Up to 24 tonnes of flour and 200 litres of diesel have been spilt across the M5 in a crash.
Previously-bagged flour was left strewn across the southbound carriageway in a collision between a lorry and a car towing a caravan at about 04:30 BST.
One lane remains closed between junctions 14 and 15 in Gloucestershire amid a clean-up operation.
Highways England said work to clear the flour, debris and fuel spillage will be completed overnight.
Avon and Somerset Police said there were no reports of serious injury.
Jack Tappin, from Highways England, said the articulated lorry "ended on its side in lanes one and two" following the collision, damaging a safety barrier,
Motorists have been advised that one lane will remain closed between junction 14 and junction 15 overnight.