Police are due to close the M4 in both directions to allow an air ambulance to land after a car overturned.
The crash happened on the eastbound carriageway just before 07:00 BST between junction 35 for Pencoed in Bridgend county and junction 34 for Miskin, Rhondda Cynon Taff.
South Wales Fire and Rescue Service and ambulances are at the scene, but it is not known if anyone has been injured.
Traffic reports said there may also be another car involved.
End of Twitter post by @EvilAwkeye