As someone who has been travelling to Tenby regularly since the 70s, I can say with confidence that more capacity on the road reduces congestion. Anyone who has queued at St Clears crossroad for an hour waiting for the lights to change, or was in the queue to go over the pontoon bridge at Carmarthen will agree that getting to Pembrokshire is a piece of pee compared to those days and that's with the increased volume of cars on the road.

Population increases, greater affordability of cars, higher employment levels(pre covid), home shopping, availability of recreational facilities and time to pursue them, fear of being mugged walking etc. drives congestion, not wider roads.