Roadworks costing £76m start today on the A23 in West Sussex between Handcross and Warninglid.
The dual carriageway just south of Gatwick Airport and on the main road between London and Brighton will be widened to three lanes in each direction.
The work will take two years to complete.
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Perfect conditions for runners saw some fast times around the famous Portsmouth route, and around 3 million pounds raised for charity.