The transport minister said the widening scheme would improve safety
A major project to widen a section of the A23 between Crawley and Brighton is to go ahead.
The scheme for a three-lane carriageway in each direction between Handcross and Warninglid in West Sussex will cost between £76m and £105m.
The Department for Transport said widening the road would improve journey time and safety.
Initial work will start in the summer with the project due for completion in early 2014.
A public inquiry was held in June 2009 amid concerns the scheme would pass through an ancient woodland.
Since then, the government has accepted the changes recommended by the planning inspector.
Transport Minister Chris Mole said he was pleased to see the scheme go through its "final planning stages".