State Highway 1 on the Brynderwyn Hills is likely to remain closed for most of the day after a crash.
The crash involved a fuel tanker and a car and the scene needs to be cleared, the Transport Agency said.
Two people received minor injuries.
The petrol tanker was fully laden and would take time to remove from the road, the agency said.
Motorists are advised to plan for an extra 25 minutes on their journeys to allow for for detours.