Work to make it quicker and easier to reach the Tesco Extra supermarket in Chichester begins on Monday.
West Sussex County Council is starting a major road improvement scheme along Cathedral Way and Fishbourne Road East.
Traffic travelling from Chichester along Cathedral Way will not be able to turn right to access Tesco while the works starts.
Motorists will have to drive around the A27 Fishbourne Roundabout and back onto Cathedral Way instead.
The work is anticipated to be finished by late April.
Disruption is expected.
Shoppers will still be able to access the Tesco Extra throughout the works.
Cathedral Way will stay open throughout the duration – except for 10 nights across the 14-week period– but the carriageway will have to be narrowed to keep the workforce safe.
Work during daytime hours will take place between 7.30am and 5.00pm.
Motorists are advised to find alternative routes during peak hours to avoid congestion.
A public drop-in event explaining the scheme is being held in the Tesco foyer between 10.00am and 12 midday on Wednesday 7 January 2015.
Engineers from the County Council and contractor Balfour Beatty will be on hand to answer any questions the public may have.
A public display will also remain in Tesco for the duration of the works for people to view.