Oxfordshire County Council is supporting calls for a full A34 interchange to the north of Abingdon - claiming it is the best way to reduce traffic in the town.
Ian Hudspeth, the council's cabinet member for transport, has written to the Government's Highways Agency asking it to expand the Lodge Hill junction.
At present, drivers can only join the northbound carriageway, or leave the southbound carriageway there.
Vale of White Horse councillors agreed last month to campaign for a four-way junction at Lodge Hill.
The county council says the number of vehicles on Abingdon's roads rose by six per cent in the past seven years.
Traders say the traffic has hit business, as shoppers shy away from using the town's congested roads.
Mr Hudspeth said: "One way to reduce the traffic in the town centre would be to improve the Lodge Hill junction to the A34, so that fewer people have to cross Abingdon.
"Ideally, I would like to see everybody, including the county, district and town councils, joining forces with residents, politicians of all parties, traders and any other interested parties, to speak as one on Lodge Hill."
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