Work to ease congestion at Stockbridge Roundabout near Chichester will start on 30 August and last for 12 weeks, the Highways Agency said.
The work will include widening the eastbound and westbound approaches to the roundabout, which will allow more traffic to get through the area.
A dedicated left turn for access onto the A286 from the A27 will be created.
The work will take place from 0700BST until 1900BST on the roundabout island, behind barrier guards.
Anti-skid
Routine maintenance work will also be carried out at the same time, including repairs to existing drainage and road signs.
The roundabout and the approaches will be resurfaced with an anti-skid surface, providing a safer road for drivers, the agency said.
For the safety of the workforce and road users, it will be necessary to close the A286 off peak and overnight for a period of five days while the resurfacing works are completed.
There will be a fully-signed diversion route in place and a temporary speed limit of 40mph with temporary two-way traffic lights.
Ing Fischer, Highways Agency route manager, said: "The improvements will help ease congestion around the junction. We ask drivers to pay attention to the new road layout and take notice of the warning signs and allow extra time for their journeys whilst this work is being carried out."