Work on introducing an extra lane in Parkway to ease congestion at the Army & Navy roundabout in Chelmsford was completed ahead of schedule.
The scheme was also finished within budget. Essex County Council highways chiefs described it as "challenging to deliver" because of the sheer amount of pipes and cabling under the roads.
County Hall was successful in its bid for more than £1m from central government to add to the cash already set aside for the project.
Work to replace the nearby Chelmer Viaduct is expected to start next year.
County councillor Rodney Bass, said: "The roundabout is now allowing a steady movement of a significant amount of traffic into and out of the city centre and its importance to local residents, visitors and businesses cannot be overstated.
"Safety for all drivers using the roundabout has also been greatly improved."
He added: "I want to thank everyone for their patience and understanding while this essential project was carried out."
