The council-backed study of the A350, which runs from Poole to Shaftesbury in Dorset, was carried out to see how road conditions can be improved in the area.
It also looked at the C13, B3081 and A30. More than 50% of their surface was also found to be "poor quality".
The county council is to hold a series of public meetings from Thursday to outline plans to improve the roads.
'Crucial evidence'
The council is considering road building, better traffic management measures and improving public transport.
It has long cherished plans to improve the A350, with environmental issues killing a scheme for a Melbury Abbas bypass back in 1995.
The council study showed that 78% of the A350 was of "poor quality", along with 54% of the C13/B3081/A30 corridor.
Gordon Jenkins, Dorset County Council's project manager, said: "This report provides us with some crucial evidence about the A350 corridor, and, together with local views, will inform our decisions about what should happen there in the future."
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