The A1 southbound near Banbridge has reopened.
Police asked drivers to seek an alternative route on November 23 after a crash and the dual carriageway remained closed afterwards due to "an unsafe amount of surface water".
It has since reopened.
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A PSNI spokesperson said: "The A1 southbound between Halfway Road and Kilmacure Road junctions has reopened.
"Please continue to drive with care, reducing your speed in accordance with the adverse weather conditions."
A PSNI spokesperson had said: "Vehicles have been cleared from the A1 southbound following an earlier collision; however the carriageway remains closed between Halfway Road and Kilmacure Road junctions due to an unsafe amount of surface water.
"Roads Service will attend in due course.
"Please seek an alternative route and exercise caution on your journey."
A PSNI spokesperson earlier said: "Motorists are advised that the A1 southbound is closed between Halfway Road and Kilmacrue Road junctions following a road traffic collision.
"Please seek an alternative route for your journey."
Meanwhile, a yellow weather warning for rain issued by the Met Office for Co Armagh, Co Down and Co Antrim is in place until 10am on Wednesday.
The forecaster said: "Heavy rain may lead to some flooding and disruption to travel."
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