Wales has become the first country in the UK to roll out a default 20mph speed limit for most residential roads.
The scheme begins on Sunday and most roads in Wales that are currently 30mph have become 20mph although councils have discretion to impose exemptions.
The Welsh Government has said that cutting the limit from 30mph to 20mph will protect lives and save the NHS in Wales £92 million a year.
The RAC warned drivers not to rely on satnavs for the correct speed limit and instead follow the road signs.
Any motorist caught driving over 20mph but under 30mph will initially receive advice from the police rather than face a ticket, a minister said.
Lee Waters, deputy minister for climate change, told the PA news agency: “Very much the intention initially is to educate and to speak to people and not to fine but over 30mph we will be fining and issuing points.”
The minister said he expected it would take around a month for motorists to get used to the changes but said trials of the 20mph limits had shown a typical delay of between one to two minutes for an average journey.
“Most delay doesn’t occur because of speed, it could be because of delays at junctions and traffic lights,” he said.
“We’ve all been overtaken by an idiot only to find them one space in front of us at the lights.
“A business model that assumes on somebody driving a way that is dangerous is not a very good business model.”
Councils have the power to change the default speed limit of any road locally but Mr Waters said some local authorities were rigidly sticking to the Welsh Government guidelines.
“The risk appetite of local authorities varies considerably,” he said.
“For example, the Vale of Glamorgan and Rhonda Cynon Taff have put in quite a few exceptions and exempted through roads.
“Other councils have not, and they are terrified to move away from the letter of the guidance.
“But I think confidence will grow and experience will influence that, and I think give it a year or two and it will settle down.”
The project is costing around £33 million to implement and has proved controversial, with reports of the new 20mph signs being defaced in areas including Conwy, Gwynedd, Newport, Torfaen, Wrexham and Flintshire.
The Welsh Conservatives have opposed the scheme and tabled a motion in the Senedd to halt the rollout but were defeated.
They cited Welsh Government documents that estimate the cost to the Welsh economy of increased journey times from lower average vehicle speeds at anywhere between £2.7 billion and £8.9 billion.
They want to see more “targeted measures” rather than the introduction of the default speed limit with exemptions, and likened it to the ultra-low emission zone (Ulez) in London.
Andrew RT Davies, the leader of the Conservatives in the Senedd, said: “Whilst I agree that 20mph is sensible outside heavily pedestrianised areas, such as schools, hospitals and care homes, the Labour Government’s blanket 20mph speed limit rollout across Wales is simply ludicrous.”
Natasha Asghar, the Welsh Conservative shadow transport minister, added: “Labour and Plaid Cymru have refused to listen to public opinion and are continuing to wage their anti-worker, anti-road and anti-motorist agenda.”
But organisations including Brake, Cardiff Cycling Campaign, We Are Cycling UK Cymru, Friends Of The Earth Cymru, Sustrans and Living Streets Cymru have signed a joint letter organised by the 20’s Plenty For Us campaign backing the rollout.
“It’s not just a road safety benefit. It also supports broader health, climate and societal goals such as helping the vulnerable to get about, improving social connection, reducing air and noise pollution, and more,” they said.