The plans are currently being trialed on some roads, meaning a 50mph limit could well be rolling out on roads in Leeds and Yorkshire if successful.
The changes are part of plans to tackle nitrogen dioxide pollution levels in areas where pollution is above legal limits.
The changes are being introduced on five stretches of motorways and A-roads in Wales, the Welsh government has announced.
The measures come into force within the next two months in areas where nitrogen dioxide levels are above legal limits.
The UK and Welsh governments have been ordered by the High Court to put plans in place to tackle pollution after failing to meet European Union targets.
The roads affected are the M4 at Port Talbot and Newport, A494 at Deeside, A483 at Wrexham and A470 between Upper Boat and Pontypridd.
Reducing speed limits from 70 mph to 50 mph will cut nitrogen dioxide emissions by "up to 18%", according to the Government.
RAC Foundation director Steve Gooding said: "Drivers are becoming increasingly used to a patchwork approach to speed limit setting whether that be in the interests of cutting congestion, improving safety or reducing air pollution.
"Ironically there will be stretches of road where drivers actually struggle to reach anything like 50 mph and if there's one thing worse for air quality than vehicles hurtling along at high speed, it's where they sit nose to tail in jams."
AA president Edmund King added: "If speed limits are to be reduced on parts of the M4 in Wales it should be a flexible system that varies speeds and is linked to air quality monitoring, so the speed limits are only reduced when air quality levels are exceeded."
Reduced speed limits are part of a Â£20 million Air Quality Fund which has been put to public consultation.
Ministers propose stopping or restricting the most polluting vehicles from entering Clean Air Zones to cut congestion and emissions.
Environment Minister Hannah Blythyn said: "I am committed to taking action to reduce air pollution in Wales to support a healthier future for our communities and protect our natural environment."