too high for some sections of open road.
Despite criticisms by Garda representative bodies about the haste with which the new system was introduced, Mr Brennan said the "anecdotal evidence from all over the country" was that drivers were responding to the threat of penalty points and slowing down.
However, the Garda Press Office could give not figures on speed checks or tickets issued over the weekend.
Meanwhile, a garda at one mid-Leinster station told The Irish Times the scheme was still not in operation because officers had yet to be briefed about it, and had not been issued with the necessary tickets and receipts.
If local drivers were slowing down already, it was only because of the publicity surrounding the scheme's launch, he added.
The AA also confirmed that the scheme appeared to be having an effect on drivers, but spokesman Mr Conor Faughnan said this was based on anecdotal evidence.
Mr Faughnan has called for a "national audit" of speed limits to bring them more in line with local conditions.
He is also urging the need for an information campaign to combat public confusion about the speed limits, which range from 30 and 40 miles per hour in built-up areas to 60 on open roads and dual carriageways, and 70 on motorways.
Mr Brennan said one of the indirect benefits of the penalty points scheme was that "maybe for the first time, people are taking notice of what the speed limits are".
Acknowledging that "there may be black spots and problem areas, as with any new scheme," he said implementation of the scheme was a matter for the Garda Commissioner.
He was happy with the "really big effort" put in by gardaí at the end of last week, and said that, despite criticisms, the force was completely behind the scheme.
