There are four miles of queues on the A1 northbound at Stamford following a collision between a car and a lorry.
The crash happened at about 10.30am just past the RAF Wittering turn off.
A Cambridgeshire police spokesman said the car had rolled onto its roof in the carriageway.
Police, fire and ambulance services are all on scene.
Motorists are advised to avoid the area if possible.