Drivers will face a continuing 50mph speed limit on Wales most expensive key road despite £336M upgrade and three-years late construction.
The five-mile A465 Heads of the Valleys road through the Clydach Gorge is part of a £800M widening of the dual carriageway between Monmouth and Neath Port Talbot.
Controversially, the M4 around Port Talbot was given a 50mph limit where bends were judged to be sub-standard for a motorway when it was opened more than 50 years ago.
A new 50mph limit was extended west to the Briton Ferry bridges to reduce air pollution but there have been reports the limit has never been enforced on drivers.
The Newport section of the M4 is also subject to variable speed limits which are due to traffic conditions and in recent years Welsh Government has also claimed the lower limit will improve health of local residents due to claimed reductions in emissions.