Temporary 50mph speed limits on two stretches of the M4 – introduced to cut air pollution through built-up areas – are to be made permanent.
The limit was reduced on the motorway and on stretches of A-road in December 2018 in an attempt to cut nitrogen dioxide emissions.
The Welsh Government has now confirmed the restrictions will stay permanently.
The 50mph limits are at the M4 Port Talbot, M4 Newport, A470 Pontypridd, A483 Wrexham and A494 Deeside.
A Welsh Government spokesman said nitrogen dioxide levels were above legal limits and “must be reduced”.
“It was established that a 50mph speed threshold alone was the measure that would achieve compliance in the shortest time possible,” he said.
“The current speed limit in place will be retained through permanent traffic regulation orders and remain in force to maintain air quality standards.”
In January 2018, the Welsh Government conceded a case brought by environmental campaign group Client Earth which said ministers had failed to meet EU targets to cut pollution.
Their campaigns and policy manager Andrea Lee urged the Welsh Government to do everything possible to meet air pollution legal limits quickly.
“Ministers could also send a clear signal with new legislation that would set legally binding targets to meet World Health Organization guideline levels by 2030,” she said.
Head of the British Lung Foundation in Wales Joseph Carter said he was “thrilled” the limit had been made permanent.
But he warned reduced speed limits “will only take us so far”.
“We need to see much bolder action if we’re to create the sort of change that will free us from the grip air pollution has on our health and wellbeing,” Mr Carter said.
An AA spokesman said it was “always the motorist that seemed to foot the bill”.
He said: “There is all this talk of banning fossil fuel vehicles by 2040 and that sort of thing, but the move to introducing electric vehicles which are viable for people in Wales is moving at a snail’s pace.”