The speed limit on residential roads in Wales is to change from 30mph to 20mph this weekend even though the leader of the House of Commons says the move is "absolutely insane".
Wales will become the first country in the UK to make 20mph the default speed limit on most residential roads when the new rules are imposed from Sunday.
Most roads that are currently 30mph will become 20mph, although councils have discretion to impose exemptions.
Welsh first minister Mark Drakeford said the reduction will save lives and save the NHS £92m a year.
But critics say the lower average speeds will increase journey times and cost money.
Conservative Party MP Penny Mordaunt, the Commons leader, described the rollout as "crazy".
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Mordaunt told the House of Commons on Thursday: “This is absolutely insane even by the standards of Labour’s Welsh government.
“They have ignored businesses and they have ignored the public. They are pushing ahead with this scheme despite huge opposition to it.
“But more disturbingly it is going to increase individuals’ fuel bills considerably and actually be harmful to the environment.
"There are circumstances where of course 20mph speed limits are a good idea, but having them as the default for many roads is crazy."
On Friday, the RAC warned motorists in Wales not to rely on their sat-navs after the speed limit drops from 30mph to 20mph.
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GPS companies have indicated that map updates may not happen immediately after the speed limit falls.
The RAC said motorists should “pay full attention” to signs rather than electronic devices after the change is made.
Its head of policy, Simon Williams, said: “It’s vitally important that drivers are fully aware of the arrival of the 20mph limit in Wales, and pay full attention to all road signage.
“Until sat nav systems have been fully updated, they shouldn’t rely on them to know what the speed limit is on any particular stretch of Welsh road.”
Williams said compliance with 20mph limits is “quite poor”, saying it would be “more effective to target areas where they are most needed”, such as on residential roads or in areas where there is high footfall.
He added: “Even if compliance with new 20mph limits is poor, it should lead to an overall reduction in speeds which will have a positive effect on road safety.”
On Friday, Drakeford told BBC Breakfast: “It is a major change. It will need time to bed in.
“It is not a change that is being introduced in order to make life difficult for people and therefore the enforcement authorities will approach it in that way.
“The approach will be a reasonable one in which we give people a chance to get used to the new regime.
“And then, as the police say, people who flagrantly and deliberately are not prepared to obey the rules that everybody else will be following, then that will be a different matter.”
There have been reports of the new signs being defaced in some areas, including Conwy, Gwynedd, Newport, Torfaen, Wrexham and Flintshire.
The Welsh Conservatives, who are opposed to the rollout, have cited Welsh government documents that estimate the cost to the country’s economy of increased journey times from lower average vehicle speeds at anywhere between £2.7bn and £8.9bn.
On its website, the Welsh government said: “Our assessment shows that reducing speeds to 20mph can result in an average increase of one minute per journey, nine lives saved and 98 serious injuries prevented each year.
“It is estimated that the casualty prevention savings, including the reduced impact on NHS and emergency services, could be up to £92m every year.”
On Thursday, the Evening Standard reported that the speed limit on more than half of London's roads has been reduced to 20mph.
Transport for London (TfL) said 51% of all roads in the capital now have a 20mph limit.