Pictures taken last week show trees being felled next to the road between the A280 roundabout and Station Road in Angmering, clearing the space for two extra lanes of traffic.
Work is also being done on utilities and archaeological evaluations.
The same works will also take place between Highdown Drive and the Body Shop roundabout in Littlehampton.
None of the trees being chopped down have preservation orders on them, West Sussex County Council said.
An ecologist will be on hand throughout the works to ensure any nesting birds are not disturbed and there will be a replanting scheme at the end of the project, the council added.
Preparation work was stopped while a public inquiry was held about West Sussex County Council buying land for the £25.8m scheme.
The main part of the construction is set to begin in autumn.