The £4.5 million scheme took place on the northbound carriageway.
The roadworks at junction 6 in Welyn included a new road surface, road markings, safety fencing and improved drainage.
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Highways England Project Manager, Mark Saunders, said: "The new layout has improved this busy junction as lane one now leaves the motorway at junction 6 and lanes two and three continue on to junction 7.
"This has removed the bottleneck within the junction where lanes two and three used to merge.
"The on-slip has been extended to give a greater distance for traffic to merge with existing A1 traffic."
Mr Saunders added there will be additional closures in order to erect new overhead gantry signs.
(LM)
