Congestion lasting around six hours on Great Yarmouth's roads was caused by the discovery of a "structural issue" on a bridge, National Highways has said.
From around 3pm on Tuesday the A47 was closed in both directions between the A149 and A1243 at Gapton Hall due to safety concerns on Breydon Bridge.
Norfolk Constabulary urged people to avoid the area due to the backlog of traffic.
The bridge reopened shortly before 9.30pm.
A spokesperson for National Highways said: "Breydon Bridge was subject to an emergency closure following the discovery of a structural issue.
"It was initially believed this could impact the safe operation of the bridge so a decision was made to temporarily close the structure while assessments were carried out."
Ajith Nair, project manager for the bridge's redevelopment said: "We have been carrying out repairs since September to bring the bridge up to modern standards. This unexpected issue was identified as part of those works, and to ensure the safety of those using the bridge, it was necessary to temporarily close it.
"We remain grateful for road users' patience while safety checks were carried out before the bridge was reopened."
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