The A2 carriageway from Belfast to Bangor has re-opened following an "ongoing incident".
A man collapsed at the roadside and is being treated by paramedics.
Police shut the Bangor-bound side of the road at Holywood Exchange, between Knocknagoney and Holywood, and motorists were being diverted to facilitate the ambulance.
There are severe delays along the Sydenham Bypass, the A55 Outer Ring and on the Newtownards, Albertbridge, and Holywood Roads.
There was also a crash on the Newtownards Road at Ballyhackamore around 12.30pm, between a bus and van which may have contributed to the traffic build-up.
Some motorists have reported being stuck in traffic for an hour.
Delays remain on the following routes:
Sydenham By Pass Bangor bound from the M3
A55 Outer Ring towards Holywood
Newtownards, Albertbridge, and Holywood Roads
Old Holywood Road