The 50mph speed limits and other measures such as increased signage for smooth traffic flow are aimed at reducing levels of nitrogen dioxide where emissions have found to exceed legal limits.
The measures – introduced at A494 at Deeside, A483 at Wrexham, M4 between Junctions 41 and 42, M4 between junctions 25 and 26 and A470 between Upper Boat and Pontypridd – are expected to help reduce emissions by up to 18%.
Welsh Environment Minister Hannah Blythyn said: “Current levels of nitrogen dioxide exceed the legal limit at the five road locations so we ned to act as soon as possible.
“The actions we take to improve air quality in Wales have a key role in supporting the right conditions for better health and wellbeing.”
The programme is part of the government’s new £20 million Air Quality Fund to improve air quality.