The eastbound carriageway of the Southwick tunnel is partly closed following an accident involving a digger.
The vehicle got stuck in the tunnel at 4pm after crashing into one of the matrix signs suspended from its roof, Sussex Police said.
Hundreds of vehicles were diverted from the road, part of the A27 trunk road across Sussex, while debris was cleared and the six-wheeled digger was freed.
For more than half an hour the entire eastbound section of the tunnel was closed to traffic.
At 4.50pm Sussex Police said one lane had been reopened and vehicles were being allowed through but they warned drivers to expect delays.
For up to date traffic information visit the local information section of this website.
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