Some 60,000 new speed limit signs have gone up
From Thursday, the speed limits in the Republic of Ireland, where both miles and kilometres have been used for many years, change to metric.
But there are already signs that, in striving for consistency, more confusion has been introduced.
Up until Thursday's change, speed limits in the Irish Republic have been in miles with road signs generally in kilometres.
The changes aim to rectify this discrepancy.
But thanks, in part, to pressure from road safety campaigners, the Irish Department of Transport has also used the switch as an opportunity to change actual speed limits.
Because of rounding up after mph to km/h conversion, limits on main roads in towns and cities will get a small boost.
But, more significantly, the limit on rural roads, which make up 90% of the road network, is being slashed by 10mph from 60mph (97km/h) to 50mph (80km/h).
Alan Richardson, acting chief executive of the National Safety Council, hopes that the reduction on more dangerous rural Irish roads will mean less deaths.
"Previously, we've had penalty point schemes but there wasn't a level of enforcement to make it work, " he told the BBC News website.
The switch would not work without police enforcement because "an awareness campaign won't work on it's own".
"In this country, we need a stick to beat ourselves with in order to comply," he said.
But National Police Service spokesman Sergeant Ronan Farrelly told the BBC News Website officers would do "everything humanly possible to enforce speed limits.
Speaking on Wednesday he said: "Tomorrow, we will do just as much as we have today - we're out there 24/7."
A six-week Christmas crackdown had caught 19,415 speeding motorists, saw 1,622 arrests for drink driving and found 2,818 people not wearing seatbelts, he said.
"But just because that campaign stops, we don't stop - we're still out there."
The number of dedicated traffic officers on the force would also be rising from 500 to 1,200, he added.
But while the NSC sees Thursday as a step towards greater safety, RAC Ireland has warned that the changeover has "major safety implications" for drivers.
It urged drivers to take time to learn the mph conversion of the new speed limits and to "err on the side of caution".
Commercial director Jerry Purcell said the Republic's shared land border with Northern Ireland, where speed signs were still in mph, was an added safety issue.
Most speedometers were in mph, the RAC said
"For the first few months of the changeover, we would urge drivers who live in proximity to the border to exercise extra caution and to be aware of the prevailing limits," he said.
He also urged ferries and car rental companies to let tourists who may be crossing borders know of the difference in the two jurisdictions.
Over the last fortnight, some 35,000 existing signs have been taken down and replaced with 60,000 new km/h signs.
Ironically, it is the erection of the 25,000 extra signs that is causing most confusion, according to the NSC.
Mr Richardson says that, because previously there was a lack of signs, people in some areas were "completely unaware" of speed limits on roads in their areas.
He cites the anger of parents at Lurgybrack Primary School, near Letterkenny, where a new 100km/h sign was erected directly outside, which they say is too fast.
The speed limit on the road, he says, has changed only marginally because of the change - from 60mph to 62mph.
But because previously there had been no sign, many parents had been unaware of the "unacceptable speed limit", Mr Richardson added.
"The new system changes a lot of things and hopefully one of those things will be the number of deaths on our roads."