The work, starting this month, will aim to improve the drainage system on the A27 between Dankton Lane, Sompting, and the Adur viaduct, in Lancing.
Defective pipes will be replaced and small areas of surfacing will be carried out west of Manor Road roundabout to Busticle Lane and “some technical repairs that are needed”.
The work spans a distance of two miles and the HA will work seven days a week between 9.30am and 4.30pm during the daytime and 8pm and 6am at night.
The HA said “as much of the noisier operations as possible” will be carried out during the daytime, and loud work in the evening will be completed by 11pm.
Daytime work will be carried out under single lane closures and will be undertaken on both east and westbound carriageways at the same time.
Diversion routes will be signposted, with one route south of the A27 going along the A24, A259 and A283, and the other, for through-traffic travelling towards Chichester or Brighton, along the A24 and A283.
Overnight road closures will occur on the A27 between Dankton Lane and Manor Road.
The HA said signage will “make it clear business is open as usual”.
The HA said access for residents and businesses would be maintained “as best as we can throughout the work”.
There is an HA information line, available 24 hours a day, seven days a week, on 0300 123 5000.