Lee Waters MS spoke to ITV Wales on the day the new law for speed limits of roads in built-up areas came into effect.
Wales' Deputy Minister for Climate Changes has admitted there may need to be further changes to the new 20mph laws in the coming months.
As of yesterday (September 17), 30mph roads have been reduced to 20mph where cars mix with pedestrians and cyclists. It is thought around 13,000km of road will be affected.
The move has proved controversial with a poll for ITV Wales revealed this week that just 31% of people support the policy, while 66% are opposed to it. A Senedd petition calling for the change to be reversed has received more than 70,000 signatures.
The Welsh Government say research shows that changes will reduce pollution and fatal crashes.Speaking to ITV Wales on the day to change came into effect, Lee Waters MS insisted the change will save lives.
The Deputy Minister for Climate Change, whose brief includes transport, says there may be tweaks that will have to be made as people get used to the new speed limits.
He said: "It's a big change, it will take a while for the bumps to be smoothed out and, no doubt, there'll be tweaks.
"Some roads may be better kept at 30, some that are kept at 30 may be better at 20. This will all work itself out over the coming months.
"But just as we found out when the laws changed in seat belts or breathalysers or smoking in public places or carrier bag charges.
"But once the change has been made, we don't want to go back."
According to the Highway Code, you can identify these types of roads because they will have streetlights spaced no more than 200 yards apart.
That means there won't necessarily be a 20mph road sign - you should assume a road that used to be 30mph is now 20mph.
The RAC has warned drivers not to rely on satnavs for the correct speed limit and to instead follow the road signs.
Before the change, 37% of roads in Wales were set at 30mph. From Sunday (Spetember 17), just 3% of the those roads will remain at 30mph.
Councils will be able to apply to make some 30mph roads exempt from the change.
You can see a Welsh Government map of exemptions here.
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