New speed limits come into effect from midnight tonight as the changeover to metric signage comes into force.
And despite criticism that the lead-in time for the new measures is too short, the Garda have warned motorists not to expect lenient treatment.
A publicity campaign on television, radio and in the press which includes the distribution of five million leaflets will continue until February 13 th.
"There is still a major job to remind people that they are now driving under the metric system," Mr Brian Farrell of the National Safety Council said.
He also backed the Garda's refusal to give motorists a period of grace before strictly enforcing the new limits saying some motorists would inevitably abuse the concession.
Labour transport spokeswoman, Ms Roisin Shortall, criticised the speed at which the new measures have been introduced, saying they have caused confusion among motorists.
She also pointed out that many Garda speed detection unit have not been re-calibrated to metric and the legislation for special speed limits around areas such as schools is "especially cumbersome" and will lead to delays in implementation.
"Metrification has been rushed through with insufficient time given to allow the public, local authorities, and the gardai to adjust," she said.
However, the Minister for Transport, Mr Cullen, this afternoon rejected the criticisms. "Over the past number of years we have also adapted well to the use of the metric system in a number of fields. We have been buying our petrol by the litre for a number of years and have made the change to kilometres for measuring distance," he said.
Chief Inspector John Farrelly, head of the new Garda Traffic Corps, said that because the law applies in mph until midnight, only half the available "speed guns" were converted to metric and that work on the remainder would be completed within a week.
He said there was "no provision" for giving an amnesty to drivers breaking the new limits in the coming weeks, and there was no particular need.
"Our ambition is to reduce to speed and people who drive carefully today will still be driving carefully tomorrow. It's not complicated, there are only four new speed limits ... and the public have been inundated ad nauseumwith adverts on television and radio and with millions of leaflets," Chief Insp Farrelly said.
Thousands of signs in the State have been changed since the start of the year indicating new speed limits in kilometres. The National Roads Authority (NRA) expects all of the 35,000 existing signs and 23,000 new ones to be in place tonight.
Signs on national roads will be 750mm larger and the smaller repeater signs will be 450mm bigger. They will still be a white disc encircled in red with black numerals though the typeface style will be thinner and kmh will also be displayed.
As part of the standarisation of colour coding directional signs, all motorway signs will be blue, national routes will be green and secondary routes will be white.
Changes in speed limits have been necessitated by the need to display round figures on the new kmh signs and as part of a broader strategy to reduce death and injury on the roads.
Stretches of road with a high rate of accidents, or roads around locations such as schools have been targetted as part of the new measures with some speed limits reduced to 30kmh (20 mph).
Legislation allowing for a temporary reduction in speed limits and statutory guidelines for the setting of speed limits by local authorities are also part of the new regime.
Apart from the targetted areas, limits on national roads will increase by 2 mph as 100 kmh signs replace the old 60 mph signs. Speeds of up to 75 mph will now be legal on motorways with the introduction of a 120 kmh sign.
Many secondary routes will see a reduction of 10 mph as 80 kmh signs replace the old 60 mph signs. In urban areas, apart from on designated stretches, the speed limit will increase by 1 mph as 50 kmh signs replace 30 mph.
Irish Advanced Motorists have reminded road users that the new limits do not mean that drivers should drive at a speed which compromises safety, particularly in relation to stopping distances.